| The experiment was designed according to orthogonal method, considering three factors which are growth stages, severity (moderate and severe water stress) and water stress duration (15d, 30d, 45d). Two common trees (saplings) in loess plateau, Acer truncatum and Platycladus orientalis were chosen to proceed pot-cultured experiment, determining main trees'water stress intensity in different growth stages, the rewatering effects to seedling's physiological index, biochemical index and growth after water stress. It has important theory directive significance in improving the moisture utilization rate, giving full play to the productive potential, and accelerating the plant-construction-centered artificial vegetation recovery and environment construction in the western drought area. Followings are the main research finding:(1) Saplings experienced the process from primary reaction to adaptation under different water stress. The main trends were that WSD of saplings decreased firstly, then, water potential deceased gradually in order to enhance water absorption ability. Meanwhile, the decreasing of WSD caused Cs lower. And then, Pn and Tr dereased at the same time. The maximum solar energy conversion efficincy of PSâ…¡, potential activity of PSâ…¡and primary reaction of PSâ…¡were inhibited to a certain extent. In biochemistry, osmolyte accumulation of saplings happened under water stress. However, osmotic adjustment could affect within the threshold. WSD caused cell dehydration,then led to cell membrane damage and then plasma-membrane permeability inceaseed. In different growth stages, it could affect growth height and stem diameter of saplings that effect of physiological index, biochemical index on different severity and duration of water stress finally.(2) As severity of water stress inceased, WSD, water potential, photosynthetic rate and transpiration rate all dereased to a certain extent, however, plasma-membrane permeability increased gradually. WUE under moderate water stress was higher than CK, by contrast, WUE under moderate water stress was lower than CK. Proline increased firstly and then decreased gradually, which showed that adjustment ability of Proline was not strong enough to eudure the water stress. In same severity of water stress condition, as duration time of water stress across, all the physiological characteristics underwent a course from primary reaction to adaption gradually. The effect of saplings on water stress variaed according to different growth stage, different index also existed diversity.The extent of WSD, water potential affected by water stress was early growth stage > fast growth stage> final growth stage. The variation of Osmotic Adjustment ability of saplings was early growth stage > fast growth stage> final growth stage.(3) After rewatering, compensation existed commonly during saplings restoration course, with variations of form. For instance, WSD, water potential, plasma-membrane permeability of both saplings all existed extra-compensation after morderate water stress followed by rewatreing. Plant height compensation happened after several days of rewatering. There were also indicators of the resumption of the differences in time. Compared with fast recovery of WSD and plasma-membrane permeability, recovery of water potential, photosynthetic rate and transpiration rate were slower. It could improve water absorption ability in order to maintain water balance that there was hysteresis between recorvery of water potential and WSD. Meanwhile, it could improve WUE that restoration degree of photosynthetic rate was higher than transpiration rate. Compensation effect of saplings can be stimulated though morderate water stress. Nevertheless, restoration degrees above indexes were all much lower after severe water stress, due to damage of water structure and function. The main trend of recovery was early growth stage > fast growth stage> final growth stage after rewatering of different growth stage.Meanwhile, the compensation effect of plant height and stem diameter was early growth stage > fast growth stage.
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