AbstractMaximum rates of net photosynthesis forCitrus sinensis(L.) Osbeck andCitrus paradisiMacf. cultivars were observed at the lowest temperature (22°C) under which these subtropical species were examined. Net photosynthesis decreased with higher temperatures or greater vapor pressure differences between leaf and air. The reduction in net photosynthesis due to higher temperatures was associated with reductions in mesophyll conductance to CO2; whereas, the effects of greater vapor pressure differences on net photosynthesis were associated with changes in total leaf conductance to water vapor. Actual water‐use efficiency was reduced at higher temperatures and greater vapor pressure differences, but intrinsic water‐use efficiency, calculated for a constant vapor pressure difference, increased with increases in vapor pressure difference indicating a possible adaptive mechanism for conditions of high evaporative demand. Day‐today increases in net photosynthesis, total leaf conductance to water vapor and mesophyll conductance were observed as plants were subjected to the same temperature and humidity treatments on progressive days. Growing plants under higher irradiances increased their photosynthetic rates and mesophyll conductance to CO2but leaf conductance to water vapor was unaffected by irradiance pretreatment. Consequently, acclimation to radiation can only account for a small part of the day‐to‐day changes in gas exchange. Other possible explanations for these day‐to‐day changes were discussed. Significant differences were not observed among trees of ‘Campbell Valencia’ orange, ‘Parent Navel’ orange and ‘Frost Marsh’ grapefruit in responses of net photosynthesis and leaf conductance t