• Phaseolus vulgaris;
  • carbohydrates;
  • carbon isotopes;
  • dark respiration;
  • discrimination;
  • drought

A, leaf net CO2 assimilation a, fractionation against 13C for CO2 diffusion through air b, net fractionation against 13C during CO2 fixation by Rubisco and PEPc δ13C, carbon isotopic composition Δ, discrimination against 13C during CO2 assimilation d, the term including the fractionation due to CO2 dissolution, liquid phase diffusion and also discrimination during both respiration and photorespiration DW, leaf dry weight dδ13C, the difference between CO2 respired in the dark and plant material in their carbon isotope composition dΔ, variation in modelled discrimination at a given pi/pa relative to a reference value at pi/pa = 0·7 FW, leaf fresh weight gc, leaf conductance to CO2 diffusion HPLC, high-performance liquid chromatography LMA, leaf mass per area pa, ambient partial pressure of CO2 pi, intercellular partial pressure of CO2 PEPc, phosphoenolpyruvate carboxylase PPFD, photosynthetic photon flux density RPDB, 13C/12C ratio of standard PDB RS, 13C/12C 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 δ13C in both leaf carbohydrates (starch and sucrose) and CO2 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 13C) with decreasing stomatal conductance and decreasing pi/pa 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 13C). This may be explained by increased pi/pa but other possible explanations are also discussed. Interestingly, the variations in δ13C of CO2 respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ13C of CO2 respired in the dark and sucrose, respired CO2 always being enriched in 13C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in 13C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing 13C-enriched CO2; and that (ii) this leads to 13C depletion in the remaining leaf material.