Carbon isotope discrimination and water use efficiency in interspecific Prunus hybrids subjected to drought stress.

In C plants, carbon isotope composition (δC) is influenced by isotopic effects during diffusion from the atmosphere to the chloroplasts and carboxylation reactions. This work aimed to demonstrate if δC of leaf soluble carbohydrates (δC) and of dry matter from new-growth shoots (δC) of Prunus plants subjected to a period of water deficit was related to water use efficiency (WUE). For this purpose, three interspecific Prunus hybrids rootstocks (6-5, 7-7 and G × N) were gradually subjected to drought and then rewatered. Soil water content (SWC) decreased from 26.1 to 9.4% after 70 days of water shortage, when plants reached values of predawn leaf water potential (LWP) ranging from -3.12 to -4.00 MPa. Gas exchange, particularly net photosynthetic and transpiration rates, differed among the three hybrids, leading to different values of WUE. After 70 days of drought, a significant δC increase of 5.86, 4.28 and 4.99‰ was observed in 6-5, 7-7 and G × N, respectively. Significant correlations between δC and other parameters (substomatal CO/atmospheric CO ratio, stomatal conductance and stem water potential) were found in all hybrids. The rewatering phase caused a recovery of the physiological status of the plants. The isotope composition of δC was correlated with the average WUE measured during the whole experiment. δC and δC were positively related (r = 0.87; p < 0.001). The isotopic signature was a reliable screening tool to identify Prunus genotypes tolerant to drought stress. The results suggest the possibility of using δC as an integrated indicator of level of drought stress in plants subjected to prolonged stress conditions.