The role of oxygen in thermo-photodynamic processes leading to sunscald-like damages in green tissues.

Sunscald is a physiological disorder causing, in warm climates, severe damages to green tissues. Earlier studies established the essential role of heat, light and chlorophyll in the development of the disorder. In the present work, the role of oxygen in the development of the injury was investigated. Green cucumber fruit were exposed to heat and light under controlled atmosphere, and the development of the injury was followed. No damage occurred in the absence of either heat or light. Tissues exposed to heat, light and either nitrogen or air, suffered marked electrolyte leakage, indicative of membrane damage. Chlorophyll fluorescence values of cucumbers treated with high temperatures, oxygen and light, were considerably lower than those measured for the respective dark treated tissues. Irradiated cucumbers flushed with either 100% N2 or less than 2.5% O2, lost about 50% of their chlorophyll. These tissues turned olive-green but did not bleach. At higher conc., oxygen treated tissues lost their plastid pigments and bleached within 7 hr of exposure to heat and light. A significant decrease in chlorophyll was evident also in atmosphere containing 1% CO2 in nitrogen. Malondialdehyde conc was significantly higher in the presence of O2 as compared with N2 atmosphere. The results clearly show, that sunscald damage is a result of thermo-photooxidative processes.