Role of leaf hydraulic conductance in the regulation of stomatal conductance in almond and olive in response to water stress

The decrease of stomatal conductance (g(s)) is one of the prime responses to water shortage and the main determinant of yield limitation in fruit trees. Understanding the mechanisms related to stomatal closure in response to imposed water stress is crucial for correct irrigation management. The loss of leaf hydraulic functioning is considered as one of the major factors triggering stomatal closure. Thus, we conducted an experiment to quantify the dehydration response of leaf hydraulic conductance (K-leaf) and its impact on g(s) in two Mediterranean fruit tree species, one deciduous (almond) and one evergreen (olive). Our hypothesis was that a higher K-leaf would be associated with a higher g(s) and that the reduction in K-leaf would predict the reduction in g(s) in both species. We measured K-leaf in olive and almond during a cycle of irrigation withholding. We also compared the results of two methods to measure K-leaf: dynamic rehydration kinetics and evaporative flux methods. In addition, determined g(s), leaf water potential (I-leaf), vein density, photosynthetic capacity and turgor loss point. Results showed that g(s) was higher in almond than in olive and so was K-leaf (K-max=4.70 and 3.42mmols(-1)MPa(-1)m(-2), in almond and olive, respectively) for I-leaf > a'1.2MPa. At greater water stress levels than -1.2MPa, however, K-leaf decreased exponentially, being similar for both species, while g(s) was still higher in almond than in olive. We conclude that although the K-leaf decrease with increasing water stress does not drive unequivocally the g(s) response to water stress, K-leaf is the variable most strongly related to the g(s) response to water stress, especially in olive. Other variables such as the increase in abscisic acid (ABA) may be playing an important role in g(s) regulation, although in our study the g(s)-ABA relationship did not show a clear pattern.