Impacts Of Silicon On Photosynthesis And Membrane Fatty Acid Composition And Membrane Function In Wheat Seedlings Under Cold Stress

Two winter wheat (ritium aestivum L.) cultivars, Yangmai No.158 (cold-susceptible) and Linmai No.2 (cold-resistant) were grown hydroponically to investigate the effects of exogenous silicon (Si) on photosynthetic and physiological parameters, RuBPCase and PEPCase activity in leaves, H~+-ATPase and H~+-PPase activity, fatty acid composition and fluidity of tonoplast and plasma membrane in leaves and roots of cold-stressed wheat seedlings. The main results were presented as follows:Exogenous Si significantly increased the net photosynthetic rate (Pn), stomatal conductance (Gs) and water utility efficiency (WUE) under cold stress. The RuBPCase activity significantly decreased in the seedlings of both cultivars treated with cold stress, especially in the cold-susceptible cultivar. The RuBPCase activity of the cold-susceptible cultivar subjected to cold-stress was higher in the treatment with Si than in the treatment without Si, but did not significantly differ from that of control treatment (CK, without cold stress). The effect of Si on mitigating cold stress was better in the cold-susceptible cultivar than in the cold-resistant cultivar. The Si-enhancement of the net photosynthetic rate (Pn) in wheat was closely related to Si-enhanced RuBPCase activity in wheat leaves. However, the PEPCase activity was raised under the short-term cold stress, which was significantly inhibited by exogenous Si in the cold-susceptible cultivar. Further investigation is needed to understand mechanisms for stimulating PEPCase activity under cold stress.The activities of tonoplast H~+-ATPase,H~+-PPase and plasma membrane H~+-ATPase in leaves of cold-susceptible cultivar decreased under cold stress, while in the cold-resistant cultivar, the tonoplast H~+-ATPase,H~+-PPase activities were up to a peak on the 1st day of the cold treatments and then decreased, and the plasma membrane H~+-ATPase activities increased. In roots, the activities of H~+-ATPase and H~+-PPase in tonoplast increased in both cultivars. However, the H~+-ATPase activities in plasma membrane decreased with the degree of decrease being greater in the cold-resistant cultivar than in the cold-susceptible cultivar.The activities of tonoplast H~+-ATPase,H~+-PPase and plasma membrane H~+-ATPase in leaves were enhanced by exogenous Si in both wheat cultivars under cold stress, especially in the cold-resistant cultivar. The stability of tonoplast H~+-ATPase and activities of plasma membrane H~+-ATPase in roots of both cultivars were increased by addition of Si. The addition of Si significantly increased the stability of root tonoplast H~+-ATPase in the cold-susceptible wheat, but did not in the cold-resistant cultivar compared with the non-silicon-treated plants under cold stress. And the effect of Si on the enzymes was better in the cold-susceptible cultivar than in the cold-resistant cultivar.Under cold stress, the content (mol%) of unsaturated fatty acids in leaf and root tonoplast of cold-susceptible cultivar decreased, while the content (mol%) of saturated fatty acids increased, and the ratio of unsaturated to saturated fatty acids (U/S) and the index of unsaturated fatty acids (IUFA) increased too. In the cold-resistant cultivar, however, the U/S and the IUFA of root tonoplast significantly increased, but no difference was found in the leaf tonoplast between the cold treatment and the control treatment (CK). There was no significant difference in the U/S and the IUFA of root and leaf plasma membrane in both cultivars between the cold treatment and the control treatment.Exogenous Si increased the content (mol%) of saturated fatty acids in leaf tonoplast of cold-susceptible cultivar under cold stress, while the content of unsaturated fatty acids decreased. In the cold-resistant cultivar, moreover, there was no difference in the ratio of U/S and IUFA between Si-amended and non-Si-amended cold treatments. For the root tonoplast, exogenous Si decreased the ratio of U/S and IUFA in the cold-susceptible cultivar, but increased the ratio of U/S and IUFA in the cold-resistant cultivar. No significant difference was observed in the ratios of U/S and IUFA in root tonoplast of the two cultivars used between non-Si-amended and the Si-amended cold treatments. And the Si effect on root tonoplast was better in the cold-resistant cultivar than in the cold-susceptible cultivar.In the cold-susceptible wheat cultivar under cold stress, exogenous Si increased the ratio of U/S in leaf plasma membrane and the content (mol%) of saturated fatty acids in root plasma membrane, but decreased the content (mol%) of unsaturated fatty acid and the ratio of U/S and IUFA. For the cold-resistant cultivar, the content (mol%) of all categories of fatty acids remained unchanged, and the ratio of U/S and IUFA had no significant difference in the leaf plasma membrane compared with CK. By contrast, the content (mol%) of saturated fatty acids decreased, and the content (mol%) of unsaturated fatty acids as well as the ratio of U/S and IUFA increased in the root plasma membrane. However, the ratio of U/S and IUFA in root plasma membrane was significantly lower in the Si-amended cold treatment than in the non-Si-amended treatment in the cold-resistant cultivar, but was not in the cold-susceptible cultivar.Exogenous Si decreased fluorescence intensity of leaf tonoplast and plasma membrane in the two cultivars tested under cold stress, namely, the fluidity increased. By contrast, there was little effect of Si on root tonoplast and plasma membrane fluidity. There were no significant differences in the membrane fluidity of tonoplast and plasma membrane in leaves and roots of the two cultivars used between the non-Si-amended and Si-amended cold treatments.