Different rootstocks may influence orange tree tolerance to water stress. Under controlled conditions and water deficit, CO2 assimilation rate (A), transpiration rate (E), stomatal conductance (gs), water use efficiency (WUE), leaf water potential (Yl) and relative water content (RWC), were analysed, in potted 2 years old 'Valencia' orange trees grafted on 'Rangpur' lime and Poncirus trifoliata (Trifoliata), rootstocks. There was no decrease in A, E, WUE, Yl and RWC when the substrate water potential (Ysb) was above -0.04 MPa. However, when Ysb was bellow -0,05MPa, plants grafted on both rootstocks showed different responses. In plants 'Rangpur' A, WUE and Yl started lowering one day before plants on Trifoliata and delayed two days to recover. When Ysb =-0.13MPa, A reached 4.1 mmol/m2.s and 1.8 mmol/m2.s to Trifoliata and 'Rangpur' treatments, respectively. The minimum values of A when Ysb = 0.23MPa were 1.0 and 2.0 mmol/m2.s in 'Rangpur' and Trifoliata, respectively. Maintenance of the highest values of A, gs, WUE and Yl suggested that 'Valencia' orange tree grafted on potted Trifoliata showed a higher tolerance to water stress than that grafted on 'Rangpur'.
citrus; Citrus sinensis Osbeck; photosynthesis; transpiration
