Light is essential for sunscald induction in cucumber and pepper fruits, whereas heat conditioning provides protection

Cucumber (Cucumis sativus L.) and pepper (Capsicum annuum L.) fruits were exposed to heat or light, or both. Temperature thresholds of 38-40 and 40.5-42.5°C, respectively, were found to be essential for development of injury in these two species. Light in the visible spectrum was also found to be essential, as no bleaching occurred in the dark. It is suggested that sunscald develops when photosynthesis is disturbed by excessive heat, so that light energy is re-directed into damaging photodynamic processes. When mature-green fruits were exposed to heat and thereafter to a potentiation period at room temperature, no sunscald developed, but the fruits acquired tolerance to sunscald injury. It is suggested that tolerance to sunscald is an instance of heat-conditioning which may be acquired under natural conditions through diurnal changes in temperature.