| Photooxidative damage in fruit occurs under high temperature and solar radiation conditions, and results in sunscald with surface discoloration and bleaching. Using a system that permitted the imposition of photooxidative stress under natural solar radiation, we evaluated symptoms development, photosynthetic efficiency, pigment and flavonoid composition, and antioxidant metabolites and enzymes in exocarp of immature green tomato (Lycopersicon esculentum Mill) fruit. Exposed or covered sections of detached fruit of mutants (anthocyanin absent, beta-carotene, delta, and high pigment-1) with attenuated pigment and/or antioxidant metabolite levels, and their nearly isogenic parents ('Ailsa Craig' or 'Rutgers'), were subjected to five hours of high solar irradiance, either in the presence or absence of ultraviolet (UV) radiation. Photooxidative stress on detached tomato fruit reproduced sunscald symptoms on attached fruit. Both high temperature and solar irradiance caused fruit surface discoloration with faster degradation of chlorophyll (Chl) than carotenoids (Car), leading to an increase in the Car/Chl ratio. Bleaching of the fruit surface was mostly caused by solar irradiance, whereas high temperature was responsible for most inactivation of photosynthesis.; Quercetin and kaempferol concentrations increased rapidly upon exposure to sunlight, but not to natural UV radiation, suggesting rapid photo-protection by these flavonoids in response to visible light. Ascorbate and glutathione concentrations decreased as duration of sun exposure increased, but activities of antioxidant enzymes (SOD, APX, DHAR, MDHAR, GR), as well as MDHAR and GR protein levels, increased to maintain the ascorbate and glutathione pools in its their reduced forms. These responses prevented the accumulation of lipid peroxidation products and possibly protein oxidation and DNA damage, suggesting that damage to these macromolecules might not be detectable until cellular antioxidant systems are unable to cope with increasing ROS flux during photooxidative stress.; Fruit exocarp from high pigment-1 had higher Chl and Car levels than the other genotypes, and more kaempferol, quercetin, and naringenin than 'Ailsa Craig'. Nevertheless, but it suffered photoinhibition to the same extend as its parent. This led to the conclusion that the higher ascorbate pool, APX and GR activities, and flavonoid concentrations in high pigment-1 fruit exocarp allowed it to better cope with the imposed photooxidative stress. |
