| Two wheat (Triticum aestivum.L) cultivars with different drought tolerance (drought-tolerant cultivars HF9703 and drought-sensitive cultivars SN215953) were used to study the effects of exogenous glycinebetaine (GB) on the drought tolerance and the underlying mechanisms. Wheat plants were either grown in pots or in the field. 30 or 100 mM GB containing 0.2% Tween 20 was applied to the leaves until runoff at the seedlings stage when the forth leaves were fully expanded and at the anthesis stage, respectively. Drought stress was induced by withholding irrigation after GB treatment, and the leaf water status, the content of proline and soluble sugars, the antioxidant enzyme activities as well as the photosynthetic rate in leaves were examined. We specially studied the protective mechanism of the GB on the photosystem II (PSII) under drought stress. The main results are as follows:1. Effects of foliar application of 30 mM GB on the drought tolerance of wheat seedlings.Foliar application of 30 mM GB could increase endogenous level of GB and ameliorate water status in drought-stressed wheat leaves, increasing leaf water potential (Ψw) and relative water content (RWC), mainly through accelerating the accumulation of proline and soluble sugars and thus improving the osmotic adjustment (OA).Exogenous GB improved activities of anti-oxidant enzyme (SOD and APX). The GB-treated leaves exhibited a lower electrolyte leakage andMDA content, indicating that GB could stabilize the membrane under drought stress and alleviate peroxidation of membrane lipid under drought stress. However, the improved anti-oxidant enzyme activity was not related to the water status as GB-treated plants still had a higher anti-oxidant enzyme activity even it had a similar water status as the non-GB treated seedlings.Exogenous GB could improve the net photosynthetic rate (Pn) and the above ground biomass of drought stressed wheat seedlings.2. Effects of foliar application of 100 mM GB on the drought tolerance of wheat at anthesis stage.Foliar application of 100 mM GB could increase the endogenous level of GB in wheat flag leaves at anthesis stage and could accelerate the accumulation of proline and soluble sugars, thus increasing the OA of wheat flag leaves, as it did in the seedlings. However, GB application had no effect on the leaf water status, which might be ascribed to the increase in evaporation due to the increased stomatal conductance (Gs) by GB application.Foliar application of GB could alleviate both stomatal limitations and non-stomatal limitations of wheat leaves under drought stress by improving Gs and carboxylation efficiency, thus inhibited the decline in Pn caused by drought stress. Exogenous GB could enhance tolerance of PSII to photoinhibition caused by combination of drought stress and a light intensity of 1200 μ mol m-2 s-1, which is the saturating light for the well-watered wheat. Exogenous GB could inhibit the decline of Fv/Fm under photoinhibiton conditions, mainly through two mechanisms: Firstly, GB could enhance turnover of D1 protein. GB-treated leaves exhibited a more rapid recovery of Fv/Fm than non-GB-treated leaves under a weak light intensity, even when they had been imposed to the same photoinhibition, moreover, the former recovered to a higher value eventually than the later. When treated with SM, an inhibitor of the D1protein turnover, the protective effect of GB on PSII in the detached wheat leaves was weakened, suggesting that GB could protect PSII through enhancing the turnover of D1 protein under drought stress. Secondly, GB could improve anti-oxidant enzymes (SOD and APX) activities in wheat flag leave under photoinhibiton conditions, alleviating the oxidative stress and photooxidation. When treated with DDTC, an inhibitor of SOD, the protective role of GB on PSII loss partly, indicating that GB could protect PSII through maintaining anti-oxidant enzymes activities under drought stress. The increased activities of SOD and APX by GB might stimulate the operation...
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