TY - JOUR
AU1 - Dong,, Jinlong
AU2 - Grylls,, Stephen
AU3 - Hunt,, James
AU4 - Armstrong,, Roger
AU5 - Delhaize,, Emmanuel
AU6 - Tang,, Caixian
AB - Abstract Background and Aims Soil acidity currently limits root growth and crop production in many regions, and climate change is leading to uncertainties regarding future food supply. However, it is unknown how elevated CO2 (eCO2) affects the performance of wheat crops in acid soils under field conditions. We investigated the effects of eCO2 on plant growth and yield of three pairs of near-isogenic hexaploid wheat lines differing in alleles of aluminium-resistant genes TaALMT1 (conferring root malate efflux) and TaMATE1B (conferring citrate efflux). Methods Plants were grown until maturity in an acid soil under ambient CO2 (aCO2; 400 µmol mol−1) and eCO2 (550 µmol mol−1) in a soil free-air CO2 enrichment facility (SoilFACE). Growth parameters and grain yields were measured. Key Results Elevated CO2 increased grain yield of lines carrying TaMATE1B by 22 % and lines carrying only TaALMT1 by 31 %, but did not increase the grain yield of Al3+-sensitive lines. Although eCO2 promoted tiller formation, coarse root length and root biomass of lines carrying TaMATE1B, it did not affect ear number, and it therefore limited yield potential. By contrast, eCO2 decreased or did not change these parameters for lines carrying only TaALMT1, and enhanced biomass allocation to grains thereby resulting in increased grain yield. Despite TaMATE1B being less effective than TaALMT1 at conferring Al3+ resistance based on root growth, the gene promoted grain yield to a similar level to TaALMT1 when the plants were grown in acid soil. Furthermore, TaALMT1 and TaMATE1B were not additive in their effects. Conclusions As atmospheric CO2 increases, it is critical that both Al3+-resistance genes (particularly TaALMT1) should be maintained in hexaploid wheat germplasm in order for yield increases from CO2 fertilization to be realized in acid soils. Acid-soil tolerance, biomass allocation, genotypic variation, near-isogenic lines, phosphorus deficiency, TaALMT1, TaMATE1B INTRODUCTION Climate change is a serious threat to agricultural production and food security (Long et al., 2004; IPCC, 2014). Fortunately, the causative increase in atmospheric CO2 concentration promotes plant photosynthesis and crop yield, and counteracts some of the negative impacts on food supply (Long et al., 2004; L. Wang et al., 2013). However, the magnitude of yield increase due to elevated CO2 (eCO2) differs among species, cultivars and environmental factors (Ainsworth, 2008; L. Wang et al., 2013; J. Wang et al., 2015). Yield gain under eCO2 decreases for wheat cultivars with reduced vigour during vegetative growth (Manderscheid and Weigel, 1997) and for rice grown under stressed conditions such as in cadmium-polluted soils (Wu et al., 2016). Acid soils comprise around 50 % of the area of the world’s arable lands (Kochian et al., 2015). Crops often fail to realize their full yield potential on acid soils because of poor root growth, mainly resulting from aluminium (Al3+) toxicity or P deficiency or a combination of both (Kochian et al., 2015). To date, the efflux of organic anions and the genes that underlie these mechanisms are the most thoroughly described Al3+ resistance mechanisms in hexaploid wheat (Triticum aestivum) (Ryan et al., 2011). A resistant allele of the TaALMT1 gene encodes a transporter protein conferring malate efflux, which is specifically activated by the presence of Al3+ and forms a malate–Al complex that is thought to detoxify Al3+ in rhizosphere or apoplast (Delhaize et al., 1993b; Ryan et al., 1995). Another gene that confers Al3+ resistance is TaMATE1B, which encodes a transporter protein conferring citrate efflux but is less effective than TaALMT1 in hexaploid wheat in protecting root growth from Al3+ toxicity (Ryan et al., 2009; Han et al., 2016; Dong et al., 2018). By conferring improved root elongation in acid soils, TaALMT1 has been shown to increase plant productivity and grain yield of hexaploid wheat grown in acid soils (Tang et al., 2001, 2002). There are no reports of similar field experiments with hexaploid wheat lines that possess Al3+-resistant alleles of TaMATE1B grown in acid soils, although field trials on soils without Al3+ toxicity did not show improved grain yield (Ryan et al., 2014). To date, the effect of eCO2 on grain yield of wheat lines carrying resistant alleles of TaALMT1 and TaMATE1B grown in acid soils has not been reported. Improved root growth under eCO2 could promote nutrient uptake, and thus plant productivity and yield (Kochian et al., 2004; Norby and Zak, 2011). Our previous short-term (24-d) study conducted on wheat seedlings grown in acid soils showed that eCO2 promoted root elongation of wheat lines carrying TaALMT1 and/or TaMATE1B to a greater extent than Al3+-sensitive wheat lines that had Al3+-sensitive alleles of both genes (Dong et al., 2018). This result is probably due to Al3+ resistance conferring improved root elongation on acid soils (Delhaize et al., 2012), and improved root growth allowing a greater yield response to eCO2 as observed for wheat grown in non-acid soils (Rogers et al., 1995; Kirschbaum, 2011). For Stipa krylovii grown in a Calcic-orthic Aridisol (P deficient), eCO2 decreased malate exudation of the intact roots but did not affect citrate exudation (Liu et al., 2016). In another study, malate addition slightly decreased root elongation of wild-type Arabidopsis grown in P-deficient solution, whilst citrate addition promoted root elongation (Mora-Macías et al., 2017). From these observations, it is expected that eCO2 might decrease Al3+ resistance or root elongation, and thus reduce grain yield of hexaploid wheat lines carrying TaALMT1 whilst the reverse could be true for wheat carrying TaMATE1B although one needs to be cautious in extrapolating findings across species. The current study was conducted in a free-air CO2 enrichment system (FACE) using three near-isogenic pairs of hexaploid wheat lines varying in alleles of TaALMT1 and TaMATE1B. The lines were grown to maturity in an Al3+-toxic and P-deficient acid soil. We hypothesized that eCO2 would promote root growth, and thus plant growth and grain yield of Al3+-resistant lines to a greater extent than sensitive lines when grown in acid soils. Amongst the Al3+-resistant lines, the growth and yield response would be greater in wheat lines carrying TaMATE1B compared to those carrying TaALMT1. MATERIAL AND METHODS Experimental design The experiment was a split-plot design conducted in soil columns in the soil free-air CO2 enrichment systems (SoilFACE) in Horsham, Victoria, Australia (36°44′57″S, 142°06′50″E) for 203 d from 2 June to 21 December 2016. The details of SoilFACE systems can be found in Mollah et al. (2009). The experiment consisted of two CO2 levels as the main plot (rings) and six wheat (Triticum aestivum L.) lines as the sub-plot. There were four rings as four replicates for each CO2 concentration (eight rings in total) maintained at either 400 µmol mol−1 (ambient CO2, aCO2) or 550 µmol mol−1 (eCO2). The position of each of these subplots within each ring was fully randomized. The six wheat lines were three near-isogenic pairs varying in Al3+ resistance due to differences in organic anion efflux from root tips. The three pairs of near-isogenic lines (NILs) comprised: (1) ES8 (carrying Al3+-sensitive alleles of both TaALMT1 and TaMATE1B; Al3+-sensitive line) and ET8 (carrying an Al3+-resistant allele of TaALMT1; Al3+-resistant line), (2) Egret (carrying Al3+-sensitive alleles of both TaALMT1 and TaMATE1B; Al3+-sensitive line) and Egret TaMATE1B (carrying an Al3+-resistant allele of TaMATE1B; Al3+-resistant line), and (3) EGA-Burke (carrying an Al3+-resistant allele of TaALMT1; Al3+-resistant line) and EGA-Burke TaMATE1B (carrying Al3+-resistant alleles of both TaALMT1 and TaMATE1B, Al3+-resistant line) (Delhaize et al., 1993a; Han et al., 2016). For simplicity, here we refer to Al3+-resistant lines that carry Al3+-resistant alleles as lines carrying TaALMT1 or TaMATE1B unless otherwise specified. Previous short-term experiments have shown that TaALMT1 is a more effective Al3+-resistant gene than TaMATE1B (Han et al., 2016). The experimental soil was a composite acid soil made by combining a Dermosol and a Ferrosol (Isbell, 1996), both collected from Kinglake, Victoria (Table 1). ES8 and ET8 were also grown in limed acid soils to establish the effect of eCO2 on wheat growth in the absence of Al3+ toxicity. In addition, ‘control’ columns without plants were also included in these treatments. Table 1. Sampling location and chemical properties of two experimental soils used in this study. Soil Collection sites pH (CaCl2) Total C (g kg−1) Total N (g kg−1) CaCl2-extractable Al* (mg kg−1) Olsen-P† (mg kg−1) Colwell-P† (mg kg−1) PBI† (mg P kg−1) Dermosol 37.462°S, 145.263°E 4.12 38.8 1.74 54.3 2.4 3.2 739 Ferrosol 37.474°S, 145.257°E 4.55 41.9 1.92 21.6 1.2 1.8 1095 Soil Collection sites pH (CaCl2) Total C (g kg−1) Total N (g kg−1) CaCl2-extractable Al* (mg kg−1) Olsen-P† (mg kg−1) Colwell-P† (mg kg−1) PBI† (mg P kg−1) Dermosol 37.462°S, 145.263°E 4.12 38.8 1.74 54.3 2.4 3.2 739 Ferrosol 37.474°S, 145.257°E 4.55 41.9 1.92 21.6 1.2 1.8 1095 * Soil extracted with 0.01 m CaCl2 was measured by ICP-AES. † Measurements of Olsen-P, Colwell-P and PBI (phosphorus buffer index) according to Rayment and Lyons (2011). View Large Table 1. Sampling location and chemical properties of two experimental soils used in this study. Soil Collection sites pH (CaCl2) Total C (g kg−1) Total N (g kg−1) CaCl2-extractable Al* (mg kg−1) Olsen-P† (mg kg−1) Colwell-P† (mg kg−1) PBI† (mg P kg−1) Dermosol 37.462°S, 145.263°E 4.12 38.8 1.74 54.3 2.4 3.2 739 Ferrosol 37.474°S, 145.257°E 4.55 41.9 1.92 21.6 1.2 1.8 1095 Soil Collection sites pH (CaCl2) Total C (g kg−1) Total N (g kg−1) CaCl2-extractable Al* (mg kg−1) Olsen-P† (mg kg−1) Colwell-P† (mg kg−1) PBI† (mg P kg−1) Dermosol 37.462°S, 145.263°E 4.12 38.8 1.74 54.3 2.4 3.2 739 Ferrosol 37.474°S, 145.257°E 4.55 41.9 1.92 21.6 1.2 1.8 1095 * Soil extracted with 0.01 m CaCl2 was measured by ICP-AES. † Measurements of Olsen-P, Colwell-P and PBI (phosphorus buffer index) according to Rayment and Lyons (2011). View Large Soil preparation and plant growth The soils were air-dried, passed through a 4-mm sieve and stored in plastic bins before use. Each soil column (0.60 m in height and 0.15 m in diameter) consisted of 2.2 kg of the Ferrosol mixed with 1 g CaCO3 kg−1 soil as a topsoil (0–0.10 m) and 8.8 kg mixture of the Dermosol and the Ferrosol (20: 80, w/w) as a subsoil (0.10–0.50 m). A preliminary experiment indicated the soil column gave good separation of growth parameters of the various lines in response to the soil acidity. The limed soils used the same mixtures but 5 g of CaCO3 kg−1 soil was mixed into both the topsoils and the subsoils to remove Al3+ toxicity. Basal nutrients were applied as follows (mg kg−1 soil): K2SO4, 441; MgSO4.7H2O, 122; CaCl2.2H2O, 186; KH2PO4, 112.5; CO(NH2)2, 60; CuSO4.5H2O, 6; ZnSO4.7H2O, 8; NaMoO4.2H2O, 0.4; and H3BO3, 1.04, and mixed into both topsoil and subsoil. After the columns were filled with soils, they were watered with reverse osmosis (RO) water to 100 % field capacity and allowed to equilibrate for 1 week before seeds were sown. The bottoms of columns were perforated to allow drainage during the winter and early spring. The columns were sealed from Day 144 and watered by weighing over the dry half of spring. Twenty pre-germinated seeds (1 d at 25 °C) were sown at 2-cm depth into each column and the columns were then placed into the FACE rings with the top of each column at the soil surface of the surrounding field. The seedlings were thinned twice to allow a final number of four uniform plants per column. To minimize the potential deficiencies of N and P, 1125 mg KH2PO4 (256 mg P) and 1320 mg CO(NH2)2 (616 mg N) were added to each column as five separate top-dressing applications prior to anthesis. The site has a Mediterranean climate with a cool wet winter and a hot dry summer. Soils were rain-fed during the wet season, and were watered to 100–120 % field capacity by weighing the columns twice a week during grain filling. Polyethylene beads were added to the soil surface to minimize evaporation after the first watering. CO2 treatments were stopped at the firm-dough stage of development and the plants were kept in the rings until they reached grain maturity. Over the experimental period, the weather record was taken from the Horsham Aerodrome Climatological Station (079100) which is 10 km from the experimental site (Figure S1). The total rainfall was 302 mm ranging from 9 to 91 mm monthly. The maximum monthly mean temperature ranged from 13.2 to 27.2 °C, minimum monthly mean temperature from 3.6 to 8.3 °C, and monthly mean solar radiation from 2.2 to 6.7 kWh m−2. Measurements Net photosynthetic rates, transpiration rates and stomatal conductance of flag leaves were measured at flowering in the field using a portable photosynthesis system (Li-Cor, Lincoln, NB, USA). Measurements were conducted between 0900 and 1200 h on sunny days, with the temperature ranging from 22 to 30 °C. Three flag leaves with moderate growth vigour in each column were used for measurements from pair to pair of the wheat NILs and from replicate to replicate to minimize the effect of environmental variations. Instantaneous transpiration efficiency was calculated by dividing net photosynthetic rates by transpiration rates. As the columns were sealed from Day 144 to maturity, the water input per column in this period was estimated as a combination of each water addition and rainfall, and weight loss of the column. Water-use efficiency was calculated as a division of total biomass by the total of water addition and rainfall over the entire growing season. Shoot biomass was measured on Days 67 and 85 and at crop maturity (Day 203). Tiller numbers were recorded on Days 97 and 107 and at maturity. At maturity, total tiller and ear numbers were counted, shoots were cut at ground level, and soil columns were temporarily stored at 4 °C to avoid root decomposition. The columns were disassembled and divided into three soil layers: 0–0.1, 0.1–0.2 and 0.2–0.5 m. Roots of each layer were separated from soil by washing with tap water, and then floated in RO water in a clear plastic tray and scanned using a flatbed scanner (Epson EU-35, Seiko Epson Corp., Suwa, Japan) at a resolution of 600 dots per inch. Root length was determined using WinRhizo Pro version 2003B software (Régent Instruments Inc., CA, USA). Roots with diameter <0.3 mm are defined as fine roots and those >0.3 mm as coarse roots. All the plant materials were washed thoroughly with RO water and oven-dried at 70 °C for 3 d prior to weighing. The dried heads were threshed, separated from chaff, and total grain and thousand-grain weights were recorded. Grains were finely ground using a mixer mill (MM400, Retsch GmbH, Haan, Germany). A 0.3-g subsample of ground plant material was digested with 8 mL concentrated nitric acid (HNO3) in Eppendorf reaction vials using a microwave reactive system (Multiwave 3000, Anton Paar GmbH, Graz, Austria). All digests were diluted into 50-mL tubes and analysed for P, Ca, Mg, S, Fe, Mn and Zn using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 8000, PerkinElmer, Waltham, MA, USA). The ground grains were also used for N determination using a CHNS/O analyser (EA2400, PerkinElmer), and the protein concentration was calculated by multiplying N concentration by 5.7 (Högy et al., 2009). Statistical analysis The data were analysed with linear mixed models to determine the main effect of CO2 and wheat line and their interactions in the GenStat user interface (Version 17.1, VSN International, Hemel Hempstead, UK). CO2 and line were used as fixed effects and whole plot (ring) as a random effect. Because least significant difference (LSD) multiple range tests may not have detected CO2 effects due to large differences when all lines were analysed together, Student’s two-tailed t-test was specifically performed to determine the CO2 effect of individual lines or within pairs of NILs using Microsoft Excel 2016 rather than the LSD values. The correlation between total tiller number and total root biomass/shoot biomass was done using Microsoft Excel 2016. The effects were considered significant at 5 % confidence levels (P < 0.05) and near-significant at 10 % confidence levels (P < 0.1). RESULTS Grain yield and biomass Elevated CO2 increased grain yield of all wheat lines by an average of 20 % (P = 0.027). Specifically, eCO2 increased the yield of lines ET8 (Al3+-resistant allele of TaALMT1) by 32 % (P = 0.016), EGA-Burke (Al3+-resistant allele of TaALMT1) by 30 % (P = 0.017), a line that carried both Al3+-resistant alleles of TaALMT1 and TaMATE1B (EGA-Burke TaMATE1B; 21 %, P = 0.025) and showed a similar trend for a line that carried only the Al3+-resistant allele of TaMATE1B (Egret TaMATE1B; 23 %, P = 0.099). It did not significantly affect the grain yield of sensitive lines ES8 and Egret (Fig. 1). Elevated CO2 increased the harvest index of ET8 by 22 % (P = 0.014) and tended to increase that of EGA-Burke by 7 % (P = 0.080), with no effect on that of other lines (Table S1). On average, EGA-Burke had the greatest yield, followed by EGA-Burke TaMATE1B, ET8, Egret TaMATE1B, Egret and ES8 the least. Compared to their sensitive NILs, the effects of both TaALMT1 (ET8) and TaMATE1B (Egret TaMATE1B) on grain yield of the lines grown in the acid soil (data of CO2 treatments combined) were significant, with the increases of 163 % and 109 %, respectively. However, there was no significant effect of TaMATE1B in addition to TaALMT1 (Fig. 1). Fig. 1. View largeDownload slide Grain yield of six wheat lines grown in an acid soil under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS, *, and *** denote the levels of significance at P > 0.1, P < 0.05 and P < 0.001 between CO2 levels of a single line or within a near-isogenic pair using a t-test. Means are presented and an LSD bar is also presented to compare any two means at P ≤ 0.05 within a panel. Fig. 1. View largeDownload slide Grain yield of six wheat lines grown in an acid soil under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS, *, and *** denote the levels of significance at P > 0.1, P < 0.05 and P < 0.001 between CO2 levels of a single line or within a near-isogenic pair using a t-test. Means are presented and an LSD bar is also presented to compare any two means at P ≤ 0.05 within a panel. Compared with aCO2, eCO2 did not affect total root biomass (P = 0.427). Elevated CO2 had variable effects on root biomass that were not consistent across lines. Specifically, eCO2 tended to decrease root biomass of Egret by 25 % (Fig. 2; P = 0.061) and ET8 by 27 % (P = 0.039). However, eCO2 increased root biomass in lines carrying the Al3+-resistant allele of TaMATE1B- Egret TaMATE1B by 24 % (P = 0.037) and EGA-Burke TaMATE1B by 26 % (P = 0.029). Elevated CO2 increased shoot biomass of Egret TaMATE1B by 38 % (P = 0.013) and tended to increase shoot biomass of EGA-Burke TaMATE1B by 21 % (P = 0.080) and EGA-Burke by 21 % (P = 0.093) with no significant effect on other lines. As a result, eCO2 decreased root-to-shoot biomass ratio (P = 0.015) across all lines. Elevated CO2 decreased the root-to-shoot ratio of ET8 (P = 0.015) and tended to decrease that of EGA-Burke (P = 0.067), whilst it had no effect on sensitive and TaMATE1B lines (Table S1). Total tiller number was related to total root biomass (r = 0.83, P < 0.001) and shoot biomass (r = 0.91, P < 0.001). Elevated CO2 increased total tiller number by an average of 11 % (P = 0.012). Specifically, eCO2 increased the tiller number of Egret TaMATE1B by 37 % (P = 0.041) and EGA-Burke TaMATE1B by 21 % (P = 0.037), with no significant effect on other lines. As found for grain yield, lines harbouring Al3+-resistant alleles of either TaALMT1 or TaMATE1B had greater total root biomass, shoot biomass and total tiller number when compared to their near-isogenic sister lines (Fig. 2). When compared to ES8, ET8 had 161 %, 168 % and 80 % more total root biomass, shoot biomass and total tiller number, respectively. The effect on Egret TaMATE1B was less dramatic for these parameters when compared with Egret, showing increases of 129 %, 91 % and 47 %, respectively. The effect of TaMATE1B did not appear to be additive with TaALMT1 given that EGA-Burke TaMATE1B showed no increase in these parameters relative to its near isogenic pair EGA-Burke. Root length Total root length followed a similar pattern as total root biomass in response to eCO2 (Fig. 3). Elevated CO2 tended to decrease total root length of the lines only carrying TaALMT1, ET8 by 27 % (P = 0.089) and EGA-Burke by 15 % (P = 0.010) with no significant effect on sensitive lines and lines carrying TaMATE1B. Elevated CO2 tended to decrease the total fine root length of lines only carrying TaALMT1, ET8 (P = 0.076) and EGA-Burke (P = 0.083). By contrast, eCO2 increased total coarse root length of lines carrying TaMATE1B, Egret TaMATE1B (P = 0.011) and EGA-Burke TaMATE1B (P = 0.033), but not of other lines. Again, the lines harbouring the Al3+-resistant genes conferred greater root growth on the acid soil compared to their near-isogenic pairs. When combing data for the CO2 treatments, ET8 had 253 %, 290 % and 201 % greater total root length, fine and coarse root length, respectively, than ES8. Similarly, Egret TaMATE1B showed improved root growth with increases in these parameters of 104 %, 68 % and 143 %, respectively, compared to Egret. As observed for root mass, the effect of the resistant genes was not additive on root length with EGA-Burke showing similar root growth to EGA-Burke TaMATE1B. Relative growth Growth parameters of ET8 and ES8 in the acid soil were also expressed as percentages of growth parameters in the limed soil and were referred to as relative values. Figure 4 shows that eCO2 decreased the relative root length of ES8 by 20 % (P = 0.016). Similarly, eCO2 tended to decrease relative root biomass of ES8 by 32 % (P = 0.096) and ET8 by 18 % (P = 0.004). The relative grain yield of both lines remained stable under eCO2. The relative data show the large benefit of wheat having TaALMT1 when grown in acid soil. Table S2 shows the absolute data for the various parameters and includes measurements of shoot biomass and tiller number on Days 97 and 107 and at maturity. It also shows that eCO2 increased grain yield of both ET8 and ES8 on limed soil whereas only ET8 had an increased grain yield on unlimed acid soil. DISCUSSION Elevated CO2 increased the grain yield of Al3+-resistant lines but not of Al3+-sensitive lines when grown in acid soil. More specifically, although there were no CO2 × line interactions on the components of yield due to the large variation inherent in field studies (Table S1), the greater yield increase of Al3+-resistant compared to Al3+-sensitive lines can be attributed largely to the greater ear number. This conclusion is consistent with studies showing that an increased ear number contributes to a yield increase (Hakala, 1998; Dijkstra et al., 1999; J. Wang et al., 2015). The average 20 % increase in grain yield across all lines due to eCO2 is well within the range of a previous meta-analysis that found an average increase of 24 % in grain yield under eCO2 for wheat grown on a range of soils (L. Wang et al., 2013). Similarly, a FACE study with a similar atmospheric environment to our study showed a grain yield increase of 24 % for wheat grown in the field under dryland conditions (Fitzgerald et al., 2016). TaALMT1 and TaMATE1B give the Al3+-resistant lines a greater ability to promote root growth and thus shoot growth, including potentially increased leaf area, which allows plants to respond to eCO2 to a greater extent than Al3+-sensitive lines. Our results also showed that both TaALMT1 and TaMATE1B frequently increased concentrations of nutrients in shoots, particularly P which was below the critical value (Ryan et al., 2014) at the vegetative stage when the resistant lines were compared with their sensitive near-isogenic pairs (Table S3). Amongst the Al3+-resistant lines, the increased grain yield due to eCO2 of lines carrying the Al3+-resistant allele of TaMATE1B (Egret TaMATE1B and EGA-Burke TaMATE1B) could be attributed largely to their increased growth, as indicated by plant shoot biomass and number of tillers (Fig. 2). By contrast, the increases in harvest index of lines carrying only the Al3+-resistant allele of TaALMT1 (ET8 and EGA-Burke) might have contributed to the yield increase to an extent (Table S1). The difference in growth between the lines carrying TaMATE1B and those carrying only TaALMT1 could stem from the differences in biomass allocation. Amongst the Al3+-resistant lines, eCO2 maintained biomass allocation to roots of lines carrying TaMATE1B (Egret TaMATE1B and EGA-Burke TaMATE1B) but decreased that of lines carrying only TaALMT1 (ET8 and EGA-Burke) (Table S1). More specifically, when the various root types were analysed, eCO2 consistently increased only coarse root length (probably nodal roots) in lines carrying TaMATE1B (Fig. 3). The greater improvement of net photosynthetic rates and stomatal conductance in flag leaves and water-use efficiency of lines carrying TaMATE1B under eCO2 (Table S4) are consistent with the greater promotion of root growth from eCO2 relative to lines carrying only TaALMT1. Therefore, it appears that the yield increase of the resistant lines under eCO2 could not all be attributed to improved root growth. Fig. 2. View largeDownload slide Root biomass (A), shoot biomass (B) and total tiller number (C) of six wheat lines grown to maturity in an acid soil under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS, *, and *** denote the levels of significance at P > 0.1, P < 0.05 and P < 0.001 between CO2 levels of a single line or within a near-isogenic pair using a t-test. Means are presented and an LSD bar is also presented to compare any two means at P ≤ 0.05 within a panel. Fig. 2. View largeDownload slide Root biomass (A), shoot biomass (B) and total tiller number (C) of six wheat lines grown to maturity in an acid soil under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS, *, and *** denote the levels of significance at P > 0.1, P < 0.05 and P < 0.001 between CO2 levels of a single line or within a near-isogenic pair using a t-test. Means are presented and an LSD bar is also presented to compare any two means at P ≤ 0.05 within a panel. Fig. 3. View largeDownload slide Total root length (A), fine root length (diameter <0.3 mm) (B) and coarse root length (diameter >0.3 mm) (C) of six wheat lines grown in an acid soil in the whole columns under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS, *, ** and *** denote the levels of significance at P > 0.1, P < 0.05, P < 0.01 and P < 0.001 between CO2 levels of a single line or within a near-isogenic pair using a t-test. Means are shown and an LSD bar is also presented to compare any two means at P ≤ 0.05 within a panel. Fig. 3. View largeDownload slide Total root length (A), fine root length (diameter <0.3 mm) (B) and coarse root length (diameter >0.3 mm) (C) of six wheat lines grown in an acid soil in the whole columns under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS, *, ** and *** denote the levels of significance at P > 0.1, P < 0.05, P < 0.01 and P < 0.001 between CO2 levels of a single line or within a near-isogenic pair using a t-test. Means are shown and an LSD bar is also presented to compare any two means at P ≤ 0.05 within a panel. This study thus did not support our speculation that the more Al3+-resistant a line was, the greater the root elongation would be under eCO2. This argues against our hypothesis that improved grain yield due to eCO2 would be driven by improved root growth although it appears to be the case when Al3+-resistant lines as a group are compared to Al3+-sensitive lines. The main reason for the discrepancy could be that Al3+-toxicity in the acid soil used inhibited fine root growth (Fig. 3 and Table S3), which might impair the root function for nutrient uptake. We speculate that under highly Al3+-toxic conditions, the increased carbon availability under eCO2 was more probably used for biomass accumulation in shoots and coarse roots or to form grain yield instead of improving fine root growth as occurred under non-stressed conditions. Therefore, more biomass can be translocated to grains of the lines carrying only TaALMT1 when less biomass was allocated to the roots. Lines carrying TaMATE1B also formed more tillers under eCO2 than lines carrying only TaALMT1 (Tables 1 and S5). However, the increased tillers were unable to develop into effective tillers (Table S1) resulting in a greater number of infertile tillers (haying off). The greater mass of nodal roots associated with greater tiller initiation might limit biomass allocation to grain to some extent as well (Watt et al., 2008). Tausz-Posch et al. (2015) observed similar carbon allocation to non-effective tillers in wheat cultivar ‘Silverstar’ and speculated that the greater early vegetative growth decreased the amount of water available for grain filling later in the growing season (van Herwaarden et al., 1998). However, eCO2 decreased the water consumption and slightly increased soil water content (Table S4 and Fig. S2). It is therefore unlikely that the tiller abortion resulted from water deficit. Because lines carrying TaMATE1B enhanced carbon allocation to roots under eCO2, the early tillers possibly could not receive enough carbon to complete their development, resulting in tiller abortion. Alternatively, TaMATE1B might respond to other soil factors apart from Al3+ toxicity, which favoured biomass allocation to the roots under eCO2. This is the first study to report that the introgression of TaMATE1B into an Al3+-sensitive hexaploid wheat cultivar promoted grain yield of wheat in the field in an acid soil (Fig. 1). The introgression of TaMATE1B into a sensitive hexaploid wheat cultivar was far less effective than TaALMT1 in enhancing acid-soil tolerance when considering root and shoot growth parameters at maturity (Figs 2 and 3). Despite TaMATE1B being less effective than TaALMT1 in conferring improved shoot and root biomass in an acid soil, TaMATE1B was as effective as TaALMT1 in increasing grain yield (Fig. 1). The greater harvest index and reduced biomass allocation to roots resulting from TaMATE1B (Table S1) might have resulted in greater relative biomass allocation to grain and thus grain yield while still maintaining sufficient root growth. Despite the benefit of each gene acting independently, they did not appear to be additive because in the EGA-Burke background, which has TaALMT1, plant growth and grain yield were not increased by the inclusion of TaMATE1B. Similarly, there was no additive effect of the gene in a short-term study using the lines grown in a Ferrosol (Han et al., 2016). However, a previous short-term chamber study using a very Al3+-toxic Dermosol showed that there was an additive effect on root length (Dong et al., 2018), suggesting that an additive effect might only be realized in highly Al3+-toxic soils. The increases in shoot biomass, tiller number and root biomass of ES8 and ET8 at maturity when grown in limed soils (Table S2, absolute data) indicate that eCO2 increased plant performance, which is consistent with previous studies for wheat grown on non-acid soils (Long et al., 2004; L. Wang et al., 2013). The relative data (Fig. 4) show the benefit of having TaALMT1 on various plant growth parameters including grain yield when ET8 and ES8 were grown on an acid soil, as previously found (Tang et al., 2002). However, eCO2 decreased the acid-soil tolerance of sensitive line ES8 and tended to decrease that of resistant line ET8 when grown in acid soils, as indicated by the general decreases in relative root length, relative root biomass and relative total biomass (Fig. 4). This is consistent with a previous short-term study using a controlled-environment chamber showing that eCO2 decreased acid-soil tolerance of ES8 (Dong et al., 2018). Reduced relative root biomass of ET8 might be due to the reduced rates of carbon fixation in this study (t-test, P = 0.033, Table S4), which probably resulted from the photosynthetic acclimation by longer-term eCO2 exposure or lower CO2 concentration, resulting in less carbon input into root elongation (Kirschbaum, 2011) or less antioxidants to counteract Al3+ toxicity (Pietrini et al., 2016). Fig. 4. View largeDownload slide Relative root length (A), relative root biomass (B), relative grain yield (C) and relative total shoot biomass (D) (as a percentage of the limed soil) of ES8 and ET8 grown in an acid soil under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS and * denote the levels of significance at P > 0.1 and P < 0.05 between CO2 levels of a single line using a t-test. Data are means ± s.e. The genotypic variation was significant on all these parameters (P < 0.001). Fig. 4. View largeDownload slide Relative root length (A), relative root biomass (B), relative grain yield (C) and relative total shoot biomass (D) (as a percentage of the limed soil) of ES8 and ET8 grown in an acid soil under two CO2 concentrations (aCO2, 400 µmol mol−1; eCO2, 550 µmol mol−1). NS and * denote the levels of significance at P > 0.1 and P < 0.05 between CO2 levels of a single line using a t-test. Data are means ± s.e. The genotypic variation was significant on all these parameters (P < 0.001). In our study, eCO2 did not affect grain quality in terms of protein and mineral concentrations (Table S6). This contrasts with findings that eCO2 decreases grain concentrations of protein (Taub et al., 2008) and minerals in various crops (Loladze, 2014; Myers et al., 2014) as well as wheat (Högy and Fangmeier, 2008). Environmental or species variations may have contributed to these differences (Högy and Fangmeier, 2008; Fernando et al., 2014). CONCLUSIONS This FACE study demonstrated that eCO2 increased the grain yield of wheat lines carrying only TaALMT1 (ET8 and EGA-Burke) or TaMATE1B (Egret TaMATE1B and EGA-Burke TaMATE1B) but not of sensitive lines (ES8 and Egret) when grown in an Al3+-toxic soil. The increased grain yield could not be attributed to improved root growth as eCO2 reduced or did not affect the total root length and biomass of lines carrying only TaALMT1 (ET8 and EGA-Burke). By contrast, eCO2 improved the coarse root length and tiller formation in lines carrying TaMATE1B, which may have resulted in a greater abortion of tillers and limited the biomass allocation to grains relative to lines carrying only TaALMT1. The introgression of TaMATE1B into Egret increased grain yield in the acid soil to a similar level as ET8 which has TaALMT1. This was despite TaMATE1B being a less effective Al3+-resistance gene than TaALMT1 based on the ability to protect root growth. However, despite the Al3+-resistance genes conferring improved grain yield when present individually, their effects were not additive. As atmospheric CO2 inevitably increases in the future, wheat breeders should maintain the resistant genes (TaALMT1 and TaMATE1B) in their germplasm, but be prudent in using TaMATE1B as a source of Al3+ resistance in Al3+-toxic soils. Further studies are needed to ascertain the reliability of the respective resistant genes and to assess the lines grown in the field on various acid soil sites over multiple seasons under a range of climatic conditions. SUPPLEMENTARY DATA Supplementary data are available online at https://academic.oup.com/aob and consist of the following. Table S1: Root-to-shoot biomass ratio, harvest index and the yield components of six wheat lines grown to maturity in an acid soil under two CO2 concentrations. Table S2: Shoot biomass, tiller number and growth parameters of two wheat lines grown in an acid soil with or without lime under two CO2 concentrations. Table S3: Concentration of nutrients in shoots of plants of six wheat lines grown in an acid soil under two CO2 concentrations. Table S4: Net photosynthetic rates, stomatal conductance, instantaneous transpiration efficiency of the flag leaf at the flowering stage, and water input and water-use efficiency of six wheat lines grown in an acid soil under two CO2 concentrations. Table S5: Shoot biomass and tiller number of six wheat lines grown in an acid soil at various growth stages under two CO2 concentrations. Table S6: Concentrations of protein and nutrients in grains of six wheat lines grown in an acid soil under two CO2 concentrations. Figure S1: Rainfall, daily solar radiation, daily maximum temperature and daily minimum temperature over the experimental period. Figure S2: Water content of the acid soils in which six wheat lines were grown in an acid soil under two CO2 concentrations from Day 138 to Day 192 of growth. ACKNOWLEDGEMENTS We thank Mel Munn, Liana Warren, Roger Perris and Russel Argall for management of the SoilFACE experiment. 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TI - Elevated CO2 (free-air CO2 enrichment) increases grain yield of aluminium-resistant but not aluminium-sensitive wheat (Triticum aestivum) grown in an acid soil
JF - Annals of Botany
DO - 10.1093/aob/mcy171
DA - 2019-02-15
UR - https://www.deepdyve.com/lp/oxford-university-press/elevated-co2-free-air-co2-enrichment-increases-grain-yield-of-N6RBa5lDkJ
SP - 461
VL - 123
IS - 3
DP - DeepDyve
ER -