Abstract
All light from the sun contains radiation in the ultraviolet (UV) range, wavelengths between 200 and 400 nm that may be harmful to living organisms. Exposure of plant tissues to UV-B radiation (200-400 nm) may result in the formation of reactive oxygen species (ROS), highly reactive chemical species that are capable of damaging biological macromolecules such as DNA, proteins and the lipids of cellular membranes. To detoxify ROS and prevent damage from occurring, plants maintain a battery of antioxidants and associated enzymes. Polyamines are small, aliphatic amines that are found in plants that also function in the stress response by protecting DNA, stabilizing cellular macromolecules and aiding the dissipation of excess energy in photosystem II (PSII). Antioxidants and polyamines are also important in human metabolism and may play a role in preventing the development of several chronic diseases including cancer, diabetes and cardiovascular disease. As antioxidant compounds and polyamines are accumulated in plants under conditions of UV-B stress, there is scope to increase the nutritional value of plant foods by exposing crops to UV-B. The aim of the current experiments was to investigate the effects of UV-B on the activity of antioxidants and polyamine accumulation in lettuce (Lactuca sativa L.), a leafy vegetable commonly grown in horticultural set-ups that reduce UV-B exposure. The ability of plant material from different UV-B environments to protect human colon cells from oxidative injury was also investigated. Exposure to UV-B increased the activity of the antioxidative enzymes superoxide dismutase, catalase, ascorbate peroxidase, glutathione peroxidase and glutathione reductase as well as increasing the accumulation of the low-molecular weight antioxidants ascorbate and glutathione. Oxidative damage in UV-B exposed plants was reflected in increases in protein carbonyl and lipid hydroperoxide contents, and in increased oxidation of cellular ascorbate and glutathione pools. However, plants acclimatized to UV-B as the experiment progressed with markers of oxidative damage decreasing after one week of exposure. The response of lettuces to UV-B radiation also varied between varieties, the red-leafed cultivar ‘Red Salad Bowl’ having lower levels of oxidative damage and recovering more fully than other cultivars.
Polyamines were also accumulated in response to UV-B radiation, especially free and conjugated forms putrescine and spermidine. Accumulation of spermine however, increased as UV-B exposure progressed and a higher proportion of spermine was accumulated as bound-spermine than was the case for other polyamines. Two pathways for polyamine biosynthesis exist in plants, starting from either ornithine decarboxylase (ODC) or arginine decarboxylase (ADC). ODC activity was not altered by UV-B. ADC activity was up-regulated in UV-B exposed plants, and the localization of this enzyme in chloroplasts suggests a role for polyamines in stabilizing PSII during UV-B stress. The accumulation of antioxidants and polyamines in UV-B exposed lettuces shows there is considerable scope for the nutritional quality of this crop to be improved through horticultural practices that expose lettuces to UV-B, especially if plants are allowed sufficient time for acclimation to UV-B stress before harvest.