Studies On Physiological Ecology Of Foxtail Millet (Setaria Italica (L.) Beauv.) Under Enhanced UV-B Radiation And Different Nitrogen Conditions

Enhanced UV-B radiation resulting from ozone depletion in stratosphere with increase of human's activities can harm plants greatly. Not only photosynthetic organization and membrane system in plants cells can be hurt but also both contents and metabolism of protective matter and system of protective enzymes can be changed by it, evidently. In addition, enhanced UV-B radiation also affects important metabolism of some elements such as carbon and nitrogen and then influence growth and development of plants directly or indirectly. As a necessary element and one of restrictive factor, nitrogen status can affect growth, development and resistance of plants, importantly. Therefore, UV-B sensitivity of plants can be changed with change of nitrogen status in environment.It was considered that enhanced UV-B sensitivity of plants in reproduction period was higher than them in vegetative period. Anthesis and pollination of plants can be affected by not only enhanced UV-B radiation but also physiological status of leaves in this period. Reasonable nitrogen supply will help plants to raise its ability against UV-B radiation and keep its normal metabolism.In this paper, effects of simulating enhanced UV-B radiation on some physiological indices such as the contents of photosynthetic pigments and flavonoids, the activities of phenylalanine ammonia-lyase (PAL) and protective enzymes in leaves, the activities of key enzymes related to nitrogen metabolism in leaves and roots in foxtail millet (Setaria italica (L.) Beauv.) which is a kind of C₄ crop grown under different nitrogen conditions were studied in order to pursue how levels of nitrogen supply was selected reasonably to help plants of foxtail millet to resist UV-B radiation and keep their normal growth and development in the reproduction period when them were stressed by enhanced UV-B radiation. The main results were as follows:1) In the florescence period and the grouting period when plants of foxtail millet grown under two different nitrate nitrogen conditions (1.875 mmol·L^-1å’Œ15 mmol·L^-1) were stressed by two different intensities of enhanced UV-B (4.29 kJ·m^-2·d^-1 and 7.12 kJ·m^-2·d^-1) , whether they had been treated by enhanced UV-B radiation or not, the content of chlorophyll in leaves of foxtail millet grown under higher nitrogen condition (15 mmol·L^-1) was increased significantly (p<0.05) compared to that grown under lower nitrogen condition(1.875 mmol·L^-1); The content of carotenoid and the carotenoid / chlorophyll ratio in leaves of the former was increased and decreased respectively compared to the latter, significantly in most of these periods. In this period, no more evident effects of enhanced UV-B radiation on the contents of photosynthetic pigments in leaves of foxtail millet grown under lower nitrogen condition was observed in this experiment, while the contents of photosynthetic pigments in leaves of foxtail millet grown under higher nitrogen condition were more sensitive to enhanced UV-B radiation only in the mid-term of the florescence period and the mid-term of the grouting period but the former had no significant correlation with intensities of the latter At the same time, in the florescence period and the grouting period, whether they had been treated by enhanced UV-B radiation or not, PAL activity in leaves of foxtail millet grown under lower nitrogen condition was higher than that grown under higher nitrogen condition, significantly in most of these periods. Moreover, the content of flavonoids in leaves of the former was increased compared to the latter significantly in the grouting periods. Meanwhile, PAL activity in leaves of foxtail millet which was treated by enhanced UV-B radiation grown under lower nitrogen condition was increased significantly in the mid-term of the florescence period and in the mid-term and the end of the grouting period and its PAL activity was significant correlated to intensities of enhanced UV-B radiation in the mid-term of the florescence period and in the mid-term of the grouting period; in the beginning and the mid-term of the grouting period its content of flavonoids was also increased significantly by enhanced UV-B radiation and its content of flavonoids was significant correlated to intensities of enhanced UV-B radiation in the beginning and the mid-term of the grouting period. Furthermore, in whole treatment period, PAL activity in leaves of foxtail millet grown under higher nitrogen condition was always increased significantly by enhanced UV-B radiation and the former was significant correlated to intensities of the latter in the grouting period; while the contents of flavonoids in leaves of that was increased significantly by enhanced UV-B radiation in the end of the grouting period and it was significant correlated to intensities of enhanced UV-B radiation in the end of the grouting period. These results showed that lower level of nitrogen supply is beneficial to accumulating of flavonoids and raising ratio of carotenoid / chlorophyll in leaves of foxtail millet, and then it reduced enhanced UV-B radiation sensibility and enhanced ability against UV-B radiation of foxtail millet in the florescence period and the grouting period.2)In conditions of the above, whether foxtail millet had been treated by enhanced UV-B radiation or not during whole treatment period, superoxide dismutase (SOD) activity in leaves always kept ascending and SOD activity in leaves of that grown under lower nitrogen condition was higher than that grown under higher nitrogen condition, significantly in almost whole treatment period; while catalase (CAT) activity in leaves of foxtail millet kept dynamically changing with the addition of treatment time and CAT activity in leaves of that grown under lower nitrogen condition was lower than that grown under higher nitrogen condition, significantly in part of the whole period. In addition, effects of enhanced UV-B radiation on SOD and CAT activity of foxtail millet were significantly in whole treatment period and in the mid-term and the end of florescence period, respectively. Then, Both enhanced UV-B radiation and levels of nitrogen supply affected peroxidase (POD) activity in leaves of foxtail millet more evidently, but its changes was irregularly for which was grown under different nitrogen condition and treated by different UV-B radiation. As to ascobate peroxidase(ASP) activity in leaves of foxtail millet, it could not be affected by nitrogen condition. Although it could be affected by enhanced UV-B radiation significantly in part of the whole period, its changes was irregularly yet. Electrolyte leakage of cell membrane in leaves of foxtail millet was not affected by nitrogen condition in whole treatment period and was affected by enhanced UV-B radiation only in the beginning and the mid- term of the grouting period more obviously. Analysis of correlation showed that there were no regular correlation between intensities of enhanced UV-B radiation and activities of protective enzymes and electrolyte leakage of cell membrane in leaves of foxtail millet. These results indicated that there was a close relationship between electrolyte leakage of cell membrane in leaves of foxtail millet which could keep stability in the florescence period and the grouting period and rising of SOD activity continuously and dynamic stability of CAT activity in leaves of that in the same period. Consequently, Adjustment of activities of the two enzymes by suitably decreasing level of nitrogen supply could raise resistance against UV-B radiation of foxtail millet.3) From the heading period to the grouting period, plants of foxtail millet grown under two different nitrogen conditions (the same above) spent 40 days on being treated by enhanced UV-B radiation (7.12 kJ·m^-2·d^-1) and some indices related to nitrogen metabolism in leaves and roots of them were determined. Before the treatment of enhance UV-B radiation started, with regard to foxtail millet grown under higher nitrogen condition, its contents of nitrate nitrogen and soluble protein in leaves, its contents of nitrate and ammonia nitrogen in roots, its activities of nitrate reductase (NR), glutamate dehydrogenase (GDH) and glutamate synthase (GOGAT) in leaves were increased significantly compared to that grown under lower nitrogen condition. After 40 days of the treatment, whether they had been treated by enhanced UV-B radiation or not, as to foxtail millet grown under higher nitrogen condition, its content of nitrate nitrogen in roots, its contents of ammonia nitrogen and soluble protein in leaves, its activities of GDH and glutamine synthase (GS) in leaves were higher than that grown under lower nitrogen condition significantly. And then, when foxtail millet grown under higher or lower nitrogen condition was accepted by treatment of enhanced UV-B radiation, activity of GPT in leaves of that grown under higher nitrogen condition was increased significantly compared to that grown under lower nitrogen condition. At the same time, enhanced UV-B radiation significantly increased content of ammonia nitrogen in leaves of that grown under lower nitrogen condition and activity of GS in leaves of that grown under higher nitrogen condition. Furthermore, activity of GDH in leaves of foxtail millet was decreased by enhanced UV-B significantly. After estimating effects of enhanced UV-B radiation and different nitrogen condition on nitrogen metabolism in leaves and roots of foxtail millet by respective means of physiological indices related to nitrogen metabolism before and after treatment of enhanced UV-B radiation , It might be found that they were more easily affected by nitrogen but only activity of GDH in leaves of foxtail millet grown under lower nitrogen condition and activity of GS in leaves of foxtail millet grown under higher nitrogen condition were significantly affected by enhanced UV-B radiation. The results suggested that raising level of nitrogen supply was benefit to its vegetative and reproductive growth of foxtail millet when it grew under enhanced UV-B radiation from the heading period to the grouting period, if only nitrogen metabolism of it could be considered.In all, there is some difference between reproductive growth and increase of resistance of foxtail millet grown under enhanced UV-B radiation in its reproduction period. Then it is necessary to find a suitable level of nitrogen supply in order to balance their different requirement in nitrogen.