The Effect of Water Stress on Leaf Aquaporin Expression in Sweet Orange Trees

Water movement through cell membranes is facilitated by water channels called aquaporins (AQPs). These membrane proteins belong to the major intrinsic protein (MIP) family and play a central role in plant water relations. There are increasing evidences that AQPs are involved in the regulation of water transport in many physiological processes such as stomatal movement. It is also known that different environment stresses affect AQP expression and activity and as a result the cell membrane water permeability is modified. Under water stress conditions plants key concern is to minimize water loss to maintain plant water balance. It is well know that one of the main mechanisms of plants to avoid water stress is the reduction of stomatal conductance. According to this information the main objective of this study was to elucidate if water stress modify the expression of leaf AQPs in sweet orange seedlings. Two year-old seedlings of Marrs Early sweet orange grafted onto C22 rootstock (Sunki mandarin x Swingle trifoliate orange) were grown in a greenhouse (Oct.–Nov. 2012). Plants were subjected to three different water regimes: control (well watered plants), 75% or 50% of the dose used to water control plants was applied 2–3 times per week. Control plants were kept under field capacity during the growing period. Leaf water potential and stomatal conductance were measured weekly. Six weeks after the beginning of the treatments, when significant differences in leaf water potential and stomatal conductance were observed among treatments, samples of xylem exudates were collected and the abscisic acid (ABA) content was determined. The expression of five citrus AQPs was quantified by reverse transcription-PCR in fully developed leaves. A decrease in water potential and stomatal conductance, and an increase in ABA content in the xylem sap were observed as the water stress treatment was more severe. The expression of AQPs was affected by the water stress treatment: the expression of one of the five AQPs tested was reduced by the most severe water stress treatment (50%) so that it reduced water transport in the leaf, which could explain the lower stomatal conductance and water potential observed in this treatment with respect to the control and to the moderate water stress.