To clarify the underlying physiological mechanism of gibberellic acid (GA) in cold tolerance, the effects of exogenous GA 3 on malondialdehyde (MDA), osmoregulation substances and endogenous hormones levels in the tiller nodes of two wheat cultivars, namely, Dongnongdongmai 1 (a cold insensitive cultivar) and Jimai 22 (a cold sensitive cultivar), were investigated at three periods of cold winter (0,-10,-25°C). The results showed that low concentrations of GA 3 (0.1 and 1 µM) decreased the

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... nous GA concentration in both cultivars, but only increased the abscisic acid (ABA)/GA ratio in Dongnongdongmai 1. High concentrations of GA 3 (10 and 100 µM) increased the MDA level, retarded the accumulation of soluble protein and sugar in both cultivars, but decreased the content of ABA and the ratio of ABA/GA only in Dongnongdongmai 1 and had no influence on those in Jimai 22. The re-greening rate of Dongnongdongmai 1 decreased as the concentration of exogenous GA 3 increased. Correlation analyses showed that MDA was negatively correlated with re-greening rate, while soluble protein, sugar ABA content, and ABA/GA ratio were positively correlated with re-greening rate. In conclusion, low exogenous GA 3 level could decrease endogenous GA content and elevate ABA/GA ratio and soluble protein content, which help to improve cold tolerance. However, high exogenous GA 3 level decreased the ABA content and ABA/GA ratio, resulting in lower soluble sugar and protein content and aggravated oxidative damage, and finally weakened cold tolerance. The endogenous GA metabolism and ABA/GA balance play central roles in exogenous GA 3 mediated cold tolerance.