Spermine Alleviates Drought Stress in White Clover with Different Resistance by Influencing Carbohydrate Metabolism and Dehydrins Synthesis

The objective of this research was to analyse whether ameliorating drought stress through exogenously applied spermine (Spm) was related to carbohydrate metabolism, dehydrins accumulation and the transcription of genes encoding dehydrins in two white clovers (drought-susceptible cv. ‘Ladino’ and drought-resistant cv. ‘Haifa’) under controlled drying conditions for 10 days. The results show that the application of Spm effectively alleviates negative effects caused by drought stress in both cultivars. Exogenous Spm led to accumulation of more water-soluble carbohydrates (WSC), sucrose, fructose and sorbitol in both cultivars under drought stress, and also significantly elevated glucose content in leaves of drought-resistant cv. ‘Haifa’, but had no effect on drought-susceptible cv. ‘Ladino’. Accordingly, the key enzyme activities of sucrose and sorbitol metabolism changed along with the application of Spm under drought stress. Spm induced a significant increase in sucrose phosphate synthase (SPS) or sorbitol dehydrogenase (SDH) activity, but decrease in sucrose synthetase (SS) activity when two cultivars were subjected to drought. In addition, the improved accumulation of dehydrins induced by exogenous Spm coincided with three genes expression which was responsible for dehydrins synthesis. But Spm-induced transcript level of dehydrin genes increased earlier in cv. ‘Ladino’ than that in cv. ‘Haifa’. Thus, these results suggest that ameliorating drought stress through exogenously applied Spm may be associated with increased carbohydrate accumulation and dehydrins synthesis. There are differences between drought-susceptible and -resistant white clover cultivars related to Spm regulation of WSC metabolism and dehydrins expression.