| Wheat (Tritivum aestivum, L) is the second-largest food crop in China and its yield impacts not noly people's living standards but also state economic development directly. During grain filling stage, wheat plants often suffer from heat and high irradiation stress, which causes damage to photosynthetic apparatus and early senescence of leaves. As a result, grain yield declines due to organic nutrition deficiency. In this paper, D1 protein and photosynthetic performance of wheat leaves under heat and high irradiation stress and the regulating effect of salicylic acid (SA) were studied. The aim is to provide experimental fonndation for increasing resistance of wheat photosynthetic apparatus to heat and high irradiation stress.Using wheat variety"Yunong 949"as material, the experiments were conducted on experimental farm of Henan Agricultual University. Wheat seeds were planted in pots, sand plates and field respectively. Foliar sprayings of 0.5mmol·L-1 SA were done the 20th day after anthesis (grain filling stage) in field and pot experiments or the fourth leaf stage in sand culture. Water was sprayed in control. Then the plants were exposed to irradiance of 1600μmol·m-2·s-1 provided by 1000W tungsten lamp and temperature of 35℃for 4 hours. Physiological and biochemical characteristics of flag leaves were determined at different times after treatment. The main results were as follows:1. Content and phosphorylation lecel of D1 protein were increased by application of SAHeat and high irradiation stress led to degradation of D1 proteins and photoinhibition of photosynthesis. Foliar application of SA was effective to inhibit degradation of D1 proteins and alleviate photoinhibition of photosynthesis under stress. The content of D1 protein was increased to levels of control and fast turnover was maintained after dark recovery. It was indicated that PSⅡreactive center was protected by SA.Phosphorylated D1 protein had the same trend with non-phosphorylated D1 protein. Application of SA stabilized D1 proteins of wheat leaves under the stress. When the stress lasted for 2 or 4 hours, phosphorylated D1 protein levels in the SA-treated leaves were higher than that in control after dark recovery, which was beneficial to D1 protein turnover.2. Ability to scavenge active oxygen of wheat leaves was increased by application of SAThough spraying SA didn't change the'increase-and-decrease'trends of APX and SOD activities, their activities in Sa-treated leaves were higher than those in control. Besides, SA decreasded content of H202 in cells. In a word, the foliar application of SA made the leaves maintain higher ability to scavenge active oxygen, alleviate accumulation of free radicals, relieve harmful effects to biological membrane and retard membrane lipid peroxidation. Therefore, plasma membrane permeability became proper and leakage of substances from cells were avoided. Consequently, damage to photosynthesis apparatus was prevented and regular electron transport was maintained.3. Photosynthetic potentiality and efficiency of wheat leaves were improved by application of SAUnder the stress of heat and high irradiation, electron transport rate (ETR) through PSⅡd ecreased sharply because of degradation of proteins in PSⅡreaction center. The foliar spraying of SA brought down the extent of retardation of ETR in the photosynthetic membrane. Moreover, SA had effect on repair of electron transport chains after dark recovery.Pn (net photosynthetic rate) of wheat leaves decreased dramatically along with stress time, and spraying SA alleviated the downtrend of Pn to some extent. The effect was more evident after dark recovery.Spraying SA maintained higher Fv/Fm (the maximal photochemical efficiency of PSII) ,ФPSⅡ(the actual photochemical efficiency of PSII) and qP (the photochemical quenching coefficient) under stress. SA also increased photosynthetic potentiality and efficiency of wheat leaves by protecting photosynthetic appratus from damage to some degree and maintaining stability of PSII reaction center and higher turnover efficiency.4. Contents of osmptic regulation substances were rised by application of SAThe foliar application of SA increased contents of proline and soluble sugars in wheat leaves. SA also promoted the protein synthesis at beginning of stress and inhibited protein degradation at late stage of stress. Therefore, protein contents in wheat were stable owing to the effect of SA.In general, foliar spraying of SA could inhibit degradation and keep higher phosphorylation level of D1 protein effectively under heat and high irradance stress. Accordingly, higher photo-chemical activity, ETR of whole chain and PSⅡand Pn were kept under stress condition. Furthermore, content and phosphorylation level of D1 protein increased rapidly after dark recovery. ETR through PSⅡand PSⅡprimary photochemical efficiency almost reached dark adaptation level. It was shown that stability of PSⅡreaction center and repair of photosynthetic apparatus could be improved by SA. |
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