Acclimation of Lactuca sativa to increased UV irradiation at various selenium levels

Abstract

The effects of selenium (Se) and pre-acclimation on photosynthesis in UV-stressed Lactuca sativa were studied by comparing the functional and structural responses of the photosynthesis apparatus in non-acclimated and pre-acclimated leaves monitored during a 40-day period. The photosynthetically active photon flux density (390 m mol m-2 s-1) and other environmental factors in the greenhouse were similar in both treatments. The non-acclimated plants were grown under white light, and the pre-acclimated plants were subjected to UV irradiation for 1 min at 0.5 W m-2 intensity after sprouting. Half of the plants received Se fertilization (0.01mg kg-1 soil). After 9 days the non-acclimated plants were also subjected to UV treatment at an intensity of 1.5 W m-2. The following parameters were measured to monitor the responses: fluorescence, proteins, glutathione peroxidase activity (GSH-Px), carotenoids and ultrastructure. Ultrastructural investigations revealed that the plants grown with Se developed more rapidly than those without added Se. A thicker epidermis began to be formed in response to the short UV irradiation episode in the pre-acclimation treatment. The additional Se further thickened the epidermis. The Fv/Fm parameters revealed that in the beginning of the irradiation treatment the leaves were relatively sensitive to UV light irrespective of whether they were pre-acclimated or not. The repair mechanism of PSII normalized the Fv/Fm values within 5 to 7 days. The intensity and duration of UV irradiation affected the acclimation process. The pre-acclimated leaves had a lower GSH-Px activity than the non-acclimated ones, and they showed a lower GSH-Px activity also after additional UV treatment. However, later on the activity decreased in the leaves irrespective of UV treatment and was low in all 30-day-old plants. The antioxidative capacity seemed to be affected more by the aging of the leaves than by environmental factors. Developmental stage affected acclimation of individual leaves. Added Se had a dual function: it accelerated development but also retarded the ageing process through its antioxidative function. In addition the changes in the carotenoid synthesis might reflect the antioxidative function of Se. If Se was not added, UV irradiation increased of lutein and b -carotene concentrations. Addition of Se seemed to stabilize the concentration of lutein and b -carotene at a constant level that was not affected by irradiation treatment.