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

The variations in delta(13)C in both leaf carbohydrates (starch and sucrose) and CO2 respired in the dark from the cotyledonary leaves of Phaseolius vulgaris L, were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in C-13) 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 C-13). This may be explained by increased p(i)/p(a), but other possible explanations are also discussed. Interestingly, the variations in delta(13)C 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 delta(13)C Of CO2 respired in the dark and sucrose, respired CO2 always being enriched in C-13 compared with sucrose by approximate to 6%. The whole leaf organic matter was depleted in C-13 compared with leaf carbohydrates by at least 1%. These results suggest that: (i) a discrimination by approximate to 6% occurs during dark respiration processes releasing C-13-enriched CO2; and that (ii) this leads to C-13 depletion in the remaining leaf material.