| The changes of leaf water potential, relative water content, the content of soluble protein, proline and malondialdehyde, relative permeability of plasma membrance and gas exchange parameters were measured by means of the simulated pot experiments. The abilities of drought resistance of Syringa oblate, Caranana microphylla, Spiraea ussuriensis, Hippophae rhamnoides in the course of seedling were studied by spraying four contents of different plant growth regulators including ascorbic acid, salicylic acid and paclobutrazol. The results showed as foliowed.(1) The three kinds of plant growth regulators could increase the abilities of drought resistance of all the shrub seedlings via different ways. Under the condition of drought, all the plant growth regulators could effectively postpone the fall of leaf water potential, relative water content, the content soluble proteins and chlorophyll and net photosynthesis and the occurs of lipid peroxidation and membrance permeability to some extent. In addition, the content of proline in leaves was increased. (2) The most suited kind and content of the plant growth regulators in enhancing the abilities of drought resistance were different for the four kinds of shrub seedlings. 5.5x10-3 - 8.5x10-3mol/L ascorbic acid and 0.7x104 - 1.0x10-4 mol/L salicylic acid were the most appropriate for Syringa oblate, 8.5x10-3mol/L ascorbic acid and 0.4x10-4 - 0.7x10-4mol/L salicylic acid for Caranana microphylla, 1.53x10-3 - 2.21x10-3mol/L paclobutrazol for Spiraea ussuriensi and 0.4x10-4- 1.0x10-4mol/L salicylic acid for Hippophae rhamnoide. The results could establish a definite base for directional cultivating and afforestation in arid and semiarid areas.
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