Induction of sunscald damage in tomatoes under natural and controlled conditions

Typical sunscald injury was induced in mature-green tomato fruit under controlled conditions. During the initial phase, the induction period, exposure to temperatures of 40°C or higher was necessary. Symptoms of sunscald became visible during the subsequent incubation period at room temperature. A relatively short induction period at high temperatures, e.g. 18 h at 45.1°C, was equivalent to more prolonged induction at more moderate temperatures, e.g. 28 h at 40.8°C. In addition to heat, visible light was necessary for the development of typical sunscald symptoms. The light treatment was most effective when applied both during the induction and the subsequent incubation period, resulting in 100% of fruit with sunscald. When illumination was restricted to the induction period, 13% of the fruit escaped injury, as compared to 66% of healthy fruits when light was applied during incubation only. It was concluded that sunscald is caused by the concurrent action of two external factors, heat and light, suggesting a mechanism of diversion of energy from the photosynthetic pathway into abnormal photodynamic reactions.