delta C-13 of CO2 respired in the dark in relation to delta C-13 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 v ulgaris L, were investigated during a progressive drought. As expected, suc rose 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 d ehydration cycle. Thereafter, when stomata remained closed and leaf net pho tosynthesis was near zero, the tendency was reversed: the carbohydrates bec ame lighter (depleted in C-13). This may be explained by increased p(i)/p(a ), but other possible explanations are also discussed. Interestingly, the v ariations in delta(13)C Of CO2 respired in the dark were correlated with th ose of sucrose for both well-watered and dehydrated plants. A linear relati onship was obtained between delta(13)C Of CO2 respired in the dark and sucr ose, respired CO2 always being enriched in C-13 compared with sucrose by ap proximate to 6%. The whole leaf organic matter was depleted in C-13 compare d with leaf carbohydrates by at least 1%. These results suggest that: (i) a discrimination by approximate to 6% occurs during dark respiration process es releasing C-13-enriched CO2; and that (ii) this leads to C-13 depletion in the remaining leaf material.