Abstract
In this study, the physiological response of potted apple trees to combined drought and heat stress was evaluated. After establishing different levels of soil water availability, the trees were exposed to a five-day simulated heatwave with daily maximum temperatures of 40°C. Stem water potential, leaf gas exchange, chlorophyll fluorescence, and tree transpiration were monitored before, during and after the combined application of heat and water stress, therefore providing insights into the extent and rapidity of the recovery. Drought caused stomatal closure that limited net photosynthesis and transpiration both at leaf and at tree level, leading to structural damage through leaf loss. On drought-stressed plants, chlorophyll fluorescence was significantly reduced by heat stress, suggesting additional leaf damage although net photosynthesis was not lower than under drought stress alone. On the other hand, well-watered trees showed low midday stem water potentials and high transpiration rates during the heatwave, while net photosynthesis was not affected. Water use efficiency of well-watered trees at 33°C was reduced to 60 % of that at 23°C. After the heatwave, transpiration rate in well-watered trees immediately declined to pre-stress levels, underscoring the strong atmospheric control on transpiration in apple trees. In drought-stressed trees, predawn stem water potential reached pre-stress values already on the first day of recovery. Stomatal conductance, net photosynthesis, and chlorophyll fluorescence, however, required a longer period to recover, indicating that drought stress induced transient hydraulic limitations. Nevertheless, all parameters fully recovered within five days after the end of the heatwave, showing that apple trees can withstand periods of combined heat and drought stress. The key role of water in modulating the response to heat stress highlights the need for improved irrigation management in apple orchards under climate change.