| Two cassava varieties of South China 205 and South China 8 were used in this experiment under potted condition. and at the phase of five to six fully expanded leaf, seven solution treatments of water,40,60,100 mg/L Salicylic Acid (SA) and 0.1,0.2,0.5 mg/L Brassinolide (BR)were applied to the foliage of cassava seedlings, then the leaf samples were collected to assay under 5 different conditions which were 21.5%-23.0%,16.0%-17.5%, 11.5%-13.0%,8.5%-10.0% and 21.5%-23.0% soil water content respectively. Determination of leaf photosynthesis, then analyzed the activities of Superoxide Dismutase (SOD) and Peroxidase (POD), relative electronic conductivity, the content of malondialdehyde (MDA), proline (Pro), and soluble protein (Prot) of seedling leaf from 2 cassava varieties, measured the height and strain path. The Purpose of the study was to investigate the effects SA and BR at drought resistance in cassava under water stress. The main results were as follows:(1) Different Concentrations of SA and BR treatment had significant influence on cell membrane permeability in cassava leaf. For example, the cell membrane permeability in South China 205 leaf which treated with 100 mg/L SA decreased by 26.7% with heavy water stress (soil water content 8.5%-10.0%). With moderate water stress, the cell membrane permeability in leaf of South China 8 with 100 mg/L SA treated and 0.2 mg/L BR treated decreased by 29.9% and 25.1%, respectively.(2) Different Concentrations of SA and BR treatment had increased the content of Prot. Concentrations between the treatments were also significant.100 mg/L SA treatments had gotten the best effect. At the same time, SA and BR treatment promoted accumulating in Pro which has osmotic and adjustive ability. In addition, the content of Pro in 2 cassava leaf with 100 mg/L SA treated and 0.2 mg/L BR treated were also higher than other treatment with drought stress.(3) Different Concentrations of SA and BR treatment induced the increase the further increase of SOD and POD activity, alleviated the increase of leaf MDA content under drought stress.The optimal concentration in South China 205 was 100 mg/L SA and 0.2 mg/L BR, and compared with drought stress control on heavy water stress they obviously reduced MDA content by 24.5% and 21.3%, remarkably increased SOD activity by 35.4% and 33.4%, POD activity by 31.1%and 24.3%, And optimal concentrations in South China 8 were 60 mg/L,100 mg/L SA and 0.1 mg/L BR. (4) Different Concentrations of SA and BR treatment improved leaf photosynthesis in cassava seedlings under drought stress. However exogenous SA and BR alleviated the decrease of leaf net photosynthetic rate (Pn), induced the further decline of leaf light compensation point (LCP), light saturation point (LSP) under drought stress, thus maintained higher photosynthetic metabolism level to improve plant resistance to drought stress, the optimal concentrations for 0.2 mg/L BR treatment had gotten the best effect of two cultivars, increased Pn by 65.4% and 55.0%. The SA and BR all treatments of South China 205, increased LCP and LSP by 9.2%-39.4% and 2.9%-36.3%; and the SA and BR all treatments of South China 8, increased LCP and LSP by 10.4%-38.4% and 2.8%-41.1%.(5) Different Concentrations of SA and BR treatment improved the growth of seedlings. 100 mg/L SA and 0.2 mg/L BR treatment were the best to the seedlings.(6) The above results showed that, SA and BR may increase the stability of cell membrane and protective enzyme activities in leaves, enhance the Osmotic Adjustment ability of cassava seedlings under water stress, increase the net photosynthetic rate (Pn) and improve the leaf photosynthesis, which were the key factors of alleviating the growth inhibition of cassava seedlings under water stress. |
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