Effects Of Exogenous Ca²⁺ On Turnover Of Chloroplast D1 Protein And The Photosynthesis In Wheat Under High Temperature And Strong Light Stress

Wheat is the most important crop in northern China and its abundance has a direct impact on people's living standards and economic development. However, in grain-filling stage, wheat often suffered from heat and high light stress, leading to decrease of grain yield because of destruction of photosynthetic apparatus and deficiency of assimilates. Therefore, the study about effect of heat and high-light on the photosynthetic apparatus of wheat and the regulation has an important theoretical and practical significance. Studies have shown that Ca²⁺ is not only a middle element in plants but also a second messenger that coupled extracellular signal and intracellular physiological and biochemical response. In this paper, we studied effects of Ca²⁺ on antioxidant metabolism, osmotic adjustment, membrane stability, chlorophyll fluorescence, electron transport rate and D₁ protein of wheat leaves under heat and high-light stress. The aim is to clarify its role in protection of the photosynthetic apparatus and provide theoretical basis for increasing resistance of wheat leaves to heat and high-light stress. The main results are as follows:1 Exogenous Ca²⁺ increased the active oxygen scavenging capacity of wheat leaves under high temperature and strong light stress.SOD activity is related to concentration of its substrate. At the beginning of stress, SOD activity increased due to rising of O₂^- content, but SOD activity decline with the stress time and extent. APX had similar trend with SOD activity. Although spraying Ca²⁺ did not change dynamics of SOD and APX activity, SOD and APX activity in Ca²⁺-treated leaves was always higher than that in water-treated leaves. At the same time, Ca²⁺ inhibited the decrease of CAT activity.Therefore, Ca²⁺ reduced membrane lipid peroxide , permeability of protoplast membranes and prevented ion leakage from cell under heat and strong light stress.2 Exogenous Ca²⁺ increased the ability of osmotic adjustment in wheat leaves under heat and strong light stress .Spraying Ca²⁺ on wheat leaves increased the accumulation of free proline and soluble sugar under heat and high light stress. At the subsequent recovery stage, Ca²⁺ can effectively inhibit the reduction of proline content. Soluble sugar content also increased with the stress time. From the total soluble protein gel electrophoresis patterns, it was seen that 90KD protein band disappeared when stress lasted to 3h and contents of some small molecular weight proteins decreased significantly. However, soluble protein content of Ca²⁺ treatment was much more stable.3 Exogenous Ca²⁺ increased the potential and the efficiency of photosynthesis in wheat leaves under heat and strong light stress.The results showed that heat and high irradiance stress led to disruption of photosynthetic electron transport, decrease of PSâ…¡photochemical efficiency, inactivation of PSâ…¡reaction center and reduction of net photosynthetic rate. Spraying 10 mmol·L^-1 of CaCL₂ on wheat leaves before the heat and high irradiance stress could effectively inhibit the loss of the D₁ protein , maintain a higher rate of the the whole electron transfer rate, PSâ… , PSâ…¡electron transfer rate, Pn, Fv / Fm,ФPSâ…¡, qP and lower Fo. After 3h of recovery in dark, the electron transfer rate and chlorophyll fluorescence of Ca²⁺ treatment recovered, but those in water-treated leaves did not. It was indicated that inactivation was reversible in Ca²⁺ treatment and irreversible in water treatment. Briefly, exogenous Ca²⁺ increased the potential and the efficiency of photosynthesis in wheat leaves under high temperature and strong light stress .4 Exogenous Ca²⁺ inhibited degradation of D₁ protein and increased phoshporylated protein (D₁*) in wheat leaves under heat and high light stress.D₁ protein content decreased significantly by introduction of D₁ protein inhibitor before high temperature and light stress. After 3h of stress, D₁ protein content was less than 50% of control. D₁ protein content in water treatment also decreased after 3h stress, but its content is still significantly higher than that in inhibitor-treatment. D₁ protein content of water treatment after subsequent recovery had not returned to pre-stress level. It was indicated that de novo synthesis of D₁ protein may be a longer-term process after its destruction. In Ca²⁺ -treated leaves, D₁ protein content did not significantly decrease during stress and returned to pre-stress level after subsequent recovery. The trend of phosphorylated protein (D₁*) is similer to D₁ protein.Generally, on the one hand, exogenous Ca²⁺ could increase proline and soluble sugar contents of wheat leaves, enhance stability of membrane lipids and reduce electrolyte leakage under heat and strong light stress. On the other hand, exogenous Ca²⁺ could increase SOD, APX and CAT activities to strengthen its ROS scavenging capacity. Therefore, higher rate of whole chain electron transport, PSâ… and PSâ…¡electron transport rate, Pn, Fv / Fm,ФPSâ…¡, qP and lower Fo were maintained by application of Ca²⁺.