δ¹³C of CO₂ respired in the dark in relation to δ¹³C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

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 Δ , 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 d δ 13 C, the difference between CO 2 respired in the dark and plant material in their carbon isotope composition d Δ , 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 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.