ABSTRACT

   Hydraulic conductance determines the capacity of water transport affecting plant water relations as well as the water and nutrient absorption efficiency. Differences in citrus rootstock hydraulic conductance have been related to xylem anatomy and root distribution. Additionally, hydraulic conductance can be the main factor determining fruit development and the plant response to stress conditions. Water relations can be affected by abiotic stresses such as drought stress, salinity and anoxia. Recent studies show that, in non-citrus species, aquaporins are significantly involved in plant water relations and environmental stimuli can regulate their expression. This, in turn, may be regulated by the cytosolic pH.
   Therefore, the main objective of this thesis was to study the hydraulic conductance of citrus rootstocks, determining its influence on the transpiration, factors that regulate it and its role in the plant response to abiotic stresses.
   In the experiments, seedlings of different rootstocks and plants grafted on different rootstocks were used. Differences in the water absorption capacity between rootstocks were observed. Hydraulic conductance had a marked influence on the plant transpiration and was related to both the expression of aquaporins as xylem anatomy. On the other hand, results showed that, under water stress conditions hydraulic characteristics could determine the degree of plant tolerance to this stress. By subjecting plants to salt stress, differences between citrus genotypes in aquaporin expression were observed, thereby determining their salinity tolerance. Finally, acidosis induced by anoxic stress caused down-regulation of aquaporin expression, thus decreasing hydraulic conductance which results in stomatal closure.