Journal of Experimental Botany

LEE, R., B.;CLARKSON, D., T.

doi: 10.1093/jxb/37.12.1753pmid: N/A

Abstract The short-lived radio-isotope nitrogen-13 (half-life 10 min) was used as a tracer in studying fluxes of N in the roots of intact barley plants. After supplying the plants with 13N-nitrate for 30 min, efflux of 13N into an unlabelled (wash) solution was followed under steady-state conditions for a further 10 min. Tests with ion exchange resins suggested that all of the 13N released during this period was in the form of nitrate. In addition to nitrate from a surface film of solution and from the free space of the roots, efflux from another compartment was detected, tentatively identified as the cytoplasmic nitrate pool. In plants grown with nitrate as the only external N-source, efflux from this compartment decreased with a rate constant about 0·17 min−1 (half-time ∼4 min). Adding ammonium sulphate to the wash solution alone did not significantly affect either the initial rate, or the rate constant, of efflux of 13N from these roots. However, 13N efflux decreased more rapidly (rate constant about 0·32 min−1, half-time ∼2·2 min) in roots grown in, and subsequently washed with, solution containing ammonium nitrate. In barley plants grown with 1·5 mol m−3 nitrate, the cytoplasmic nitrate pool was estimated to contain about 2% of the total nitrate in the roots, corresponding to a cytoplasmic nitrate concentration ∼26 mol m−3. Nitrate efflux was equivalent to almost 40% of nitrate influx in the roots of these plants. Ion transport, nitrate, ammonium, efflux analysis, compartmentation This content is only available as a PDF. © Oxford University Press

LEE, R., B.;DREW, M., C.

doi: 10.1093/jxb/37.12.1768pmid: N/A

Abstract Influx of nitrate into the roots of intact barley plants was followed over periods of 1–15 min using nitrogen-13 as a tracer. Based on measurements taken over 15 min from a range of external nitrate concentrations (0·2–250 mmol m−3), the kinetic parameters of influx, Imax and Km, were calculated. Compared with plants grown in the presence of nitrate throughout, plants that had been starved of N for 3 d showed a significantly greater value ofImax for 13N-nitrate influx (by a factor of 1·4–1·8), but a similar value of Km (12–14 mmol m−3). Pre-treating N-starved plants with nitrate for about 5 h further increased the subsequent rate of 13N-nitrate influx, but had little effect in the unstarved controls. Allowing for this induction of additional nitrate transport, the difference in rates of nitrate influx in control and N-starved plants was sufficient to account for the previously-observed difference in net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, but with ammonium as N-source, both Imax and Km for subsequent 13N-nitrate influx were significantly decreased (by about one-half) compared with the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulation of net uptake of nitrate is discussed. Ion transport, nitrate, influx, kinetic parameters, N-deficiency This content is only available as a PDF. © Oxford University Press

BISHOP, H., T.;THOMPSON, R., G.;AIKMAN, D., P.;FENSOM, D., S.

doi: 10.1093/jxb/37.12.1780pmid: N/A

Abstract 11CO2 was offered to leaves of sunflower, corn and ryegrass and 13N2 to root nodules of alfalfa and alder. Movement of the tracers out of the feed region was monitored along stems or petioles using geiger tubes. Fluctuations in radioactivity were not detected as statistically significant from random tracer decay in the ‘background’ section of the time-activity profiles before mass-flow commenced, but became highly significant in the mass-flow sections. These pulses of radioactivity could be followed from one detector to the next over 1–3 cm and were analysed for periodicity by cross-correlation and auto-correlation computer programmes. Periodicity was only rarely detected in 11C runs, but was evident in many 13N experiments. Speed of pulse movement (microfronts) were measured, both visually and by computer cross-correlations, and compared with rates found by the ‘moving intercept’ of mass flow. Microfront speeds were faster. Speeds of 11C movement were comparable with those reported for phloem, but 13N movements were often much higher, suggesting xylem movements. Fine structure pulses indicate that movements of 11C photosynthate or 13N compounds are rapid, erratic and far more complex than expected by a simple Münch pressure flow mechanism. 11C, 13N, plant stems, radioactive pulse This content is only available as a PDF. © Oxford University Press

ENGELS,, CH.;MARSCHNER,, H.

doi: 10.1093/jxb/37.12.1795pmid: N/A

Abstract Potato plants (Solanum tuberosum L.) were grown in water culture in a controlled environment. Cooling (+8°C) of individual tubers decreased their growth rates and increased the growth rates of non-cooled tubers of the same plant. The carbohydrate concentration in non-cooled and cooled tubers did not differ significantly, but 14C-import from labelled photosynthate was lower in cooled than in non-cooled tubers. The markedly lower conversion rate of ethanol-soluble 14C to starch in cooled, in comparison to non-cooled tubers, was not associated with significant differences in the in vitro activities of starch synthase, ADPG-pyrophosphorylase and starch phosphorylase under standard assay conditions (+30°C). However, the Q10-values of the enzymes differed in vitro in the temperature range between 30°C and 8°C, leading to a marked decrease in the activity ratio of ADPG-pyrophosphorylase/starch phosphorylase in cooled tubers. In tubers differing in growth rates without manipulation, 14 d after tuber initiation significant positive correlations were found between 14C-concentration of tuber tissue and the in vitro activities of starch synthase and ADPG-pyrophosphorylase and a significant negative correlation between 14C-concentration and starch phosphorylase. In contrast, in tubers which were analysed 5 d after initiation, there were only small differences between tubers in growth rate, 14C import and the activity ratio ADPG-pyrophosphorylase/starch phosphorylase. From various direct and indirect evidence it is concluded that the growth rate of individual tubers, and thus the sink strength, is at least in part controlled by the activity of starch synthesizing enzymes. Potato tuber, cooling, starch synthesizing enzymes This content is only available as a PDF. © Oxford University Press

ENGELS,, CH.;MARSCHNER,, H.

doi: 10.1093/jxb/37.12.1804pmid: N/A

Abstract Potato plants (Solanum tuberosum L.) were grown in water culture in a controlled environment. The growth rates of individual tubers were closely reflected by their 14C-content 20 h after 14CO2 had been applied to the aerial parts of the shoot for 4 h. The 14C-content of the tuber (sink strength) was significantly correlated to the 14C-concentration of the tuber tissue (14C g−1 fr. wt.=sink activity). The sink activity, which differed between individual tubers by up to a factor of 10, was also closely related to the conversion rates of 14C into the starch and the remainder as well as to the 14C-content in the ethanol soluble fraction. This indicates the simultaneous use of photosynthate for growth and storage in the growing tubers. No preference of photosynthate utilization for either of these processes could be detected in relation to the sink activity of the tubers. Tubers with high sink activity imported 14C-labelled photosynthate at higher rates although their tissue contained higher concentrations of reducing sugars and sucrose than the tissue of tubers with low sink activity. Despite the close relationship between sink activity and the rate of starch synthesis (14C-conversion into starch), no significant correlation was found between sink activity and the actual starch concentration of the tissue. The application of zeatin riboside directly onto individual tubers increased their growth rates in comparison to non-treated tubers of the same plant. The results indicate the importance of both growth and storage processes for the regulation of sink activity in young potato tubers. Potato tuber, 14C-photosynthate partitioning, zeatin riboside application This content is only available as a PDF. © Oxford University Press

ENGELS,, CH.;MARSCHNER,, H.

doi: 10.1093/jxb/37.12.1813pmid: N/A

Abstract Potato plants (Solanum tuberosum L.) were grown in water culture. About 14 d after tuber initiation no significant differences were found between apical and basal tuber parts in 14C-uptake and partitioning into various fractions from 14C-labelled photosynthate. Thus, the fresh weight of these tubers could be used as a parameter for the sink size. The 14C-content per tuber (sink strength) 20 h after 14CO2-supply to the foliage was significantly correlated with the tuber fresh weight. No correlation was found between the 14C-concentration of the tuber (sink activity; ct. min− g− fr. wt.) and tuber fresh weight. Consequently, tuber fresh weight (sink size) per se must have been a factor which influenced sink strength. Stolon parameters characterizing their growth prior to tuber initiation (e.g. stolon volume) and their capacity for photosynthate transport (diameter, length) were measured at the time of tuber initiation. Significant correlations were found between these stolon parameters and subsequent growth of individual tubers. Anatomical studies on the proportion of various tissues in the cross sectional area of stolons support the idea of a negative relation between growth of individual tubers and transport resistance in the phloem of the stolons. It is concluded that in the initial phase of tuber growth, mainly factors outside of the tuber determine its growth rate. In later stages of tuber growth, when the sink strength increases, the competing strength of individual tubers for photosynthate is dominated mainly by factors within the tuber itself, such as their sink size and sink activity. Potato tuber, sink size, tuber initiation, transport resistance This content is only available as a PDF. © Oxford University Press

COLLINS, ROSEMARY, P.;JONES, M., B.

doi: 10.1093/jxb/37.12.1823pmid: N/A

Abstract Growth and production of the temperate C4 species Cyperus longus L. was measured throughout a growing season in an established plot in Eastern Ireland. The maximum standing live biomass reached was 2·5 kg m−2. Estimates of unit leaf rate (ULR) and leaf area index (LAI) were made. The product of these quantities gave the crop growth rate (CGR) each week. C. longus was found to maintain high values of LAI throughout the summer, with a maximum value of about 13 in early August. CGR reached a peak in early July. The optimum LAI was 11·6. Temperatures at five levels in the plant canopy, and the amount of solar radiation intercepted by the canopy were measured continuously during the summer. The mean daily rate of leaf extension was positively correlated with the mean daily air temperature above the canopy but the temperature coefficient of the process was low compared with other temperate species. The percentage of solar radiation intercepted by the canopy increased rapidly in early summer, and canopy closure had occurred by mid-June. Rates of net photosynthesis were measured on young and old leaf material in situ at the time of peak LAI. In young leaves the maximum rates of net photosynthesis were higher than those published for a range of temperate C3 species, but similar to those found in another temperate C4 species, Spartina townsendii. C4 photosynthesis, leaf growth, productivity This content is only available as a PDF. © Oxford University Press

YAMASHITA,, T.;FUJINO,, A.

doi: 10.1093/jxb/37.12.1836pmid: N/A

Abstract Changes in the contents of ribulose 1,5-bisphosphate carboxylase (RuBPCase) and other cellular constituents were measured in the leaves remaining on mulberry shoots after removal of the top one-third of shoots (old) which had developed since the spring and shoots (young) which developed following shoot harvest on 1 July. Top pruning of the young summer shoots increased the contents of chlorophyll, total soluble protein, RuBPCase, RNA and starch in the leaves. In the old shoots that had developed since the spring, top-pruning in the late summer produced no appreciable change except in the content of RNA. The content of RuBPCase of the young summer shoots doubled as a result of top-pruning; this was the most conspicuous change in the cellular constituents examined. The increase in RuBPCase protein accounted satisfactorily for the increase in total soluble protein. RuBP carboxylase, shoot top-pruning, mulberry (Morus alba) This content is only available as a PDF. © Oxford University Press

WARD, DAVID, A.;BUNCE, JAMES, A

doi: 10.1093/jxb/37.12.1842pmid: N/A

Abstract A dual-surface leaf chamber was used to investigate the responses of net photosynthesis and leaf conductance to independent changes in the humidity environments of the upper and lower surfaces of leaves of sunflower and soybean. In sunflower decreasing the humidity around the upper leaf surface while maintaining that of the lower surface constant and high reduced both the photosynthetic rate and the conductance of the lower surface. These reductions could not be attributed to changes in bulk leaf water potential since the transpiration rate of the whole leaf remained constant. Similarly, the reductions were not related to localized water deficits in the lower epidermis or lower mesophyll since the transpiration rate of the lower surface was reduced. Possible mechanisms whereby the gas exchange characteristics of the lower leaf surface of sunflower respond to the humidity environment of the upper surface are discussed. In contrast to sunflower, the photosynthetic rate of the lower surface of soybean was insensitive to the humidity environment of the upper surface. In leaves of sunflower grown under a moderate temperature and a medium light level, simultaneous decreases of humidity at both leaf surfaces reduced the photosynthetic rate of the whole leaf without affecting the substomatal partial pressure of CO2. In contrast, with leaves developed under a cool temperature and a high light level, both the photosynthetic rate and the substomatal partial pressure of CO2 were reduced. Evidently, the occurrence in sunflower of the response pattern suggesting a non-stomatal inhibition of photosynthesis by low humidity depends upon the environment during growth. The possibility that this non-stomatal inhibition may be an artifact due to an error in the assumption of water vapour saturation within the leaf airspace is considered. Vapour pressure deficit, photosynthesis, conductance, non-stomatal inhibition, Helianthus annuus, Glycine max This content is only available as a PDF. © Oxford University Press

EAMUS,, DEREK

doi: 10.1093/jxb/37.12.1854pmid: N/A

Abstract Intact plants of okra (Hibiscus esculentus) were chilled at 6°C in the light, and leaf diffusion resistance (LDR) and leaf water potential measured. The response of the LDR of excised leaves to fresh weight loss and, separately, exogenous abscisic acid (ABA) supply, was also studied at 6°C and 30°C. The influence of two pre-treatments upon these measurements was studied. The two pre-treatments consisted of the imposition of a period of water stress at 30°C prior to measurement (followed by re-watering) or the spraying of the leaves with ABA. It was found that plants that had been grown in a high humidity environment continuously (designated control plants) had stomata that were very unresponsive to both water loss from the leaves and to exogenous ABA at both temperatures (6°C and 30°C). Chilling the control plants resulted in rapid wilting and concomitant decline in leaf water potential. A pre-treatment of water stress prior to chilling did not alter or reduce the rate of development of chilling injury, nor did the pre-treatment increase the responsiveness of stomata at 6°C to water loss or exogenous ABA. However, spraying the leaves with ABA prior to chilling reduced the severity and delayed the onset of chilling injury. Stomatal response to water stress and exogenous ABA was increased by the spraying pre-treatment. These results are discussed in relation to previous studies of the phenomenon of stomatal locking open at low temperature and the effect of pre-treatments upon the development of chill-resistance. Water stress, chilling, stomata This content is only available as a PDF. © Oxford University Press