Nature 363, 439-443 (1993) SUBSCRIBERS to Nature in the United States and Canada will have received an incorrect version of this letter, in which colour Figs 2 and 3 have been transposed. In copies of Nature received by the rest of the world, the figures are correct as shown. Also, the symbol á£e used in the first sentence of the second new paragraph on page 442 should be á£r, the oxygen isotope composition of respired CO2.

Ten groundnut genotypes were grown under adequately irrigated conditions or subjected to drought during the pod filling phase (83-113 days after sowing) in a medium deep Alfisol at the ICRISAT Centre during the 1986-1987 post-rainy season. Crop growth was measured in both treatments, but transpiration (T) and water-use efficiency (W) were quantified only in the drought treatment. Leaf samples from both treatments were assayed for discrimination against (CO2)-C-13 fixed in leaves (DELTA) to examine the relationships between DELTA, crop growth, and W under field conditions.The shoot dry matter accumulated during the period of drought (Y) ranged from 72-150 g m-2 and was closely related to transpiration. This indicates scope for selection of traits and practices to increase T. Water-use efficiencies ranged from 1.38-2.50 g kg-1 and were inversely related to DELTA in eight out of the 10 genotypes. For the other two genotypes, there was evidence that T was under-estimated by field measurements. Water-use efficiency and transpiration were not correlated suggesting that these two traits might be combined through breeding. Variation between genotypes was greatest for the partitioning of total dry matter to pods (73%), followed by water-use efficiency (31%) and transpiration (29%). Crop growth rates were negatively related to DELTA under irrigated conditions but not under drought.

Pod yield response of two spanish (McCubbin and Red Spanish) and two virginia (Virginia Bunch and Q18801) cultivars were compared under a range of irrigation treatments applied at different growth stages on a Xanthozem soil in a subtropical environment in south-east Queensland. Detailed growth and soil water use measurements were taken on a fully irrigated treatment and a treatment which received no rainfall after 83 days after planting (DAP). Soil water deficits occurring during the flowering to the start of pod growth phase (R/I) significantly reduced pod yields (range, 17-25%) relative to the well-watered control plots (I/I) for all cultivars. Where crops were irrigated until 83 DAP, then crop water deficits occurred throughout the pod growth phase (I/R), a significant cultivar by irrigation treatment interaction was observed for pod yield. The greatest reduction in yield occurred when severe stress occurred during the pod filling phase (Sh). Significant cultivar variation in pod yield was apparent. Differences in pod yield within this treatment were analysed in terms of a simple framework where pod yield is a function of transpired water (T), transpiration efficiency (TE) and harvest index ( H ) . Estimates of TE derived from measurements of carbon isotope discrimination in leaves indicated only small variation in TE, and suggest this trait contributed little to pod yield variation in the cultivars used in this experiment. Variation in pod yield among the four cultivars was largely a result of differences in harvest index characteristics.

Transpiration efficiency of dry matter production (W), carbon-isotope discrimination () and dry matter partitioning were measured on six sunflower (Helianthus annuus L.) genotypes grown for 32 days in a glasshouse. Two watering regimes, one well watered (HW) and the other delivering half the water used by the HW plants (LW), were imposed. Four major results emerged from this study. (1) There was significant genotypic variation in W in sunflower and this was closely reflected in Δ for both watering treatments. (2) The low watering regime caused a decrease in Δ but no change in W; nonetheless the genotypic ranking for either Δ or W was not significantly altered by water stress. (3) A positive correlation between W and biomass accumulation occurred among genotypes of HW plants. (4) Q, the ratio of total plant carbon content to leaf area, was positively correlated with W and negatively correlated with Δ. These results are discussed with reference to the connection between transpiration efficiency and plant growth. In short, Δ can be used to select for W among young vegetative sunflower plants. However, selection for W may be accompanied by changes in other important plant growth characteristics such as Q.

Theory and empirical evidence have demonstrated that 13C discrimination (∆) by leaves of C3 plants may be associated with intrinsic water‐use efficiency or productivity. Consequently, selection for ∆ has potential value in breeding plants with improved adaptation. This study evaluates genotypic differences in ∆ among cowpea [Vigna unguiculata (L.) Walp.] and genotype ✕ drought level interactions, and compares ∆ measured in leaves and grains. Sixty cowpea accessions were subjected to drought by growing them on stored soil moisture in six randomized blocks, and genotypes differed significantly (P < 0.001) in leaf ∆. Seventeen cowpea accessions were grown under weekly irrigation (wet) and stored moisture (dry) conditions in four randomized‐split blocks. Genotypes differed significantly in ∆ under both conditions using either leaves or grains. Plants under dry conditions had lower ∆, which theory predicted could be associated with 62% higher water‐use efficiency. The same plants showed a 62% higher ratio of CO2 assimilation rate to leaf diffusive conductance. Genotypic rankings for ∆ were similar under wet and dry conditions for most genotypes, but a significant (P < 0.05) genotype ✕ drought interaction was observed, which was mainly due to one genotype. Correlations between ∆ in grain and subtending leaves were highly significant (P < 0.001), but two genotypes exhibited substantial differences in ranking for ∆ determined in grain compared with leaves. Genotypic differences were more readily detected in leaves than grains with broad sense heritabilities of 0.76 and 0.35, respectively. Heritabilities were similar under wet and dry conditions.

Carbon-isotope discrimination (Delta) is used to distinguish between different photosynthetic pathways. It has also been shown that variation in Delta occurs among varieties of C(3) species, but not as yet, in C(4) species. We now report that Delta also varies among genotypes of sorghum (Sorghum bicolor Moench), a C(4) species. The discrimination in leaves of field-grown plants of 12 diverse genotypes of sorghum was measured and compared with their grain yields. Discrimination varied significantly among genotypes, and there was a significant negative correlation between grain yield and Delta. The variation in Delta may be caused by genetic differences in either leakiness of the bundle-sheath cells or by differences in the ratio of assimilation rate to stomatal conductance. At the leaf level, the former should be related to light-use efficiency of carbon fixation and the latter should be related to transpiration efficiency. Both could relate to the yield of the crop.

Online carbon isotope discrimination (Delta) and leaf gas exchange measurements were made with control and salt-stressed Zea mays and Andropogon glomeratus, two NADP-ME type C(4) grasses. Linear relationships between Delta and p(i)/p(a) (the ratio of intercellular to atmospheric CO(2) partial pressure) were found for control plants which agreed well with theoretical models describing carbon isotope discrimination in C(4) plants. These data provided estimates of phi, the proportion of CO(2) fixed by phosphoenolpyruvate carboxylase which leaks out of the bundle sheath and the component of fractionation due to diffusion in air. Salt-stressed plants had wider variation in Delta for the same or less range in p(i)/p(a). Additional work indicated Delta changed independently of p(i)/p(a) in both water- and salt-stressed plants, suggesting a possible diurnal change in phi as plant water status changed linked to a decrease in the activity of the C(3) photosynthetic pathway relative to C(4) pathway activity. The possible effect of stress-induced changes in phi on organic matter delta(13) C of C(4) plants is apt to be most apparent in chronically stressed environments.

Short-term discrimination in assimilation of stable isotopes of carbon was measured for leaves of the C3 speciesPhaseolus vulgaris L. cv. Hawkesbury Wonder andFlaveria pringlei Gandoger, the C4 speciesAmaranthus edulis Speg., and the C3−C4 intermediate speciesPanicum milioides Nees ex. Trin,Flaveria floridana Johnson, andFlaveria anomala B.L. Robinson. Discriminations in the C3 and C4 species were similar to those expected from theoretical considerations. When ambient CO2 pressure was 330 μbar the mean discriminations in the C3 species andPanicum milioides were similar, whereas the mean discriminations inF. floridana andF. anomala were less than discrimination in C3 species andPanicum milioides. When ambient CO2 pressure was 100 μbar the mean discriminations inPanicum milioides andF. anomala were greater, and that inF. floridana was less, than that inPhaseolus vulgaris. We conclude that the pattern of discrimination inPanicum milioides is consistent with the presence of a glycine shuttle; inF. floridana andF. anomala, discrimination is consistent with the presence of a C4 pathway coupled with the operation of a glycine shuttle.

In order for plants to grow, they must fix carbon. Carbon usually enters the leaves as carbon dioxide, diffusing through pores in the epidermis called stomata. Increased stomatal conductance, g, of leaves causes an increase in the partial pressure of CO2 inside the leaves, p i . This usually causes an increase in the rate of CO2 assimilation, A, but also allows a greater rate of transpirational water loss, E. Such an action by a plant is a gamble, because while it increases the likelihood of growth and reproductive success, it also increases the probability of desiccation and death (Cowan 1986).

Abstract A negative correlation between water‐use efficiency (W), defined as the ratio of moles of carbon in the plant to moles of water transpired, and carbon isotope discrimination (Δ) was established for barley in pot experiments using 12 cultivars. The correlation was strong in two independent experiments in four different controlled environment where ambient temperature and vapour pressure deficit were varied and plants were either well‐watered or given limited amounts of water. Variation among cultivars was found in both Δ and W and rankings of both parameters, according to cultivar, were similar in different environments. Limiting water usually increased water‐use efficiency of plants. Total dry matter can be substituted for moles of carbon when calculating water‐use efficiency but the correlation between W and Δ were calculated using the carbon content of dry matter. There were differences varied significantly among cultivars. Despite these differences, correlations were also large between whole plant W and Δ of any of the plant parts. The amount of dry matter partitioned into reproductive growth varied genetically, as did the effect of stress on the partitioning. Growth, W and Δ of barley were compared with theory derived from gas exchange properties and with other literature. The effect on W of variation in vapour pressure deficit in these experiments was removed by multiplying W by vapour pressure deficit to derive the parameter, k(Pa mol C/mol H2O). This allowed comparisons among experiments with different vapour pressure deficits. The mean k for these barley cultivars was similar to that calculated by others for grasses. However, variation was found, and, in contrast with previous work which treats k as a species constant, we conclude that there is promise in selecting for increased k.

We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Variation in water-use efficiency (W, g of total dry matter produced/kg water used), and its correlation with cultivar isotope discrimination in leaves (Δ) was assessed in peanut plants grown in small canopies in the field. Plants were grown in separate minilysimeters that were both embedded in the ground and positioned above the crop. Differences among cultivars were found in W and and the relationship between W and Δ was compared for plants grown in open and closed canopies. Genetic variability in W in plants grown in the field under non-limiting water conditions was demonstrated, with Tifton-8, of Virginia habit, having the highest W (3.71 g/kg) and Rangkasbitung, an Indonesian cultivar of Spanish habit, the lowest (2.46 g/ kg). Variability in W was due to variation in total dry matter production more than that of water use. A strong negative correlation was found between Δ and W, and also between Δ and total dry matter. The relationship between whole plant W, including roots, and Δ was stronger than that between shoot W, without roots and Δ. The improvement occurred because of variation among cultivars in the root to shoot ratio. This highlights the importance of taking account of root dry matter in studies concerning W. There were significant differences in W and Δ between plants in pots above-ground compared to pots in the ground, with above-ground plants having significantly lower values of both W and Δ. The ranking of W and Δ among cultivars was not affected by the contrast in environment, which suggests these parameters are under strong genetic control. Total above-ground dry matter yield at maturity was negatively correlated with Δ, while pod yield was not. It appears a negative association between harvest index and Δ may exist; however not all cultivars used in this and other studies follow this response. Both water-use efficiency, Wand total dry matter production are negatively correlated with Δ in leaves of peanut plants grown in small canopies in the field. Measurement of Δ may prove a useful trait for selecting cultivars with improved W and total dry matter yield under field conditions.

Carbon isotope discrimination (Delta) was analyzed in leaf starch and soluble sugars, which represent most of the recently fixed carbon. Plants of three C(3) species (Populus nigra L. x P. deltoides Marsh., Gossypium hirsutum L. and Phaseolus vulgaris L.) were kept in the dark for 24 hours to decrease contents of starch and sugar in leaves. Then gas exchange measurements were made with constant conditions for 8 hours, and subsequently starch and soluble sugars were extracted for analysis of carbon isotope composition. The ratio of intercellular, p(i), and atmospheric, p(a), partial pressures of CO(2), was calculated from gas exchange measurements, integrated over time and weighted by assimilation rate, for comparison with the carbon isotope ratios in soluble sugars and starch. Carbon isotope discrimination in soluble sugars correlated strongly (r = 0.93) with p(i)/p(a) in all species, as did Delta in leaf starch (r = 0.84). Starch was found to contain significantly more (13)C than soluble sugar, and possible explanations are discussed. The strong correlation found between Delta and p(i)/p(a) suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of p(i)/p(a) weighted by CO(2) assimilation rate, over a day. Because p(i)/p(a) has a negative correlation with transpiration efficiency (mol CO(2)/mol H(2)O) of isolated plants, Delta in starch and sugars may be used to predict differences in this efficiency. This new method may be useful in ecophysiological studies and in selection for improved transpiration efficiency in breeding programs for C(3) species.

The discrimination, Δ, against 13C in carbon accumulation by different genotypes of peanut was investigated by looking at changes in composition of isotopes of CO2 during CO2 assimilation and at the 13C/12C ratio in dry matter. The short term (minutes) discrimination, which was measured as CO2 passed over leaves in a gas exchange cuvette, was positively related to pi/pa, the ratio of the pressure of CO2 inside the leaves to the pressure of CO2 around the leaves, which is independently related to transpiration efficiency, W. Heritability of Δ was studied in plants grown in pots and in the field. Crosses were made of cultivars with contrasting Δ and W. For these genotypes, small Δ (and large W) seemed to be dominant in the F1 progeny. Severe drought stress slightly decreased Δ and W in F2 plants but, for parental genotypes in another experiment, Δ decreased but W increased when they were less severely stressed. A strong negative phenotypic correlation (r = -0.78) was found between W (g DM/ kg water used) and Δ in the leaves of well-watered potted plants of F2 progeny from a cross between two cultivars with contrasting W and Δ. Transpiration efficiency varied significantly among cultivars of peanut irrespective of whether plants were drought-stressed or well-watered. Transpiration efficiency in the potted F2 plants had a heritability of 34% while Δ had a heritability of 53%. The broad sense heritability of Δ in fieid-grown cultivars was 81% and there was no significant genotype × environment interaction for Δ. The high heritability of Δ and its strong genetic correlation with W indicates that Δ is a suitable selection criterion for W in peanut breeding programs.