TY - JOUR
AU - Calderini, Daniel, F
AB - Abstract Background and Aims The pericarp weight comprises <17 % of wheat grain weight at harvest. The pericarp supports the hydration and nutrition of both the embryo and endosperm during early grain filling. However, studies of the pericarp and its association with final grain weight have been scarce. This research studied the growth dynamics of wheat pericarp from anthesis onwards and its relationship to final grain weight under contrasting plant densities and night warming. Methods Two spring wheat cultivars contrasting in kernel weight (Bacanora and Kambara) were sown in field conditions during seasons 2012–13 and 2014–15. Both genotypes were grown under contrasting plant density (control, 370 plants m–2; and low plant density, 44 plants m–2) and night temperatures, i.e. at ambient and increased (>6 °C) temperature for short periods before and after anthesis. From anthesis onward, grains were harvested every 3 or 4 d. Grain samples were measured and the pericarp was removed with a scalpel. Whole grain and pericarp fresh and dry weight were weighed with a precision balance. At harvest, 20 grains from ten spikes were weighed and grain dimensions were measured. Key Results Fresh weight, dry matter and water content of pericarp dynamics showed a maximum between 110 and 235 °Cd. Maximum dry matter of the pericarp ranged between 4.3 and 5.7 mg, while water content achieved values of up to 12.5 mg. Maximum values and their timings were affected by the genotype, environmental condition and grain position. Final grain weight was closely associated with maximum dry matter and water content of the pericarp. Conclusions Maximum pericarp weight is a determinant of grain weight and size in wheat, which is earlier than other traits considered as key determinants of grain weight during grain filling. Better growing conditions increased maximum pericarp weight, while higher temperature negatively affected this trait. Kernel, seed dynamic, dry matter, water content, grain size, grain yield INTRODUCTION Global wheat production is challenged by the need to increase grain yield at a rate of 2.4 % year–1 to feed the growing human population (Ray et al., 2013). This aim should be achieved mainly by accelerating genetic gains (Hall and Richards, 2013). Grain yield is the product of the number (GN) and weight of grains (GW); therefore, the breeders’ efforts should improve either of them or both (Qin et al., 2015). However, trade-off between GW and GN should be carefully considered because negative associations between the main yield components have been reported recently in wheat studies (Bustos et al., 2013; García et al., 2014; Quintero et al., 2018; Rivera-Amado et al., 2019), highlighting the need to counteract this negative association in the drive to increase grain yield in breeding programmes. The increase of GW is seen as one strategy for increasing grain yield per se in wheat but also to counteract the trade-off with GN. However, the present understanding of the genetic and physiological determinants of GW is still limited (Brinton and Uauy, 2019). In this regard, maternal tissues of grains have been found to be key to GW determination in different crop species such as wheat (Calderini and Reynolds, 2000; Yu et al., 2015; Simmonds et al., 2016; Benincasa et al., 2017; Reale et al., 2017), barley (Radchuk et al., 2011), sorghum (Yang et al., 2009) and sunflower (Lindström et al., 2006, 2007; Rondanini et al., 2009; Castillo et al., 2017). In grasses, several studies demonstrated that maternal tissues of grains are closely associated with the final individual GW (IGW) in wheat (Calderini et al., 1999; Ghiglione et al., 2008; Xie et al., 2015; Brinton et al., 2017) but, although there is agreement about the importance of these tissues for GW determination, they have been only partially researched, lacking information of detailed studies on maternal tissue dynamics from anthesis onward and their impact on final GW, especially in a key crop such as wheat. This is possibly due to the anatomy of the pericarp since maternal tissues are fused with the thin seed coat in caryopsis fruits (Hemery et al., 2010). From floral initiation to physiological maturity, the maternal tissues of wheat grains show a complex development and growth (Ghiglione et al., 2008; Ferrante et al., 2010). For example, they give rise to a wide ovary wall during floral development (Benincasa et al., 2017; Reale et al., 2017), which covers internal structures (Sainiab et al., 1983; Ferrante et al., 2010; Chakrabarti et al., 2011). These tissues, which will become the pericarp after ovule fertilization, are sensitive to environmental conditions before anthesis such as temperature (Calderini et al., 1999), photoperiod (González et al., 2005) and nitrogen availability (Ferrante et al., 2010), affecting floral growth (Guo et al., 2015) and carpel weight (Hasan et al., 2011). After anthesis, maternal tissues include several layers such as the outer and inner pericarp, testa and nucellar epidermis (Hemery et al., 2010), where each layer has its own cellular organization (Antoine et al., 2003) and chemical composition (Barron et al., 2007). Maternal tissues during early grain filling, especially the pericarp, are the main components of the wheat grain. The outer pericarp represents >70 % of grain dry matter at anthesis, but its relative weight falls rapidly during grain growth (Schnyder et al., 1993). Functionally, the pericarp covers the seed tegument, endosperm and embryo tissues of wheat grains (Brinton et al., 2017), and controls the hydration and nutrition of grains during the coenocytic stage, e.g. the water transport into the endosperm cavity (Wang and Fisher, 1994), the synthesis of organic compounds (Caley et al., 1990; Fujita and Taira, 1998) and the temporal storage of starch (Calderini and Reynolds, 2000; Xiong et al., 2013; Yu et al., 2015). The enlargement of pericarp cell walls and the relationship with grain size suggest that there is a link between cell expansion of maternal tissues, grain water content and grain volume which controls the final GW of wheat (Lizana et al., 2010; Hasan et al., 2011; Muñoz and Calderini, 2015). Even so, it is unknown how environmental conditions modulate the pericarp growth dynamics and its subsequent impact on the size and weight of grains. The GW is affected by different management and environmental factors, which include plant density and temperature. For example, the availability of resources per plant positively modulates grain yield, yield components such as GN per spike and Thousand grain weight (Whaley et al., 2000). This is why plant densities were assessed in the present study to evaluate the sensitivity of maternal tissues and their relationship to final GW. On the contrary, temperature is a major environmental factor that affects both the development and growth of grain crops (Porter and Gawith, 1999), where increased temperatures at both pre-anthesis and post-anthesis have a negative impact on final GW of wheat and, in turn, on grain yield (Calderini et al., 1999; García et al., 2015). Higher night temperatures (Peng et al., 2004) as well as more episodes of heatwaves are expected as a consequence of climate change (McKersie, 2015; Alexander, 2016; FAO, 2016). Therefore, higher night temperatures are a key to evaluate the response of maternal tissue to future climate conditions of wheat cropping systems. However, little is currently known about how and to what extent increments in temperature could affect the maternal tissues of wheat grains during grain filling and their relationship to the final GW. In the present study, the sensitivity of the pericarp and GW to increased night-time temperature was evaluated. The aim of this research was, therefore, to study the development and growth of the wheat pericarp from anthesis onward and their relationship to final GW in grains set at different positions within the spike under contrasting plant densities and increased night temperatures. MATERIALS AND METHODS Plant material and experimental conditions Two spring wheat genotypes developed by the CIMMYT of contrasting GW [Bacanora (Bac): medium GW and Kambara (Kam): high GW], but with similar phenology and grain yield potential, were evaluated in two experiments carried out under field conditions in the experimental station of Universidad Austral de Chile in Valdivia (E.E.A.A.), Chile (39°47'S, 73°14'W, 19 m asl). These experiments were accomplished in the experimental station of Universidad Austral de Chile during seasons 2012–13 (Exp. 1) and 2014–15 (Exp. 2) in a Duric Hapludand soil of southern Chile. The experiments were set up in a split-plot design with three replicates in the field, where the main plots were subject to different plant densities or night warming treatments, and the sub-plots were assigned to different genotypes. Experimental treatments were conceived to affect both GW and maternal tissues. In Exp. 1, two plant density treatments were carried out for both genotypes: (1) a conventional plant density (control or Con) of 370 plants m−2 and (2) a low plant density (LR) of 44 plants m−2. In the conventional plant density treatment or control (Con), plots consisted of 13 rows of 2.5 m long, 0.15 m distance between rows and 0.18 m between seeds. In the LR plots, seeds were sown in a squared arrangement of 0.15 m × 0.15 m, where plots consisted of 15 rows of 6.0 m long and 0.15 m distance between rows and seeds. In Exp. 2, three night warming treatments were evaluated; one control at ambient temperature and two timings of warming treatments increasing night temperature, i.e. at pre-anthesis (T°Pre) and post-anthesis (T°Post). All night warming treatments were sown under a conventional plant density of 370 plants m−2. In the T°Pre treatment, night air temperature was increased by 6 °C above the ambient temperature during a period from 10 d before anthesis to anthesis, while in the T°Post treatment, night air temperature was also elevated by 6 °C above the ambient temperature from anthesis to 16 d after anthesis (DAA). The periods of higher temperature were chosen taking into account that during the 10 d before anthesis the carpels of the ovary grow rapidly (Calderini et al., 1999) and from anthesis to 16 DAA grain enlargement occurs (Hasan et al. 2011). To increase the crop temperature, wood frame chambers (18 × 3.0 × 1.25 m) were covered with transparent polyethylene (100 μm) in both warming treatments. With the aim of increasing night temperature, the chamber lids were closed at 19.00 h and removed at 08.00 h every day during the treatment. The removal of the chamber lids was done to prevent increasing the temperature during the day and to avoid the interception of solar radiation by the polyethylene film. The temperature within the chambers was increased by electric heaters controlled by thermal sensors placed at 0.25 m above the ground surface, connected to a temperature regulator as in Lizana and Calderini (2013). Air temperature within the chambers was recorded using four data loggers (Smart Button, ACR data logger). Two sensors were placed at 0.25 m above the ground surface, and the other two at 1.0 m. Ambient temperature was registered in a weather station (Davis Vantage Pro, USA) close to the experiment (<50 m). In both experiments, plots were fertilized at sowing with 150 kg N ha−1, 300 kg P2O5 ha−1 and 150 kg K2O ha−1. An additional fertilization of 150 kg N ha−1 was applied at tillering across treatments and experiments. Diseases were controlled by recommended insecticide and fungicide. Weeds were removed by hand or by chemical applications. Bird attack was prevented through setting fishing nets about 2.5 m above the ground supported by wooden stakes and wire ties (<0.05 m diameter) and by spraying repellent (BIRD SHIELD, BirdShield Repellent Corporation). Plots were periodically irrigated to complement rainfall during the growing seasons every 3 d after the last rainfall [approx. 10 min per plot, i.e. approx. 20 L m–2 (Supplementary data Fig. S1)]. It is also important to take into account that the Andisoils, where the experiments were carried out, have a total crop available water content between 180 and 200 mm (Dörner et al., 2015). Therefore, the combination of rainfall, irrigation and soil water storage capacity avoided water stress during all experiments. Phenology, plant sampling and measurements The crop phenology of wheat was recorded according to the decimal code scale (Zadoks et al., 1974). The timing of physiological maturity was estimated when grains of wheat reached a water concentration of 37 % (Calderini et al., 2000). Thermal time was estimated as the average between maximum and minimum daily temperatures (°Cd) considering a base temperature of 0 °C. From anthesis onwards, grain dimensions, fresh and dry weight of individual grains corresponding to two grain positions (GPs) closest to the rachis (G1 and G2) of two central spikelets were measured in four main shoot spikes per plot every 3 d (Exp. 2) or 4 d (Exp. 1). Length, width and height of grains were recorded immediately after sampling the spikes using an electronic caliper (6 inch/150 mm Digital Calipers, China). Afterwards, the fresh weight of grains was measured using an electronic balance (Mettler Toledo, XP205DR, Greifensee, Switzerland). In these grains, the outer pericarp was removed to record fresh and dry matter. To sample the pericarp, a shallow incision with a scalpel was carefully made along the dorsal side from the base to the apex of the grain, without damaging the internal tissues. The outer pericarp was carefully peeled from the inner pericarp using tweezers (Fig. 1). The endosperm and embryo remained wrapped inside the inner pericarp. In a few cases, when tissues of the inner pericarp were attached to the outer pericarp, the inner tissues were carefully removed by tweezers. Immediately afterwards, the outer pericarp and the others tissues (embryo, endosperm and endocarp) were weighed on the same electronic balance. The outer pericarp was sampled up to 36 DAA because after that time the fusion of the pericarp and the seed layers prevented us from accurately separating the tissues. Fig. 1. Open in new tabDownload slide Photo and schematic drawing of the wheat grain structures. Top photo (A), wheat grain dissection at 15 DAA. Bottom drawing (B), schematic diagram of wheat grain at 15 DAA. Adapted from Yu et al. (2015). Fig. 1. Open in new tabDownload slide Photo and schematic drawing of the wheat grain structures. Top photo (A), wheat grain dissection at 15 DAA. Bottom drawing (B), schematic diagram of wheat grain at 15 DAA. Adapted from Yu et al. (2015). Dry weights of grains and pericarps (dry matter) were measured, after drying the samples at 65 °C in an oven for 48 h, with the same electronic balance. The water content of grains and pericarps was estimated as the difference between fresh and dry weight. A minimal handling effect was found when this was measured (Supplementary data Fig, S2). At harvest, ten additional spikes of similar development and size were sampled from each plot from the central spikelets of the main shoot spikes to record GW and dimensions in two proximal GPs closest to the rachis (G1 and G2). Calculations and statistical analysis The timecourse of fresh weight (FWP), dry matter (DMP) and water content (WCP) of pericarp in thermal time units (°Cd) after anthesis were fitted with a logistic equation [eqn (1)] using the software TableCurve 2D v5.01.02. y=a+4×b×n(1+n)2 ; n=e(−(X−c)d)(1) where y is the pericarp weight (mg), X is the thermal time from anthesis onward, a is the minimum value (mg), b is the difference between maximum and minimum values (mg), c is the thermal time (°Cd) when the pericarp reached its peak and d is the parameter proportional to the thermal time during which most of the maximum pericarp weight is formed. The rate and duration of grain filling and grain length (GL) were fitted by using a bi-linear broken-stick equation [eqns (2) and (3)] with an unknown breaking point as in Calderini et al. (2000): y=a+b×X ; if(X≤c)(2) y=a+b×c ; if(X>c)(3) where y is the GW (mg) or the GL (mm) studied, X the thermal time from anthesis, a is the intercept, b is the slope which estimates the rate of grain filling (mg °Cd–1) or the rate of grain elongation (mm °Cd–1), and c is the breaking point showing the time of physiological maturity (mg) or maximum GL (mm). Water content of grains (WCG) was calculated using a tetra-linear broken-stick equation [eqns (4)–(7)] with three unknown breaking points, adapted from Lizana et al. (2010): y=a+b×X ; if(X<c)(4) y=b×c ; if(c<X<d)(5) y=d+e×X ; if(d<X<f)(6) y=e×f ; if(X>f)(7) where y is the grain water content (mg) studied, X is the thermal time from anthesis, a is the intercept, b is the slope which estimates the rate of water accumulation (mg °Cd–1), c is the breaking point of stabilized water content (mg), d is the water content during the hydric plateau (mg), e is the thermal time (°Cd) to physiological maturity and f is the dehydration rate (mg °Cd–1). The volume of grains at harvest was calculated with grain dimensions (length, width and height), assuming the grain is an ellipsoid [eqn (8)], where π = 3.1416, a = 0.5 × length, b = 0.5 × width and c = 0.5 × height as in Hasan et al. (2011). Grain volume=43×(π×a×b×c)(8) Data recorded in Exps 1 and 2 were subject to an analysis of variance (ANOVA) by INFOSTAT software, 2018 (Di Rienzo et al., 2011). Linear regression analyses were used to assess the degree of association between variables. RESULTS Weather conditions and crop phenology in the experiments Weather conditions were similar between Exps 1 and 2 from seedling emergence to physiological maturity, i.e. the difference in average air temperature was <1.7 °C between experiments (Table 1). In Exp. 2, average daytime temperature was similar between both night warming treatments and the control, as the difference was <0.8 °C (Fig. 2). In this experiment, the night warming treatments increased night-time temperature by 4.5 and 5.7 °C over the control in the T°Pre and T°Post treatments, respectively (Fig. 2). As a consequence of the night warming treatments, the average temperature of the booting–anthesis period was increased by 5.3 °C over the control and the grain filling period was 2.9 °C above the control due to the different duration of each phenological phase (Table 1). Table 1. Mean temperature (°C) from seedling emergence to booting (Em–Bo), booting to anthesis (Bo–An) and from anthesis to physiological maturity (An–PM) in control and night warming treatments between 10 d before anthesis to anthesis (T°Pre) and from anthesis to 16 d after anthesis (T°Post) in Experiments 1 and 2 . . Mean temperatures (°C) . . . Experiment . Treatment . Em–Bo . Bo-An . An–PM . 1 Control 11.8 13.2 14.6 2 Control 10.1 13.2 14.3 T°Pre 10.1 18.5 14.3 T°Post 10.1 13.2 17.2 . . Mean temperatures (°C) . . . Experiment . Treatment . Em–Bo . Bo-An . An–PM . 1 Control 11.8 13.2 14.6 2 Control 10.1 13.2 14.3 T°Pre 10.1 18.5 14.3 T°Post 10.1 13.2 17.2 Night temperature was increased from 19.00 h to 08.00 h. Open in new tab Table 1. Mean temperature (°C) from seedling emergence to booting (Em–Bo), booting to anthesis (Bo–An) and from anthesis to physiological maturity (An–PM) in control and night warming treatments between 10 d before anthesis to anthesis (T°Pre) and from anthesis to 16 d after anthesis (T°Post) in Experiments 1 and 2 . . Mean temperatures (°C) . . . Experiment . Treatment . Em–Bo . Bo-An . An–PM . 1 Control 11.8 13.2 14.6 2 Control 10.1 13.2 14.3 T°Pre 10.1 18.5 14.3 T°Post 10.1 13.2 17.2 . . Mean temperatures (°C) . . . Experiment . Treatment . Em–Bo . Bo-An . An–PM . 1 Control 11.8 13.2 14.6 2 Control 10.1 13.2 14.3 T°Pre 10.1 18.5 14.3 T°Post 10.1 13.2 17.2 Night temperature was increased from 19.00 h to 08.00 h. Open in new tab Fig. 2. Open in new tabDownload slide Average temperature above the control under environmental conditions of night warming treatments during the whole day (24 h), daytime (08.00 h to 19.00 h) and night-time (19.00 h to 08.00 h) at pre-anthesis (T°Pre) and post-anthesis (T°Post) in Experiment 2. Data are means ± s.e. (n = 4). Fig. 2. Open in new tabDownload slide Average temperature above the control under environmental conditions of night warming treatments during the whole day (24 h), daytime (08.00 h to 19.00 h) and night-time (19.00 h to 08.00 h) at pre-anthesis (T°Pre) and post-anthesis (T°Post) in Experiment 2. Data are means ± s.e. (n = 4). As expected, crop phenology of Bac and Kam was similar in each experiment, and differences between genotypes were <2 d up to anthesis (Fig. 3). In Exp. 1, grain filling duration (i.e. from anthesis to physiological maturity) ranged between 38 and 51 d, whereas differences between genotypes reached up to 6 d (P < 0.001), especially due to the lengthening of the LR treatment (Fig. 3). This treatment extended (P < 0.05) the grain filling phase by 5 d in Kam without changes in Bac (Fig. 3). In Exp. 2, the T°Pre warming treatment accelerated the development rate of both genotypes, reaching anthesis about 2 d earlier than controls, while the T°Post treatment shortened (P < 0.01) the grain filling period by 3 d (Bac) and 5 d (Kam) relative to the control (Fig. 3). Interaction (P < 0.01) between genotype and the T°Post treatment was found in this experiment. Fig. 3. Open in new tabDownload slide Phenological phases of wheat genotypes Bacanora (Bac) and Kambara (Kam) from seedling emergence to physiological maturity in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Bars show the duration of each phase: from seedling emergence to booting (open bars), booting to anthesis (grey bars) and anthesis to physiological maturity (closed bars). Arrows show the start and the end of thermal treatments in Experiment 2. The standard error of the means are not shown because symbols are smaller than the bar lines. Fig. 3. Open in new tabDownload slide Phenological phases of wheat genotypes Bacanora (Bac) and Kambara (Kam) from seedling emergence to physiological maturity in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Bars show the duration of each phase: from seedling emergence to booting (open bars), booting to anthesis (grey bars) and anthesis to physiological maturity (closed bars). Arrows show the start and the end of thermal treatments in Experiment 2. The standard error of the means are not shown because symbols are smaller than the bar lines. Weight and size of grains at harvest A wide range of IGW was recorded in this study across experiments, genotypes, GP, plant densities and warming treatments, ranging from 50.3 to 74.8 mg of proximal GPs G1 and G2 (Table 2). For example, GW between genotypes ranged up to 33.3 % (P < 0.001), and G2 was up to 9.2 % heavier than G1 in Exp. 2 (P < 0.001). The LR treatment increased (P < 0.01) IGW between 6.9 and 12.1 % over the control treatment (Table 2), while night warming decreased (P < 0.05) IGW up to 12.0 % relative to the control and averaged across the heating assessed periods in Exp. 2 (Table 2). Interestingly, both the T°Pre and T°Post treatments negatively affected IGW, but genotypes showed different sensitivity in one period of the night warming treatments as a similar sensitivity of IGW was found between Bac and Kam in T°Pre (IGW decreased 5.1 and 4.4 %, respectively), but different under T°Post, when a negligible sensitivity was shown by Bac (decrease: 0.5 %) and a high sensitivity in Kam (decrease: 11.4 %) (Table 2). Table 2. Individual dry weight of grains (mg), volume (mm3) and length (mm) of wheat grains at harvest from two proximal (G1 and G2) grain positions to the raquis of central spikelets of wheat genotypes Bacanora and Kambara under control plant density (Con: 370 plants m–2), low plant density (LR: 44 plants m–2), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post) in Experiment 1 (season 2012–13) and Experiment 2 (season 2014–15) . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . Dry weight (mg) . Volume (mm3) . Length (mm) . Temperature . Dry weight (mg) . Volume (mm3) . Length (mm) . Bacanora G1 Con 50.5 46.4 6.82 Con 53.7 40.1 6.79 LR 56.6 52.9 7.16 T°Pre 50.3 36.7 6.52 T°Post 53.2 39.8 6.66 G2 Con 54.6 50.3 6.85 Con 57.2 43.6 6.88 LR 59.6 56.6 7.28 T°Pre 55.0 42.1 6.77 T°Post 57.2 43.5 6.79 Kambara G1 Con 66.7 64.7 7.62 Con 70.0 52.2 7.39 LR 72.8 69.9 7.88 T°Pre 67.1 50.3 7.19 T°Post 62.5 47.3 7.03 G2 Con 70.0 66.7 7.83 Con 73.0 54.1 7.69 LR 74.8 71.5 8.07 T°Pre 69.6 51.2 7.41 T°Post 64.2 49.0 7.21 s.e. 1.8 1.9 0.10 1.2 0.9 0.06 Plant density/temperature (T) ** *** ** * * *** Genotype (G) *** *** *** *** *** *** Grain position (GP) *** ** ** *** *** *** T × G n.s. n.s. n.s. *** ** ** T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP n.s. n.s. n.s. ** *** n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . Dry weight (mg) . Volume (mm3) . Length (mm) . Temperature . Dry weight (mg) . Volume (mm3) . Length (mm) . Bacanora G1 Con 50.5 46.4 6.82 Con 53.7 40.1 6.79 LR 56.6 52.9 7.16 T°Pre 50.3 36.7 6.52 T°Post 53.2 39.8 6.66 G2 Con 54.6 50.3 6.85 Con 57.2 43.6 6.88 LR 59.6 56.6 7.28 T°Pre 55.0 42.1 6.77 T°Post 57.2 43.5 6.79 Kambara G1 Con 66.7 64.7 7.62 Con 70.0 52.2 7.39 LR 72.8 69.9 7.88 T°Pre 67.1 50.3 7.19 T°Post 62.5 47.3 7.03 G2 Con 70.0 66.7 7.83 Con 73.0 54.1 7.69 LR 74.8 71.5 8.07 T°Pre 69.6 51.2 7.41 T°Post 64.2 49.0 7.21 s.e. 1.8 1.9 0.10 1.2 0.9 0.06 Plant density/temperature (T) ** *** ** * * *** Genotype (G) *** *** *** *** *** *** Grain position (GP) *** ** ** *** *** *** T × G n.s. n.s. n.s. *** ** ** T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP n.s. n.s. n.s. ** *** n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. n.s., not significantly different at P < 0.05; *, ** and ***, different at P < 0.05, < 0.01 and < 0.001, respectively. Open in new tab Table 2. Individual dry weight of grains (mg), volume (mm3) and length (mm) of wheat grains at harvest from two proximal (G1 and G2) grain positions to the raquis of central spikelets of wheat genotypes Bacanora and Kambara under control plant density (Con: 370 plants m–2), low plant density (LR: 44 plants m–2), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post) in Experiment 1 (season 2012–13) and Experiment 2 (season 2014–15) . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . Dry weight (mg) . Volume (mm3) . Length (mm) . Temperature . Dry weight (mg) . Volume (mm3) . Length (mm) . Bacanora G1 Con 50.5 46.4 6.82 Con 53.7 40.1 6.79 LR 56.6 52.9 7.16 T°Pre 50.3 36.7 6.52 T°Post 53.2 39.8 6.66 G2 Con 54.6 50.3 6.85 Con 57.2 43.6 6.88 LR 59.6 56.6 7.28 T°Pre 55.0 42.1 6.77 T°Post 57.2 43.5 6.79 Kambara G1 Con 66.7 64.7 7.62 Con 70.0 52.2 7.39 LR 72.8 69.9 7.88 T°Pre 67.1 50.3 7.19 T°Post 62.5 47.3 7.03 G2 Con 70.0 66.7 7.83 Con 73.0 54.1 7.69 LR 74.8 71.5 8.07 T°Pre 69.6 51.2 7.41 T°Post 64.2 49.0 7.21 s.e. 1.8 1.9 0.10 1.2 0.9 0.06 Plant density/temperature (T) ** *** ** * * *** Genotype (G) *** *** *** *** *** *** Grain position (GP) *** ** ** *** *** *** T × G n.s. n.s. n.s. *** ** ** T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP n.s. n.s. n.s. ** *** n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . Dry weight (mg) . Volume (mm3) . Length (mm) . Temperature . Dry weight (mg) . Volume (mm3) . Length (mm) . Bacanora G1 Con 50.5 46.4 6.82 Con 53.7 40.1 6.79 LR 56.6 52.9 7.16 T°Pre 50.3 36.7 6.52 T°Post 53.2 39.8 6.66 G2 Con 54.6 50.3 6.85 Con 57.2 43.6 6.88 LR 59.6 56.6 7.28 T°Pre 55.0 42.1 6.77 T°Post 57.2 43.5 6.79 Kambara G1 Con 66.7 64.7 7.62 Con 70.0 52.2 7.39 LR 72.8 69.9 7.88 T°Pre 67.1 50.3 7.19 T°Post 62.5 47.3 7.03 G2 Con 70.0 66.7 7.83 Con 73.0 54.1 7.69 LR 74.8 71.5 8.07 T°Pre 69.6 51.2 7.41 T°Post 64.2 49.0 7.21 s.e. 1.8 1.9 0.10 1.2 0.9 0.06 Plant density/temperature (T) ** *** ** * * *** Genotype (G) *** *** *** *** *** *** Grain position (GP) *** ** ** *** *** *** T × G n.s. n.s. n.s. *** ** ** T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP n.s. n.s. n.s. ** *** n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. n.s., not significantly different at P < 0.05; *, ** and ***, different at P < 0.05, < 0.01 and < 0.001, respectively. Open in new tab Similar to IGW, the sources of variation affected grain volume as the LR treatment increased this trait (P < 0.001); and night warming reduced grain volume (P < 0.05) (Table 2). The GW was linearly associated with grain volume in each experiment (Exp. 1: y = 0.95x + 6.46, R2 > 0.99; P < 0.001; and Exp. 2: y = 1.35x – 0.68, R2 = 0.99; P < 0.001). Additionally, GL was also affected by the genotype (P < 0.001) and GP in both experiments. For example, G2 was up to 4.1 % longer than G1 in Exp. 2 (Table 2). Low plant density and warming treatments impacted on GL, taking into account that LR increased (P < 0.01) this trait, while higher night temperature decreased (P < 0.001) the length of grains (Table 2). Time course of grain pericarp and its physiological variables Pericarp dynamics of fresh weight, dry matter and water content showed consistent patterns across genotypes, plant densities, night warming treatments and GP in both experiments (Fig. 4). All three pericarp weights rapidly increased, reaching their highest values by 110–235 °Cd after anthesis, followed by a fast drop until 300–400 °Cd, with little variation afterwards (Fig. 4; Tables 3 and 4). Pericarp dynamics were fitted by a logistic equation, where the coefficient of determination fluctuated between 0.70 and 0.99 (most of them reaching R2 ≥ 0.90). In addition, key indicators of pericarp weight dynamics such as maximum FWP, DMP and WCP, and the timing to reach them were affected by the sources of variation assessed in the present study (Fig. 4; Tables 3 and 4). Table 3. Maximum fresh weight (MFWP), dry matter (MDMP) and water content (MWCP) of pericarp from two proximal (G1 and G2) grain positions to the raquis of central spikelets of wheat genotypes Bacanora and Kambara under control plant density (Con: 370 plants m–2), low plant density (LR: 44 plants m–2), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post) in Experiment 1 (season 2012–13) and Experiment 2 (season 2014–15) . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . MFWP (mg) . MDMP (mg) . MWCP (mg) . Temperature . MFWP (mg) . MDMP (mg) . MWCP (mg) . Bacanora G1 Con 12.22 4.41 8.06 Con 13.43 4.40 9.23 LR 14.86 (21.7) 4.68 (6.1) 10.20 (27.7) T°Pre 13.97 (4.0) 4.45 (1.1) 9.74 (5.5) T°Post 14.61 (8.8) 4.31 (-2.0) 9.80 (6.2) G2 Con 12.52 4.27 8.30 Con 13.87 4.56 9.56 LR 13.96 (11.5) 4.55 (6.5) 09.93 (19.7) T°Pre 14.13 (1.9) 4.48 (-1.6) 9.85 (3.1) T°Post 14.67 (5.8) 4.43 (-2.7) 10.13 (6.0) Kambara G1 Con 14.25 5.10 09.38 Con 16.84 5.51 11.60 LR 15.57 (9.3) 5.57 (9.2) 10.14 (8.1) T°Pre 16.42 (-2.5) 5.08 (-7.8) 11.69 (0.8) T°Post 16.94 (0.6) 4.85 (-12.0) 12.02 (3.6) G2 Con 15.09 5.18 10.04 Con 17.66 5.72 12.15 LR 16.16 (7.1) 5.70 (10.1) 10.51 (4.6) T°Pre 16.91 (-4.2) 5.26 (-8.0) 12.00 (-1.3) T°Post 17.44 (-1.2) 5.01 (-12.5) 12.47 (2.6) s.e. 0.29 0.11 0.20 0.29 0.08 0.22 Plant density/temperature (T) ** ** ** n.s. * n.s. Genotype (G) ** *** * *** *** *** Grain position (GP) n.s. n.s. 0.06 n.s. *** ** T × G n.s. n.s. n.s. n.s. * n.s. T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP * * * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . MFWP (mg) . MDMP (mg) . MWCP (mg) . Temperature . MFWP (mg) . MDMP (mg) . MWCP (mg) . Bacanora G1 Con 12.22 4.41 8.06 Con 13.43 4.40 9.23 LR 14.86 (21.7) 4.68 (6.1) 10.20 (27.7) T°Pre 13.97 (4.0) 4.45 (1.1) 9.74 (5.5) T°Post 14.61 (8.8) 4.31 (-2.0) 9.80 (6.2) G2 Con 12.52 4.27 8.30 Con 13.87 4.56 9.56 LR 13.96 (11.5) 4.55 (6.5) 09.93 (19.7) T°Pre 14.13 (1.9) 4.48 (-1.6) 9.85 (3.1) T°Post 14.67 (5.8) 4.43 (-2.7) 10.13 (6.0) Kambara G1 Con 14.25 5.10 09.38 Con 16.84 5.51 11.60 LR 15.57 (9.3) 5.57 (9.2) 10.14 (8.1) T°Pre 16.42 (-2.5) 5.08 (-7.8) 11.69 (0.8) T°Post 16.94 (0.6) 4.85 (-12.0) 12.02 (3.6) G2 Con 15.09 5.18 10.04 Con 17.66 5.72 12.15 LR 16.16 (7.1) 5.70 (10.1) 10.51 (4.6) T°Pre 16.91 (-4.2) 5.26 (-8.0) 12.00 (-1.3) T°Post 17.44 (-1.2) 5.01 (-12.5) 12.47 (2.6) s.e. 0.29 0.11 0.20 0.29 0.08 0.22 Plant density/temperature (T) ** ** ** n.s. * n.s. Genotype (G) ** *** * *** *** *** Grain position (GP) n.s. n.s. 0.06 n.s. *** ** T × G n.s. n.s. n.s. n.s. * n.s. T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP * * * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. Relative change (%) from the control is shown in parentheses. n.s., not significantly different at P < 0.05; *, ** and *** different at P < 0.05, < 0.01 and < 0.001, respectively. Open in new tab Table 3. Maximum fresh weight (MFWP), dry matter (MDMP) and water content (MWCP) of pericarp from two proximal (G1 and G2) grain positions to the raquis of central spikelets of wheat genotypes Bacanora and Kambara under control plant density (Con: 370 plants m–2), low plant density (LR: 44 plants m–2), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post) in Experiment 1 (season 2012–13) and Experiment 2 (season 2014–15) . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . MFWP (mg) . MDMP (mg) . MWCP (mg) . Temperature . MFWP (mg) . MDMP (mg) . MWCP (mg) . Bacanora G1 Con 12.22 4.41 8.06 Con 13.43 4.40 9.23 LR 14.86 (21.7) 4.68 (6.1) 10.20 (27.7) T°Pre 13.97 (4.0) 4.45 (1.1) 9.74 (5.5) T°Post 14.61 (8.8) 4.31 (-2.0) 9.80 (6.2) G2 Con 12.52 4.27 8.30 Con 13.87 4.56 9.56 LR 13.96 (11.5) 4.55 (6.5) 09.93 (19.7) T°Pre 14.13 (1.9) 4.48 (-1.6) 9.85 (3.1) T°Post 14.67 (5.8) 4.43 (-2.7) 10.13 (6.0) Kambara G1 Con 14.25 5.10 09.38 Con 16.84 5.51 11.60 LR 15.57 (9.3) 5.57 (9.2) 10.14 (8.1) T°Pre 16.42 (-2.5) 5.08 (-7.8) 11.69 (0.8) T°Post 16.94 (0.6) 4.85 (-12.0) 12.02 (3.6) G2 Con 15.09 5.18 10.04 Con 17.66 5.72 12.15 LR 16.16 (7.1) 5.70 (10.1) 10.51 (4.6) T°Pre 16.91 (-4.2) 5.26 (-8.0) 12.00 (-1.3) T°Post 17.44 (-1.2) 5.01 (-12.5) 12.47 (2.6) s.e. 0.29 0.11 0.20 0.29 0.08 0.22 Plant density/temperature (T) ** ** ** n.s. * n.s. Genotype (G) ** *** * *** *** *** Grain position (GP) n.s. n.s. 0.06 n.s. *** ** T × G n.s. n.s. n.s. n.s. * n.s. T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP * * * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . MFWP (mg) . MDMP (mg) . MWCP (mg) . Temperature . MFWP (mg) . MDMP (mg) . MWCP (mg) . Bacanora G1 Con 12.22 4.41 8.06 Con 13.43 4.40 9.23 LR 14.86 (21.7) 4.68 (6.1) 10.20 (27.7) T°Pre 13.97 (4.0) 4.45 (1.1) 9.74 (5.5) T°Post 14.61 (8.8) 4.31 (-2.0) 9.80 (6.2) G2 Con 12.52 4.27 8.30 Con 13.87 4.56 9.56 LR 13.96 (11.5) 4.55 (6.5) 09.93 (19.7) T°Pre 14.13 (1.9) 4.48 (-1.6) 9.85 (3.1) T°Post 14.67 (5.8) 4.43 (-2.7) 10.13 (6.0) Kambara G1 Con 14.25 5.10 09.38 Con 16.84 5.51 11.60 LR 15.57 (9.3) 5.57 (9.2) 10.14 (8.1) T°Pre 16.42 (-2.5) 5.08 (-7.8) 11.69 (0.8) T°Post 16.94 (0.6) 4.85 (-12.0) 12.02 (3.6) G2 Con 15.09 5.18 10.04 Con 17.66 5.72 12.15 LR 16.16 (7.1) 5.70 (10.1) 10.51 (4.6) T°Pre 16.91 (-4.2) 5.26 (-8.0) 12.00 (-1.3) T°Post 17.44 (-1.2) 5.01 (-12.5) 12.47 (2.6) s.e. 0.29 0.11 0.20 0.29 0.08 0.22 Plant density/temperature (T) ** ** ** n.s. * n.s. Genotype (G) ** *** * *** *** *** Grain position (GP) n.s. n.s. 0.06 n.s. *** ** T × G n.s. n.s. n.s. n.s. * n.s. T × GP n.s. n.s. n.s. n.s. n.s. n.s. G × GP * * * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. Relative change (%) from the control is shown in parentheses. n.s., not significantly different at P < 0.05; *, ** and *** different at P < 0.05, < 0.01 and < 0.001, respectively. Open in new tab Table 4. Thermal time to maximum fresh weight (TMFWP), dry mater (TMDMP) and water content (TMWCP) of pericarp from two proximal (G1 and G2) grain positions to the raquis of central spikelets of wheat genotypes Bacanora and Kambara under control plant density (Con: 370 plants m–2), low plant density (LR: 44 plants m–2), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post) in Experiment 1 (season 2012–13) and Experiment 2 (season 2014–15). . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Temperature . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Bacanora G1 Con 135.4 154.5 122.7 Con 140.3 167.6 128.6 LR 111.0 167.3 123.9 T°Pre 123.2 143.9 114.4 T°Post 141.5 156.8 134.3 G2 Con 149.0 170.2 137.4 Con 153.6 180.6 142.0 LR 132.1 169.1 110.0 T°Pre 133.6 155.3 124.2 T°Post 152.8 171.0 146.7 Kambara G1 Con 155.5 187.3 130.8 Con 194.1 220.6 181.5 LR 198.5 220.3 183.9 T°Pre 166.4 201.7 153.1 T°Post 151.4 162.1 146.1 G2 Con 182.1 208.2 167.0 Con 209.3 235.3 197.3 LR 215.5 231.8 205.4 T°Pre 182.8 217.9 168.8 T°Post 164.2 181.6 158.3 s.e. 7.5 5.7 7.3 4.3 5.0 4.1 Plant density/temperature (T) n.s. * n.s. ** ** ** Genotype (G) ** *** ** *** *** *** Grain position (GP) ** *** ** *** *** *** T × G * n.s. * * * * T × GP n.s. * * n.s. n.s. n.s. G × GP n.s. n.s. * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Temperature . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Bacanora G1 Con 135.4 154.5 122.7 Con 140.3 167.6 128.6 LR 111.0 167.3 123.9 T°Pre 123.2 143.9 114.4 T°Post 141.5 156.8 134.3 G2 Con 149.0 170.2 137.4 Con 153.6 180.6 142.0 LR 132.1 169.1 110.0 T°Pre 133.6 155.3 124.2 T°Post 152.8 171.0 146.7 Kambara G1 Con 155.5 187.3 130.8 Con 194.1 220.6 181.5 LR 198.5 220.3 183.9 T°Pre 166.4 201.7 153.1 T°Post 151.4 162.1 146.1 G2 Con 182.1 208.2 167.0 Con 209.3 235.3 197.3 LR 215.5 231.8 205.4 T°Pre 182.8 217.9 168.8 T°Post 164.2 181.6 158.3 s.e. 7.5 5.7 7.3 4.3 5.0 4.1 Plant density/temperature (T) n.s. * n.s. ** ** ** Genotype (G) ** *** ** *** *** *** Grain position (GP) ** *** ** *** *** *** T × G * n.s. * * * * T × GP n.s. * * n.s. n.s. n.s. G × GP n.s. n.s. * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. n.s., not significantly different at P < 0.05; *, ** and ***k different at P < 0.05, < 0.01 and < 0.001, respectively. Open in new tab Table 4. Thermal time to maximum fresh weight (TMFWP), dry mater (TMDMP) and water content (TMWCP) of pericarp from two proximal (G1 and G2) grain positions to the raquis of central spikelets of wheat genotypes Bacanora and Kambara under control plant density (Con: 370 plants m–2), low plant density (LR: 44 plants m–2), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post) in Experiment 1 (season 2012–13) and Experiment 2 (season 2014–15). . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Temperature . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Bacanora G1 Con 135.4 154.5 122.7 Con 140.3 167.6 128.6 LR 111.0 167.3 123.9 T°Pre 123.2 143.9 114.4 T°Post 141.5 156.8 134.3 G2 Con 149.0 170.2 137.4 Con 153.6 180.6 142.0 LR 132.1 169.1 110.0 T°Pre 133.6 155.3 124.2 T°Post 152.8 171.0 146.7 Kambara G1 Con 155.5 187.3 130.8 Con 194.1 220.6 181.5 LR 198.5 220.3 183.9 T°Pre 166.4 201.7 153.1 T°Post 151.4 162.1 146.1 G2 Con 182.1 208.2 167.0 Con 209.3 235.3 197.3 LR 215.5 231.8 205.4 T°Pre 182.8 217.9 168.8 T°Post 164.2 181.6 158.3 s.e. 7.5 5.7 7.3 4.3 5.0 4.1 Plant density/temperature (T) n.s. * n.s. ** ** ** Genotype (G) ** *** ** *** *** *** Grain position (GP) ** *** ** *** *** *** T × G * n.s. * * * * T × GP n.s. * * n.s. n.s. n.s. G × GP n.s. n.s. * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. . . . Experiment 1 . . . . Experiment 2 . . . Genotype . Grain position . Plant density . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Temperature . TMFWP (°Cd) . TMDMP (°Cd) . TMWCP (°Cd) . Bacanora G1 Con 135.4 154.5 122.7 Con 140.3 167.6 128.6 LR 111.0 167.3 123.9 T°Pre 123.2 143.9 114.4 T°Post 141.5 156.8 134.3 G2 Con 149.0 170.2 137.4 Con 153.6 180.6 142.0 LR 132.1 169.1 110.0 T°Pre 133.6 155.3 124.2 T°Post 152.8 171.0 146.7 Kambara G1 Con 155.5 187.3 130.8 Con 194.1 220.6 181.5 LR 198.5 220.3 183.9 T°Pre 166.4 201.7 153.1 T°Post 151.4 162.1 146.1 G2 Con 182.1 208.2 167.0 Con 209.3 235.3 197.3 LR 215.5 231.8 205.4 T°Pre 182.8 217.9 168.8 T°Post 164.2 181.6 158.3 s.e. 7.5 5.7 7.3 4.3 5.0 4.1 Plant density/temperature (T) n.s. * n.s. ** ** ** Genotype (G) ** *** ** *** *** *** Grain position (GP) ** *** ** *** *** *** T × G * n.s. * * * * T × GP n.s. * * n.s. n.s. n.s. G × GP n.s. n.s. * n.s. n.s. n.s. T × G × GP n.s. n.s. n.s. n.s. n.s. n.s. n.s., not significantly different at P < 0.05; *, ** and ***k different at P < 0.05, < 0.01 and < 0.001, respectively. Open in new tab Fig. 4. Open in new tabDownload slide Time course of pericarp fresh weight (FWP), dry matter (DMP) and water content (WCP) of first (G1: A, C, E and G) and second (G2: B, D, F and H) grain positions closest to the rachis from central spikelets of wheat genotypes Bacanora and Kambara in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). Fig. 4. Open in new tabDownload slide Time course of pericarp fresh weight (FWP), dry matter (DMP) and water content (WCP) of first (G1: A, C, E and G) and second (G2: B, D, F and H) grain positions closest to the rachis from central spikelets of wheat genotypes Bacanora and Kambara in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). In both experiments, and across GP, the highest (P < 0.001) maximum DMP was recorded in Kam (the heavier GW cultivar) when control treatments were considered (Table 3). LR increased (P < 0.01) the maximum DMP up to 10.1 % over the control (Table 3). On the other hand, night warming treatments had a negative effect on the maximum DMP, decreasing this trait between 1.6 and 12.5 % in Exp. 2, with the exception of G1 of Bac under night warming before anthesis (Table 3). The water content of the pericarp was the main component of pericarp fresh weight from anthesis onward. Similar to DMP dynamics, the maximum WCP was reached during the early period of grain filling, achieving up to 12.2 g in control plants (Fig. 4; Tables 3 and 4). Water concentration of pericarp diminished rapidly until 200 °Cd after anthesis to achieve up to 65 % from that point onward (Supplementary data Fig. S3). The fresh weight of pericarp is a function of dry matter and water content, where the dynamics showed a similar behaviour to that recorded by the other pericarp traits. Parameters estimated in the WCP and FWP dynamics such as the maximum weight and the time to achieve it showed differences between genotypes (Kam > Bac). In addition, the T°Pre treatment reduced the thermal time to maximum WCP (G1, –11.0 %; and G2, –12.5 %) and FWP (G1, –12.2 %; and G2, –13.1 %) in Bac, while the T°Post treatment had less effect in the same genotype (from –0.6 to 4.5 %). In Kam, both the T°Pre and T°Post treatments reduced the time to maximum WCP and FWP relative to the control by 19.6 % in WCP and 21.8 % in FWP (Table 4). When time was measured in days instead of thermal time units, the impact of night warming on the period to reach these traits was higher. For example, days to reach maximum DMP was reduced by 6.5 and 30.4 % relative to the control under the T°Pre and T°Post treatments, and to reach maximum WCP, fewer days were recorded than in the control in the T°Pre and T°Post by 8.3 and 27.5 %, respectively (data not shown). Relationships among grain weight/size, pericarp traits and dynamics During the grain filling period, the DWP:IGW ratio dropped from 80 to <15% during the early 300 °Cd of grain filling (data not shown). At harvest, the DMP:IGW ratio ranged between 0.74 and 0.88, and was affected by genotype (Kam > Bac; P < 0.01) and GP (G2 > G1; P < 0.01). A linear association (P < 0.001) was found between IGW at harvest and maximum DMP (Fig. 5A). Physiological determinants of IGW such as the rate and duration of grain filling (Supplementary data Table S1) also showed a linear relationship with maximum DMP (Fig. 5B, C), as well as volume (Fig. 5D), stabilized water content (Fig. 5E), water accumulation rate (Fig. 5F) and GL at harvest (y = 0.82x + 3.21, R2 = 0.80 and P < 0.001). In addition, other key traits for GW such as grain filling rate (y = 6.48e–3x + 3.83e–2, R2 = 0.64 and P < 0.001), water accumulation rate (y = 6.38e–3x + 8.28e–2, R2 = 0.22 and P < 0.05) and maximum GL estimated (y = 0.22x + 5.30; R2 = 0.34 and P < 0.01) were linearly associated with maximum WCP (see Supplementary data Table S1). However, two associations instead of only one were found when both grain filling duration and grain volume were plotted against maximum DMP (Fig. 5C, D), suggesting that these relationships were affected by the experimental season. For a similar range of maximum WCP, grain filling duration was 82.2 °Cd longer in Exp. 1 than in Exp. 2, averaged across the data shown in Fig. 5C. As for grain filling duration, grain volume was also higher in Exp. 1 for similar values of maximum WCP. Thus, grain volume was 30.6 % (14 mm3) higher in Exp.1 than in Exp. 2, averaged across the data. Fig. 5. Open in new tabDownload slide Relationships of the final dry weight of grains (A), grain filling rate (B), grain filling duration (C), grain volume (D), stabilized water content (E) and water accumulation rate (F) of grains with the maximum dry matter of pericarp of grains of two grain positions closest to the rachis from central spikelets of wheat genotypes Bacanora and Kambara in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). Fig. 5. Open in new tabDownload slide Relationships of the final dry weight of grains (A), grain filling rate (B), grain filling duration (C), grain volume (D), stabilized water content (E) and water accumulation rate (F) of grains with the maximum dry matter of pericarp of grains of two grain positions closest to the rachis from central spikelets of wheat genotypes Bacanora and Kambara in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). Association of grain weight/size and the pericarp growth time An overview of grain and pericarp dynamics is shown in Fig. 6 where maximum DMP was reached earlier than key grain traits such as maximum GL (Fig. 6A, D) and stabilized grain water content (Fig. 6C, F). For example, maximum DMP was achieved, ending the lag phase of grain filling, during the increase in grain water accumulation (Fig. 6C, F) and before maximum GL (Fig. 6A, D). Pericarp growth ended before 235 °Cd after anthesis (Table 4; Fig. 6) and maximum WCP was reached earlier (P < 0.001) than maximum DMP. Pericarp growth stages were affected by genotype, night warming and GP in Exp. 2 (Tables 3 and 4), reinforcing the early determination of maximum DMP/WCP and its importance for GW (Fig. 7A, D). Other characters associated with the weight and size of grains have also shown a linear relationship with the time to reach maximum DMP and WCP (Fig. 7); however, some of them were affected by the environment as two relationships instead of a single one were found (Fig. 7C, F). In addition, grain filling and water content duration had a positive association with the pericarp growth stage (data not shown). Fig. 6. Open in new tabDownload slide (A and D) Time course of grain length and the fresh weight of pericarp (FWP), (B and E) grain weight and the dry matter of pericarp (DMP), and (C and F) water content of the grain and water content of pericarp (WCP) of the second grain position (G2) closest to the rachis from central spikelets of wheat genotypes Bacanora (Bac) and Kambara (Kam) in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). Fig. 6. Open in new tabDownload slide (A and D) Time course of grain length and the fresh weight of pericarp (FWP), (B and E) grain weight and the dry matter of pericarp (DMP), and (C and F) water content of the grain and water content of pericarp (WCP) of the second grain position (G2) closest to the rachis from central spikelets of wheat genotypes Bacanora (Bac) and Kambara (Kam) in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). Fig. 7. Open in new tabDownload slide Relationship between final dry weight of grains, stabilized water content, grain volume and time to the maximum dry matter of pericarp (A–C) or time to the maximum water content of pericarp (D–F) of two grain positions closest to the rachis from central spikelets of wheat genotypes Bacanora and Kambara in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). Fig. 7. Open in new tabDownload slide Relationship between final dry weight of grains, stabilized water content, grain volume and time to the maximum dry matter of pericarp (A–C) or time to the maximum water content of pericarp (D–F) of two grain positions closest to the rachis from central spikelets of wheat genotypes Bacanora and Kambara in Experiments 1 and 2. In Experiment 1, treatments were control (Con, 370 plants m–2) and low plant density (LR, 44 plants m–2). In Experiment 2, treatments were control (at ambient temperature), night warming before anthesis (T°Pre) and night warming after anthesis (T°Post). Data are means ± s.e. (n = 3). DISCUSSION This study had the aim of elucidating to what extent maternal tissues of wheat grains, such as the pericarp and its dynamics during grain filling, are associated with the final grain size and weight. Historically, wheat research has focused on endosperm tissues and starch accumulation overshadowing the importance of the pericarp in the development and growth of wheat grains (Antoine et al., 2004; Gegas et al., 2010; Hemery et al., 2010). However, the most recent research has proposed that maternal tissues of grains are a key determinant of final GW of wheat (Hasan et al., 2011; Brinton et al., 2017; Brinton and Uauy, 2019) by setting an upper limit to endosperm accumulation (Calderini and Reynolds, 2000; Hasan et al., 2011; Simmonds et al., 2016). Studies of maternal tissues have shown associations between the final IGW of wheat and the ovary size recorded at anthesis (Calderini et al., 1999; Benincasa et al., 2017; Reale et al., 2017). The importance of the pericarp for GW has also been demonstrated in other crops such as sorghum (Yang et al., 2009) and sunflower (Lindström et al., 2006, 2007; Rondanini et al., 2009; Castillo et al., 2017), where the final IGW of sunflower showed a strong association with the ovary and pericarp weights (Castillo et al., 2017; Rondanini et al., 2009). In wheat, research has mainly focused on programed cell death of pericarp (Cejudo, 2001; Zhou et al., 2009), the temporary accumulation of starch in pericarp cells (Yu et al., 2015) and, more recently, the identification of proteins involved in cell wall polysaccharide remodelling and cell wall assembly (Mehdi et al., 2020). However, to the best of our knowledge, little is known about the time course of pericarp tissues and their relationship to final IGW of wheat. All pericarp traits recorded in this study showed similar dynamics from anthesis onwards and across genotypes and growing conditions (Fig. 4). These dynamics had three stages from anthesis onwards: (1) an early linear growth of up to 235 °C (10–12 DAA); (2) a rapid drop between approx. 200 °C and approx. 350°C (18–20 DAA); and (3) a stabilized phase from 350 °C to physiological maturity (Fig. 4). Similar to the pericarp dynamics shown in sunflower (Rondanini et al., 2009), the maximum pericarp weight of wheat was reached by 12 DAA. Remarkably, pericarp dynamics of wheat grains are different from other processes of grain growth (Figs 6 and 8), e.g. the typical sigmoid shape of grain dry matter accumulation (Gleadow et al., 1982), the hyperbolic form of GL dynamics (Rogers and Quatrano, 1983) and the tri-linear phases of grain water accumulation (Pepler et al., 2006). Fig. 8. Open in new tabDownload slide Schematic diagram of dynamics and physiological characters determining individual grain weight of wheat before and after anthesis. The diagram was divided into grain phases, maternal tissues, grain size and grain weight characters (left boxes) from booting to before harvest (upper time line). The figure shows phenological stages: double ridge (DR), booting (Bt), anthesis, physiological maturity (PM) and harvest (Hv); pericarp dynamics: fresh weight of pericarp (FWP), water content of pericarp (WCP), dry matter of pericarp (DMP); grain characters: grain length (GL), grain volume (GV), number of endosperm cells, water content (WC) and grain weight (GW). The solid lines in character dynamics indicate studied characters and dotted lines the speculative behaviour of these dynamics. Arrow connections show the relationship between characters and processes. Fig. 8. Open in new tabDownload slide Schematic diagram of dynamics and physiological characters determining individual grain weight of wheat before and after anthesis. The diagram was divided into grain phases, maternal tissues, grain size and grain weight characters (left boxes) from booting to before harvest (upper time line). The figure shows phenological stages: double ridge (DR), booting (Bt), anthesis, physiological maturity (PM) and harvest (Hv); pericarp dynamics: fresh weight of pericarp (FWP), water content of pericarp (WCP), dry matter of pericarp (DMP); grain characters: grain length (GL), grain volume (GV), number of endosperm cells, water content (WC) and grain weight (GW). The solid lines in character dynamics indicate studied characters and dotted lines the speculative behaviour of these dynamics. Arrow connections show the relationship between characters and processes. During early grain filling, the grain undergoes multiple events associated with its size and weight (Saulnier et al., 2012), such as rapid nuclear divisions of the endosperm (Nicolas et al., 1984), the development of maximum GL (Lizana et al., 2010) and the stabilization of water content (Schnyder and Baum, 1992; Pepler et al., 2006), among others (Figs 6 and 8). At the same time, each grain undertakes rapid enlargement, which demands the deposition and reorganization of carbohydrates within the cell wall (Peaucelle et al., 2012). For instance, the rapid accumulation of arabinoxylans and β-glucans during the first 10 DAA (Verspreet et al., 2013) results in a positive variation of dry biomass and water content of the tissues. In addition, the pericarp has a secondary and temporary increase of dry matter through starch synthesis (Caley et al., 1990; Fujita and Taira, 1998) and translocation of assimilates (Wang and Fisher, 1994), sustaining the demand of both the endosperm and embryo tissues (Nakamura et al. 1998) during the coenocytic stage. It seems likely that the increase of DWP soon after anthesis is due to the accumulation of structural carbohydrates (Verspreet et al., 2013) and a temporary accumulation of starch (Yu et al., 2015). An important finding of the present research has been that the higher the maximum DMP, the higher the final IGW (Fig. 5A). No less important is the fact that DMP is reached early during grain filling, and even earlier than other traits registered during the grain filling, which has been reported as key for IGW determination in wheat (Figs 6 and 8); for instance, maximum grain water content (Pepler et al., 2006), maximum GL (Lizana et al., 2010) and the end of endosperm cell division (Gleadow et al., 1982). The flux, accumulation and transport of water is a key in plant growth due to the turgor-driven expansion and cell growth (McQueen-Mason et al., 1992). In addition, key processes such as the transport of nutrients are also regulated by water, as has been widely demonstrated (Cosgrove, 2005). In the pericarp, water content is the main component of the fresh weight of maternal tissues during the period soon after anthesis (Fig. 4; Table 3). These tissues have a considerable capacity for water retention (Evers and Millar, 2002; Yu et al., 2015), which can treble the dry matter of the pericarp (Fig. 4, Table 3). The flow and accumulation of water inside the plant cell are essential to produce elongation of the cell wall, while, in contrast, low turgor pressure limits the wall creep and subsequent cell elongation (Zonia and Munnik, 2007). In our study, the water accumulation inside the pericarp and the maximum WCP were observed during the rapid grain elongation period (Fig. 6A, D). In addition, the grain elongation process slowed down or stopped when the water content diminished in the pericarp (Fig. 8). This is supported by the fact that first water flux is destined to grain elongation, and once grain elongation ends the water flux would be mainly driven inside the grain for accumulation processes (Figs 6 and 8). Regarding the relationship between pericarp weight and GW dynamics, Schnyder et al. (1993) registered an exponential drop in the relative dry matter of the pericarp from anthesis onwards as the product of exponential accumulation of carbohydrates in endosperm tissues (Gleadow et al., 1982). In our study, the outer pericarp is the main component of wheat grain during the early grain filling period, but its contribution dropped <10 % to the final IGW in the last sampling (approx. 30 DAA), and could be even lower at harvest. In sunflower fruits, the pericarp is between 20 and 25 % of the fruit dry weight (Lindström et al., 2006), and the same study showed that the volume reached by the pericarp of sunflower limits the growth of the grain fruits. Therefore, the dry matter of the pericarp without starch granules (Xiong et al., 2013) could control the wheat grain elongation, taking into account that longer grains than the control were found under low plant density conditions and, in contrast, shorter grains than the control were recorded in the night warming treatments. In the latter, the increase of night temperature could have modified, for example, air circulation and vapour pressure within the chambers; therefore, an effect of the treatment on ovary and grain tissues should not be discarded. However, the thermal treatments last for only part of the ovary or grain growing periods (10 and 16 d, respectively). Treatments used in our experiments, from genotype to plant density and thermal increase, generated a wide range of GW, modifying this trait both positively and negatively. In addition to GW, associated characters such as grain filling rate, maximum water content and GL, among others, were affected by the treatments (Supplementary data Table S1). At the same time, maternal tissues were also affected by treatments, showing changes in maximum pericarp weight, the timing to reach maximum weight and others (Fig. 4; Tables 3 and 4). Previous research has shown the sensitivity of maternal tissue to temperature (Calderini et al., 1999) and the subsequent effect on final GW. Thus, the present results reinforce the sensitivity of maternal tissue to higher temperature, but also show that crop management (plant density) can affect both maternal tissues and GW positively. Therefore, our results clearly demonstrate the association between final GW and pericarp across contrasting management and thermal conditions improving or decreasing both traits, suggesting that maximum pericarp weight is an early determinant of GW of wheat, as is schematized in Fig. 8, summarizing the main processes and traits of GW determination in this crop species. In spite of the common pattern of DMP dynamics found in our study, some differences should be considered. For example, the evaluated genotypes showed different IGW associated with maximum DMP. Therefore, there is an apparent genetic control of pericarp growth linked to the water uptake capacity of these tissues. It is probable that the different water uptake capacity between genotypes is due to the size of the ovary at pollination and the higher number of cells of these tissues (Hasan et al., 2011; Benincasa et al., 2017; Reale et al., 2017). The search for quantitative trait loci (QTLs) associated with maximum DMP could be useful for increasing IGW in breeding programmes by sampling grains at 180 °Cd after anthesis, taking into account that at this moment most of the grain dry matter corresponds to the pericarp and that sampling young growing grains is easier than that of ovaries at pollination. In addition, although close associations were found between DMP and grain dry matter, grain filling rate and grain stabilized water content (Fig. 5A, B, E, F), two relationships were plotted between grain filling duration and DMP as well as between grain volume and DMP (Fig. 5C, D). Therefore, these interactions could be due to growing conditions affecting these traits during grain filling. It is likely that the shorter grain filling duration (82.3 °Cd) and smaller grain volume (23.3 %) recorded in Exp. 2 compared with Exp. 1 could be due to the higher average maximum temperature (4 °C higher) recorded in Exp. 2 between anthesis and 25 DAA. In addition, the difference in average maximum temperature increased from anthesis to 25 DAA as the difference was even higher (5 °C) from 10 to 25 DAA. It is important to take into account that endosperm cell division of growing grains occurs from anthesis to 20–25 DAA (Gleadow et al., 1982), and this period is longer than the T°Post treatment (16 d), proving that the background temperature affected IGW under both control and increased night temperatures between seasons in this experiment, as was shown in grapevines (Sadras and Moran, 2013) and in ecological studies (Way and Oren, 2010). Conclusions The present study quantitatively evaluated the time course of the pericarp maternal tissues from anthesis onwards, where the fresh weight, dry matter and water content showed a logistic shape, reaching maximum values soon after anthesis. Interestingly, a positive association was found between final GW and both pericarp maximum dry matter and water content, highlighting the importance of maternal tissues on GW determination of wheat. Moreover, the maximum dry matter and water content of the pericarp were reached before key traits considered determinants of final GW in wheat, such as the maximum GL and the maximum grain water content. Finally, the positive associations between grain and pericarp traits reported in our study could facilitate the understanding of GW, grain size and the grain filling duration of wheat, together with possible tools for wheat breeding such as the tissue and timing associated with final GW to be sampled. SUPPLEMENTARY DATA Supplementary data are available online at https://academic.oup.com/aob and consist of the following. Table S1: rate and duration of grain filling, stabilized grain water content, water accumulation rate and water accumulation duration, maximum grain length, grain elongation rate and timing of grain elongation. Figure S1: maximum and minimum temperatures, rainfall and irrigation days during the wheat cycle. Figure S2: grain weight before and after dissection of the pericarp from anthesis onwards. Figure S3: time course of water content as a percentage within the outer pericarp in wheat grains. ACKNOWLEDGEMENTS We are very grateful to Maria Paredes, Alejandro Quintero, Paola Montecinos and staff of the Experimental Field Station and Laboratory of Physiology and Molecular Biology of Crops (Universidad Austral de Chile) for their technical assistance. FUNDING This study was partially funded by the Chilean Technical and Scientific Research Council (CONICYT) Project FONDECYT 1141048 competitive grant. J.H. held a postgraduate scholarship from CONICYT. LITERATURE CITED Alexander LV . 2016 . Global observed long-term changes in temperature and precipitation extremes: a review of progress and limitations in IPCC assessments and beyond . Weather and Climate Extremes 11 : 4 – 16 . Google Scholar Crossref Search ADS WorldCat Antoine C , Peyron S, Mabille F, et al. 2003 . 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Google Scholar Crossref Search ADS PubMed WorldCat © The Author(s) 2020. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Pericarp growth dynamics associate with final grain weight in wheat under contrasting plant densities and increased night temperature
JO - Annals of Botany
DO - 10.1093/aob/mcaa131
DA - 2020-10-30
UR - https://www.deepdyve.com/lp/oxford-university-press/pericarp-growth-dynamics-associate-with-final-grain-weight-in-wheat-9XE4Hq8VQi
SP - 1063
EP - 1076
VL - 126
IS - 6
DP - DeepDyve
ER -