Evaluation of water use efficiency: the contribution of structural vs. functional adaptations to drought stress

Abstract

The main objective was the evaluation of different strategies of water use efficiency (WUE) through the analysis of carbon and oxygen isotope composition of leaf dry matter and gas exchange measurements in three Mediterranean species with different structural and functional adaptations. Carbon isotope discrimination (D 13C) is a widely used parameter which gives an integrated measure of WUE over the leaf life-span. However, different responses are expected from species with different growth strategies and leaf longevities when subjected to a slow drying treatment. Functional adaptations to drought can occur through stomatal regulation of water loss and/or a reduction in photosynthetic capacity, which is difficult to distinguish solely from D 13C values. Oxygen isotope ratios of leaf dry matter may help to separate these effects, since they reflect the evaporative conditions. Therefore, three Mediterranean species with different strategies were studied: a drought-tolerant evergreen (Quercus coccifera) with highly sclerophyllous leaves and strong stomatal regulation of water loss, a semi-deciduous species (Cistus albidus) with more mesophyllic, highly pubescent leaves, and a fast growing perennial herb (Vinca sp.) with mesophyllic leaves and low stomatal control. Plants were exposed to a slow drying cycle under semi-controlled conditions. Water potentials, gas exchange and fluorescence parameters (maximum and effective quantum yield) as well as structural parameters (growth, specific leaf weight) were measured continuously. These results will be used to evaluate the carbon and oxygen isotope analysis and to answer the above presented questions.