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505Plant, Cell and Environment (2001) 24, 505-515 intercellular partial pressure of CO 2 ; PPFD, photosynthetic photon flux density; R, dark respiration; R VPDB , 13 C/ 12 C ratio of standard VPDB; R s , 13 C/ 12 C ratio of sample; Rubisco, ribulose 1,5 bisphosphate carboxylase-oxygenase; RWC, leaf relative water content; VPD, vapour pressure deficit; d 13 C, carbon isotopic composition. INTRODUCTIONCarbon isotope discrimination during leaf CO 2 assimilation has been extensively studied and models have been developed (Farquhar, O'Leary & Berry 1982;Evans et al. 1986). The simple version of these models, which does not include the discrimination during respiration, has been validated for many species, suggesting that the discrimination during respiration is negligible and does not significantly modify the net discrimination during on-line measurements compared to the predicted values (for a recent review see Brugnoli & Farquhar 2000). Yet, the carbon isotope signature of plant dry matter integrates not only the discrimination during net CO 2 assimilation in the light (including CO 2 diffusion from the atmosphere to the chloroplasts, carboxylation, photorespiration and day respiration) but also the discrimination that could occur during the night-time respiration. Therefore, any fractionation during the night and/or the use of heavy or light substrates for dark respiration (releasing 13 C-enriched or 13 C-depleted CO 2 compared with leaf material) should change the isotopic signature of the remaining leaf material. Moreover, when non-photosynthesizing organs are taken into account, the release of 13 C-enriched or 13 C-depleted CO 2 will further contribute to changes in whole-plant carbon isotopic signature. Henderson, von Caemmerer & Farquhar (1992) observed in some C 4 species that the discrimination determined on leaf dry matter was significantly greater than that measured on-line. Using a modelling approach, they proposed that at least a part of this difference could be explained by the fractionation during dark respiration, releasing CO 2 enriched in 13 C relative to the plant material. We obtained similar results on Phaseolus vulgaris (unpublished results) and Nicotiana sylvestris (Duranceau, ABSTRACTThe variations of d 13 C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO 2 respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well-watered conditions, CO 2 respired in the dark was 13 C-enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant 13 C-enrichment by about 6‰ in respired CO 2 compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the 13 C-enrichment in respired CO 2 increased in dehydrated N. sylvestris and decrea...

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Twenty‐one durum wheat genotypes originating from different geographic areas were grown during 3 successive years. The trials were characterised by different precipitation regimes. Carbon isotope discrimination (Δ), carbon content (CC) and ash content (ma) were assessed in the flag leaf during anthesis, then in the kernel at full maturity. Differences between the 3 years, due to water availability, were noted for Δ, ma, CC and yield. Genotypic differences were also noted within each year for all the traits studied. Some genotypes from the Middle East exhibited higher flag leaf and kernel Δ than those originating from the West of the Mediterranean basin. The kernel Δ was strongly correlated with grain yield (GY). The leaf Δ correlated with GY only under strong water limitation and with biomass production (BP) in favourable water conditions. For the flag leaf, Δ was correlated with ma and with CC. Silicon content was then assessed in the flag leaf and in the kernel on a subset of 10 genotypes differing in their Δ values. Strong positive correlations were noted between silicon content and Δ and ma for the flag leaf. However, no clear relationship was found between silicon content and GY. The results obtained in this study confirm the validity of kernel Δ as a predictive criterion for GY under water stress and suggest the possible use of kernel ma as an alternative criterion to select genotypes with higher water stress tolerance.

The variations in δ 13 C in both leaf carbohydrates (starch and sucrose) and CO 2 respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in 13 C) with decreasing stomatal conductance and decreasing p i /p a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in 13 C). This may be explained by increased p i /p a but other possible explanations are also discussed. Interestingly, the variations in δ 13 C of CO 2 respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ 13 C of CO 2 respired in the dark and sucrose, respired CO 2 always being enriched in 13 C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in 13 C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing 13 C-enriched CO 2 ; and that (ii) this leads to 13 C depletion in the remaining leaf material.Abbreviations: A, leaf net CO 2 assimilation; a, fractionation against 13 C for CO 2 diffusion through air; b, net fractionation against 13 C during CO 2 fixation by Rubisco and PEPc; δ 13 C, carbon isotopic composition; ∆, discrimination against 13 C during CO 2 assimilation; d, the term including the fractionation due to CO 2 dissolution, liquid phase diffusion and also discrimination during both respiration and photorespiration; DW, leaf dry weight; dδ 13 C, the difference between CO 2 respired in the dark and plant material in their carbon isotope composition; d∆, variation in modelled discrimination at a given p i /p a relative to a reference value at p i /p a = 0·7; FW, leaf fresh weight; g c , leaf conductance to CO 2 diffusion; HPLC, high-performance liquid chromatography; LMA, leaf mass per area; p a , ambient partial pressure of CO 2 ; p i , intercellular partial pressure of CO 2 ; PEPc, phosphoenolpyruvate carboxylase; PPFD, photosynthetic photon flux density; R PDB , 13 C/ 12 C ratio of standard PDB; R S , 13 C/ 12 C ratio of sample; Rubisco, ribulose 1,5 bisphosphate carboxylase-oxygenase; RWC, leaf relative water content; SW, leaf saturated weight; VPD, vapour pressure deficit.δ 13 C of CO 2 respired in the dark in relation to δ 13 C of leaf carbohydrates in P. vulgaris 523

The sink capacity of the stalk in Zea mays L. (cv DEA) during the elongation period was previously investigated with 13C and 15N tracing. The chase experiment described here demonstrates the different behavior of intermediary reserves for C and N remobilization until full maturity of the kernels. Carbon incorporated during stalk elongation participated mainly in cellulose formation in vegetative organs appearing after the labeling period; the remobilization to kernels was low (0.5%). Soluble carbohydrates and proteins were the main intermediary sink compounds, starch being little remobilized. N first incorporated in roots, sheaths, stalk, blades was translocated to the kernel; 42% of the labeled N were recovered in kernels where they represented 8% of the total N. Cob, husk, and shank acted first as N sinks and then as N sources during ear development. It appeared that aminoacids used for synthesis of kemel proteins have a common origin, except for glutelin G3.vegetative parts for the development of the ear and to point out use of intermediary reserves of the stalk for kernel filling. MATERIALS AND METHODS Plant Culture and Labeling ExperimentMaize plants (Zea mays L., cv Dea) were grown in a greenhouse for 45 d. Control plants were maintained in the greenhouse while 15 other plants were put in a climatic chamber to assimilate '3C02 (1.2889 13C atom %) at 450 ppm in a controlled atmosphere and '5N-nitrates (1.9643 '5N atom %) in the nutrient solution (4), for an 8-d exposure. After the labeling, 3 plants were harvested and the 12 remaining plants continued their development in the greenhouse under previous conditions. The whole experimental procedure is detailed further in a previous paper (4). The use of stable isotope labeling at the natural abundance range and its use for estimation ofthe long-term partitioning was described previously (8, 9).The contribution of C and N reserves accumulated in the shoot to the ear growth is an important factor in crop yield (12,22). In maize, redistribution of carbon and nitrogen reserves from the senescent leaves to the kernel has already been reported (6,10

Summary• Dry mass per unit of leaf area (LDM) and ash content were evaluated as alternative criteria for carbon isotope discrimination ( ∆ ) in durum wheat ( Triticum durum ) flag leaves and grains.• Using correlation analysis the relationships between the three parameters (LDM, ∆ , ash content) and productivity were determined over three consecutive years in 37 field-grown durum wheat genotypes under contrasting drought conditions.• Highly significant differences were found between years and among genotypes for all measured traits. Grain ∆ and ash content, and LDM and flag leaf ∆ were negatively correlated under nondroughted conditions. Positive correlations were found between grain yield, harvest index and both ∆ and ash content of the flag leaf under drought. No significant correlations were found between LDM and both ∆ and grain yield.• Differences in LDM do not predict variations in ∆ , whereas ash content of grain and flag leaf (under droughted conditions) might be useful in predicting ∆ and grain yield. Ash content might provide an alternative screening method in the improvement of drought tolerance and yield stability in durum wheat.

available online at httpj//www.idealibrary.com on I BE bl 0 @ Four sites with contrasting environmental stress in southeastern Brazil: relations of species, life form diversity, and geographic distribution to ecophysiological parameters Some ecophysiological parameters related to plant performance and fitness (carbon and nitrogen isotope composition and total C and N concentrations; in situ chlorophyll fluorescence measurements) were determined for over 30 species in four habitats bordering the montane Atlantic rain forest of Brazil, along a gradient of altitude and rainfall: a dry coastal forest, two areas of sandy coastal plain vegetation (restingas), and a high altitude campo. There was a considerable diversity of ecophysiological behaviour within and between the functional groups we created based on plant life-forms. For instance, both crassulacean acid metabolism (CAM) and C3 species were found in most life-forms sampled and throughout all habitats. Despite the variation in rainfall regimes, average overall water-use efficiency was similar between sites, particularly for C., species, while no clear pattern regarding nitrogen-use emerged in this respect. Acute and chronic photoinhibition were found in many species across this gradient, even in CAM plants. However, on average, chronic photoinhibition and lower energy dissipation capacity were more characteristic of plants from the restinga habitats. This suggests that, although plants colonizing these habitats have evolved features to deal with water shortage, adaptation to high light levels has not been fully achieved yet. The ecophysiological performance of some individual species in distinct habitats and in distinct microhabitats within habitats is also discussed. ! C 2001 The Linnean Society of London ADDITIONAL KEY WORDS: Atlantic forest -carbon isotopes -chlorophyll a fluorescence -dry forest -high altitude vegetation -nitrogen isotopes -sandy coastal vegetation.