Photosynthesis (A), transpiration (E), stomatal conductance (g s), leaf water potential (Y), leaf relative water content (RWC), internal CO2 concentration (Ci) and water use efficiency (WUE) have been evaluated in ´Valencia´ orange trees grafted on two rootstocks exposed to substrate desiccation. Daily measurements at the laboratory conditions (temperature = 27 ± 1oC, water vapor pressure 1,5 ± 0,3 kPa, and photon flux density = 700 mmol.m-2.s-1) have been made until A reached values near zero. After rehydration, the same variables above have been evaluated for additional four days. A and Y values have not varied within 24% and 15% of substrate water content and for RWC ranging from 90 and 80%. However, there has been a decrease in g s along with the decline on substrate water content and RWC. It is suggested that the stomatal response is directly related to the variation in substrate water content, by root-to-shoot communication. The ratio A/E (WUE) has shown a slight reduction trend as g s declined, showing that under severe stress (Y < -2,7 MPa), A has reduced more than E. The increase of CO2 internal concentration (Ci) in Y below -2,7 MPa has pointed out that, under severe stress, CO2 assimilation has declined as a result of both stomatal closure and reduced photosynthesis activity. After a 12 hour-rewatering period Y had recovered the initial values while A and g s had not yet reached the initial values, which have been obtained, though partially, only after a three day-rewatering period.
Citrus limonia Osbeck; Citrus sinensis Osbeck; water deficit; water use efficiency; Poncirus trifoliata Raf.; gas exchange
