Water-deficit tolerance in citrus is mediated by the down regulation of PIP gene expression in the roots

Water deficit (WD) is a growing problem in agriculture. In citrus crops, genetically-determined rootstock characteristics are important factors influencing plant responses to WD. Aquaporins are involved in regulating the water supply to the plant by mediating water flow through the cell membranes. Recent studies support a direct role for aquaporins in plant water relations and demonstrate their involvement in WD tolerance. This study investigates the relationship between photosynthetic and water-balance parameters with aquaporin expression levels and hydraulic conductance of roots (Kr) in conditions of moderate WD in citrus rootstocks. The plant materials used were the rootstocks Poncirus trifoliata (L.) Raf. (PT), Cleopatra mandarin (Citrus reshni Hort ex Tan.) (CM) and 030115 (a hybrid of the two former rootstocks), all grafted with the citrus variety 'Valencia Late' (C. sinensis (L.) Osb). Plants were irrigated with two differents irrigation doses (normal irrigation and moderate WD) during 70 days and leaf water potential (ψs), net CO2 assimilation (ACO2), transpiration, stomatal conductance (gs) and substomatal CO2 concentration (Ci) were measured periodically under both irrigation conditions. Kr and PIP1 and PIP2 gene expression levels in fine roots of control plants and plants subjected to WD on day 43 of the experiment were determined. Under WD conditions, the hybrid 030115 drastically reduced aquaporin expression and Kr, accompanied by a loss of plant vigour but without reducing the net CO2 assimilation (ACO2). PT maintained the same aquaporin expression level and similar Kr under WD as under normal irrigation conditions, but suffered a sharp reduction in ACO2. CM, which has lower Kr and aquaporin expression than PT under both normal irrigation conditions and WD, responded better to water stress conditions than PT. Low aquaporin levels, or down-regulated aquaporin expression, accompanied by decreased plant vigour led to decreased plasma membrane permeability, thereby facilitating water retention in the cells under water stress conditions. This may induce water stress tolerance in citrus rootstocks.