| For the sake of illustrating its adaptation mechanism to arid conditions and the suitable water-providing range, and providing a basis for establishing the key technique for water supplying practically, the study on Populus×euramericana.cv."74/76" was conducted to set up 5 pieces of soil moisture gradient with pot tests. Then the photosynthesis characteristics, the water consumption regularity, the osmotic adjustment substances, the protective enzyme activity,the mineral nutrition content and the growth and photosynthesis Products allocation under different water conditions were analyzed and compared with each other. The main conclusions were as following:1 .The basal diameter and Height Growth got faster,growth time prolonged,the width,length and area of leaves increased as the Soil Water Content (SWC) increasing,The growth indexes were remarkable at the end of controlling water. Besides, the accumulation and allocation of the photosynthesis Products were affected remarkably by the water moisture. The rate of root to shoot reduced, specific root proportion increaded,but stem proportion stabilized on some level when SWC decreased2. The range of SWC keeping the 90% upwards of maximum of net photosynthesis rate was 11. 4% — 16. 6%, and the Optimum SWC was 14.2%.The indexes of light compensator point ,Photorespiration velocity and carboxyl efficiency reduced with SWC decreased. The diurnal course of photosynthesis rate under different soil water content showed a double-peak curve on August, the peak value advanced with SWC decreased. On limitative factors of net photosynthesis rate, When SWC > 11.4%,soil water content is not a primary limitative factor, stomatic conductivity was primary limitative factor under different water disposals except 8.5%.by Ci, When SWC decreased, the content of chlorophyll increased and then fell down, it was the hightest at 11.7%,and the lowest at 17.1%,the relation between net photosynthesis rate and chlorophyll showed that therewas no significant correlation.3 .From consideration on leavers and individuals,11.5%～15.5% can be used as the range of suitable SWC .On the leaves level, the results indicated the curve of diurnal variation of transpiration rate was one-peaked, and the peak value presented itself earlier as the SWC declined, furthermore, the diurnal average transpiration rate fell down with the SWC while the difference among the 5 disposals were significant, and The main effecting factors on the transpiration rate under different water conditions were different.In the level of Single-line, the results showed that ,firstly ,the diurnal variation of transpiring rates was unfolded as a Single-peaked curve or double-peaked curve., and the peak value appeared early with the reducing of SWC. Secondly, when SWC was up to 11.7%, water consumption rate was highest in August. Whereas,the highest appeared in July if the SWC was under 11.7%. What's more, water consumption of single-line which was correlated observably with the biomass increased with the SWC. The diurnal variation of water use efficient of leaves was a double-peaked curve., the average of WUE was the highest at 10.1%,and the curve between SWC and WUE showed there was no significant on WUE in the range of 10.1%-15.3%. On individual level, WUE increased and then fell down as SWC decreasing , it was the highest at 10.1%.4 It can be considered as the beginning of causing water stress when SWC was at 10.1%.The contents of proline and MDA increased with SWC decreasing ,when SWC> 10.1,the content of proline changed slowly .Moreover, the activities of SOD, POD and CAT varied dissimilarly with SWC, owing to the concurrent functions of the three kinds of enzymes, it made reactive oxygen free radicals keep balance in the trees.
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