The effects of growth and leaf temperature on photosynthesis were evaluated in sweet orange seedlings (Citrus sinensis cv. Pera) infected with Xylella fastidiosa (the bacterium that causes citrus variegated chlorosis, CVC). Measurements of leaf gas exchange and chlorophyll a fluorescence were taken at leaf temperatures of 25, 30, 35 and 40degreesC in healthy and infected (without visible symptoms) seedlings submitted to two temperature regimes (25/20 or 35/20degreesC, day/night), not simultaneously. The CO2 assimilation rates (A) and stomatal conductance (g(s)) were higher in healthy plants in both temperature regimes. Values for A and g(s) of infected and healthy plants were higher in the 35/20degreesC regime, decreasing with leaf temperature increase. In addition, differences between healthy and infected plants were higher at 35/20degreesC, while no differences in chlorophyll a fluorescence parameters were observed except for potential quantum efficiency of photosystem II, which was higher in infected plants. Low A values in infected plants were caused by low g(s) and probably by biochemical damage to photosynthesis. The high alternative electron sink of infected plants was another effect of reduced A. Both high growth and high leaf temperatures increased differences in A between healthy and infected plants. Therefore this feature may be partially responsible for lower growth and/or productivity of CVC-affected plants in regions with high air temperature.