| The experiment was carried out in the Corn Research Garden of Shandong Agriculture University and in the Key Laboratory of Crop Biology in Shandong from 2003 to 2004. In the first year, four maize cultivars were selected to research the effects of low irradiance on grains yield of different maize cultivars in the field shading conditions. Then Nongda108 (ND108) and Yedan13 (YD13) were selected to research the effects of low irradiance (including different shading phases and different shading degrees) on grains yield and quality and its physiological characteristics of the two maize cultivars. In the potted conditions, it was carried out to research the effects of low irradiance (shading) and high temperature stress on grains yield and quality and its physiological characteristics of maize. The main results were as follows: 1 Effects of light and temperature stress on grains yield of maize Low irradiance decreased significantly grains yield of maize. Different shading phases had different effects on grains yield of maize. The effects of shading from flowering to maturity stage on grains yield of maize were most significant, those of shading from joining to flowering stage were the second, those of shading from seedling to joining stage were smaller relatively. Shading from joining to flowering stage decreased grains number mostly, and shading from flowering to maturity stage decreased 1000 kernels weight mostly. Grains yield of maize was decreased significantly as the intensity of illumination decreased. Effects of different shading phases on grains yield of maize were significantly higher than those of different intensities of illumination. Grains yield of maize was significantly decreased by increasing temperature from male tetrad to maturity stage, the effects of high temperature stress on different maize cultivars were different. The effects of shading on grains yield of maize were significant higher than those of high temperature. In shading and increasing temperature experimental conditions grains yield of maize couldn't be gotten because maize couldn't silk and flower normally. 2 Effects of light and temperature stress on grains quality of maize Low irradiance affected significantly grains quality of maize, and different shading phases had different effects on grains quality of maize. Shading from seedling to joining stage had no significant effects on grains quality of maize. While shading from joining to flowering stage, crude protein and lysine contents of maize grains were increased, ether extract and total starch contents were decreased. While shading from flowering to maturity stage, crude protein and lysine contents of maize grains were increased significantly, ether extract and total starch contents were decreased significantly. As the intensity of illumination decreased, the effects of it on grains quality were increased. Effects of different shading phases on grains quality of maize were significantly higher than those of different intensities of illumination. Maize grains quality was influenced significantly by increasing temperature from male tetrad to maturity stage. Crude protein and total starch content of maize grains were increased, ether extract, lysine and free amino content were decreased. There were difference between effects of shading and those of increasing temperature on grains quality. Because maize grains couldn't formed, and dry matter movement and partition were inhibited, crude protein, ether extract, lysine and free amino content in leaves and stalks of maize were all increased in the shading and high temperature experimental conditions. 3 Physiological mechanism of effects of light and temperature stress on grains yield and quality of maize 3.1 Effects of light and temperature stress on growth course and plant properties After shading, growth course of maize was delayed, plant height was reduced,internode length shortened, stem diameter thinned, leaf area index reduced, dry matter accumulation and activity of roots decreased, and these changes were increased as the intensity of illumination decreased. Increasing temperature accelerated the growth course of maize, shortened the grains filling period of maize. After shading, plant height of maize was reduced, leaf area index reduced, dry matter accumulating and activity of roots decreased. In the shading and high temperature experimental conditions, plant height, leaf area index, dry matter accumulating and activity of roots were decreased significantly, and lower than those of shading and increasing temperature significantly. 3.2 Effects of light and temperature stress on photosynthetic characteristics Low irradiance decreased significantly leaf area index, photosynthetic pigments content and photosynthetic rate. The effects of shading from flowering to maturity stage were most significant, and these effects were increased as the intensity of illumiantion decreased. After shading from seedling to joining and from joining to flowering stage were finished, photosynthetic pigments content and photosynthetic rate of maize leaves could renew to the level of control. Increasing temperature inhibited the growth of leaf, leaf area index was reduced significantly, photosynthetic pigments content, especially chlorophyll a content was reduced significantly. Increasing temperature for a short time could increase photosynthetic rate of maize leaves, but it was reduced as increasing temperature period prolonged. In the shading and high temperature experimental conditions, leaf area index, photosynthetic pigments content and photosynthetic rate of maize ear leaf were decreased significantly, and lower than those of shading and increasing temperature significantly. Effects of shading on leaf area index and photosynthetic rate were higher than those of increasing temperature, and effects of increasing temperature on photosynthetic pigments content were higher than those of shading. 3.3 Effects of light and temperature stress on key enzyme activity in carbon metabolism Shading and increasing temperature had different effects on key enzyme activity of leaves and grains in carbon metabolism. While shading in different phases, RuBPCaseand PEPCase activity of maize ear leaf were decreased significantly, and those decreased as the intensity of illumination decreased. After shading from seedling to joining was finished, RuBPCase and PEPCase activity of maize ear leaf could renew to the level of control. After shading from joining to flowering stage were finished, PEPCase activity of maize ear leaf could renew to the level of control, but RuBPCase activity of maize ear leaf couldn't renew to the level of control, that is to say, shading from joining to flowering stage damaged the functions of RuBPCase in maize leaf. Increasing temperature decreased RuBPCase and PEPCase activity of maize ear leaf, the effects on different maize cultivars had differences. Effects of shading on RuBPCase activity were higher than those of increasing temperature, and effects of increasing temperature on PEPCase activity were higher than those of shading. In the shading and high temperature experimental conditions, RuBPCase and PEPCase activity of maize ear leaf were decreased significantly, and lower than those of shading and high temperature significantly. While shading from seedling to joining and from joining to flowering, SS, SPS, ADPGPPase and UDPGPPase activity were all decreased significantly, and these changes were increased as the intensity of illumiantion decreased, after shading from seedling to joining was finished, those could renew to the level of control. Shading from seedling to flowering stage had no effects on SS, SPS, ADPGPPase, UDPGPPase, GBSS and SSS activity of maize grains. While shading from joining to flowering stage, SS and SPS activity of maize grains were not affected significantly, but ADPGPPase, UDPGPPase, GBSS and SSS activity of maize grains were decreased in the anaphase of forming grains, and these changes were increased as the intensity of illumination decreased. Shading from flowering to maturity stage decreased SS, SPS, ADPGPPase, UDPGPPase, GBSS and SSS activity of maize ear leaf and grains significantly, and these changes were increased as the intensity of illumination decreased. While increasing temperature from male terad to maturity stage, SS, SPS, ADPGPPase, UDPGPPase, GBSS and SSS activity of maize ear leaf and grains was decreased firstly, then increased as high temperature stress period prolonged. In the shading and high temperature experimental conditions, SS, SPS, ADPGPPase andUDPGPPase activity were decreased significantly, and lower than those of shading and increasing temeprature significantly. 3.4 Effects of light and temperature stress on key enzyme activity in nitrogen metabolism While shading NR activity of maize leaf was decreased, but it had no significant difference with control, and it was decreased as the intensity of illumination decreased. While shading GS activity of maize ear leaf and grains were decreased significantly, and those were decreased as the intensity of illumination decreased. After shading from seedling to joining and from joining to flowering stage were finished, GS activity of maize ear leaf could renew, but not renew to the level of control. Shading reduced GOGAT activity of maize ear leaf, but it had no significant effects on GOGAT activity of maize grains. Shading before flowering increased GDH activity of maize grains, but shading after flowering inhibited GDH activity of maize grains. While increasing temperature from male terad to maturity stage, GS activity of maize ear leaf was decreased firstly, then increased as high temperature stress period prolonged. Increasing temperature could increase GS activity of maize grains, and increase GOGAT activity of maize ear leaf and grains. Increasing temperature increased GDH activity of maize ear leaf firstly, then decreased as high temperature stress period prolonged, and increased that of maize grains. In the shading and increasing temperature experimental conditions, GS activity of maize ear leaf was decreased significantly, and was lower than that of shading and increasing temperature. GS activity of maize ear leaf has no significant differences with that of shading. GDH activity of maize ear leaf was increased firstly, then decreased, and lower than control significantly. 3.5 Effects of light and temperature stress on active oxygen metabolism and membrane lipid overoxidation in maize After shading, SOD, POD, CAT activity and soluble protein content were decreased, and those were decreased as the intensity of illumination decreased, effects of shading from flowering to maturity stage were the most significant. While shading from seedling to joining stage and from joining to flowering stage, MDA content of...
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