The objective of this work was to evaluate the effects of citrus variegated chlorosis (CVC) on sap flow, gas exchanges and photochemical activity in `Natal' sweet orange plants with and without CVC under field condition. Diurnal courses of sap flow, leaf water potential, CO2 assimilation rate and transpiration, stomatal conductance, potential and effective quantum efficiency of photosystem II were evaluated. The experiment was arranged in a random block design with five repetitions. Healthy plants showed sap flow values around 1.9 times higher than injured ones. Injured plants exhibited reductions of 43, 28 and 33% in CO2 assimilation rate, stomatal conductance and leaf transpiration, respectively. CVC-affected plants showed dynamic photoinhibition. Since healthy and injured plants showed similar diurnal course of effective quantum efficiency, the possible influence of photorespiration on photochemical apparatus in CVC-affected plants is discussed. Reductions in CO2 assimilation rate, transpiration and sap flow of injured plants were induced by low stomatal conductance values, which was probably due to the xylem vessel colonization by Xylella fastidiosa.