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

In C₃ plants, carbon isotope composition (δ¹³C) is influenced by isotopic effects during diffusion from the atmosphere to the chloroplasts and carboxylation reactions. This work aimed to demonstrate if δ¹³C of leaf soluble carbohydrates (δ¹³C_leaves) and of dry matter from new-growth shoots (δ¹³C_shoots) 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 δ¹³C increase of 5.86, 4.28 and 4.99‰ was observed in 6-5, 7-7 and G × N, respectively. Significant correlations between δ¹³C and other parameters (substomatal CO₂/atmospheric CO₂ 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 δ¹³C_shoots was correlated with the average WUE measured during the whole experiment. δ¹³C_leaves and δ¹³C_shoots 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 δ¹³C as an integrated indicator of level of drought stress in plants subjected to prolonged stress conditions.