| In this study, apple seedlings of a year-old Red Fuji were used as experimental material, the effects of exogenous Chitosan(CTS), Salicylic acid(SA) and Polyamines (Spermine, Spm) on growth and drought resistance of apple seedlings were studied by the pot-culture of water control. Changes of seedlings growth, photosynthetic rate, content of osmotic adjustment substances, activities of protective enzymes, relative permeability of plasma membrane and content of Malondialdehyde (MDA) were determined respectively under moderate water stress in order to ascertain the optimum concentration of CTS, SA and Spm on enhancing drought resistance of apple seedlings, compare the effects of the three exogenous matters on enhancing drought of apple seedlings, and discuss the mechanism of CTS, SA and Spm to enhance the drought resistance of apple seedlings. The results were as follows:1. Under water stress, treatmeat with exogenous CTS, SA and Spm on apple seedlings could maintain normal metabolism, promote plant growth and enhance drought resistace of the apple seedings. Under moderate drought conditions, content of chlorophyll and photosynthetic rate of apple seedlings were decreased, the growth of plant height and increase of stem-diameter were significantly inhibited. Comparing with the control of normal water,net increase of stem height and stem diameter were decreased respectively about 26.68% and 27.59%. Exogenous CTS, SA and Spm were able to increase content of chlorophyll, maintain photosynthetic rate and promote growth rate of seedlings, its effects were close to or above the normal.2. Exogenous CTS, SA and Spm could maintain or increase the activities of protective enzymes(SOD,CAT), decrease content of MDA and relative permeability of plasma membrane, prevent or decrease peroxidative degree of plasma membrane.At the same time , accumulations of organic osmotic substances that soluble sugar and free praline were increased, osmotic adjustment ability was enhanced, normal metabolism of apple seedlings was maintained, and drought resistance was increased.3. The effects of CTS, SA and Spm on growth and drought resistance of apple seedlings have relation to the concentration, the effect would enhance when the concentration of CTS increase within a certain rang, the best concentratin was 100mg/L. If the concentration continued to increase it's effects would not enhance. And the effects were increased firstly and then decreased with the SA and Spm concentration from low to high, the best concentration of SA and Spm respectively were 0.1mmol/L and 10μmol/L, but the effect of SA on decreasing relative permeability of plasma membrane and membrane lipid peroxidation of apple seedlings was not significant. The results showed that the optimum concentration of CTS, SA and Spm could enhance growth and drought resistance of apple seedlings under water stress, but there was no significant when treated with the low concentration SA and Spm on apple seedlings, and the high concentration had inhibition, when the concentration of CTS was up to a value, it's effects would not enhance.4. Under water stress, there were some differences in the effects of the optimum concentration of CTS, SA and Spm on enhancing growth and drought resistance of apple seedlings, but no significant. The effects of CTS and SA on drought resistance were better before rewatering of water stress, but the better effect of Spm was in the after rewatering of water stress and then stress again.
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