Combined Effects And Mechanism Of Shade And Drought On Growth And Physiological Of Torreya Grandis Seedlings

Light and water were the major factors for plant growth.Previous studies found that the combined effects of water and shade were different in various specious.Many reports suggested that shade could improve the negative effect of drought,however,others suggested shade would accelerated the negative effect of drought.Two-year-old seedlings of Torreya grandis Fort.Ex Lindl were taken as materials in this study.The experiment was laid out in completely randomized factorial design,with four shading levels（0%,50%,75% and 90% shading level）and two water treatments（well water 75% FC,and water stress,35 FC）.To elucidate the effects of photosynthetic characteristics and nitrogen allocation of Torreya grandis seedlings under different shading levels and the combined effects of shade and drought on growth characteristics,photosynthetic characteristics,photoprotection and protective enzyme activity of Torreya grandis seedlings.Through our results of how to adapt to different light and water interactions,we can provide some theoretical basis and technical supports for efficient cultivation of Torreya grandis seedlings.Through the determination and analysis of the various indexes in the experiments,the main results are as follows:（1）Shading treatment can effectively promote the growth of Torreya grandis seedlings.The seedlings grown under shading treatment can effectively use the lower light intensity at early morning or late afternoon in comparison to that grown at full sun light.Heavy shading（90% shading level）significantly decreased the Pmax,which may be related to in order to capture more light,more resources were used to the light protein,resulting in a decline in photosynthetic capacity.Compared with the 50% shading level,more nitrogen was allocated to the photosynthetic mechanism,such as in NCand NB,and significant higher PNUE at 75% shading level.Therefore,our results indicate that Torreya grandis seedlings likely grow better at 75% shade level for seedling cultivation.（2）Water stress resulted in significant decrease in height increment,biomass of seedlings under each shading level companied with yellow leaves.Compared with well water,the collection of biomass to root significant increased at 0% and 50% shading levels under water stress,while the biomass collection of each part at 75% and 90% shading levels slight changed.Thus,we suggested that biomass collection of various shading levels were different response to water stress.（3）Under water stress,relative water content（RWC）of Torreya grandis leaves at 50% and 75%shading levels decreased significantly less than at other shading levels.Whether well water or water stress,proline and soluble protein content of Torreya grandis leaves at 0% and 90% shading levels were significantly higher than those under 50% and 75% shading levels.Under water stress,proline and soluble protein content of Torreya grandis leaves was not changed significantly at 0% and 75%shading levels.The results showed that Torreya grandis leaves under 50% and 70% shading conditions can maintain better water state,do not need through the rapid accumulation of proline and soluble protein content to regulate water condition.（4）In addition to the 75% shading level,water stress significantly decreased Fv/Fm of Torreya grandis seedling leaves under other shading levels,especially under 0% shading level.Under water stress,the maximum photosynthetic rate（Pmax）of Torreya grandis seedlings were significantly decreased under all shading levels;shading treatments（50%,75% and 90% shading levels）significantly reduced the light compensation point（LCP）and dark respiration rate（Rd）;the light saturation point（LSP）were significantly reduced under 0% and 90% shading levels.The results showed that 75% shading level can effectively ease the photoinhibition of Torreya grandis leaves to water stress,however,water stress aggravate the photoinhibition of Torreya grandis leaves under 0%and 90% shading levels;shading treatments can reduce the minimum energy dissipation and respiration consumption of Torreya grandis leaves under water stress,is beneficial to the accumulation of carbohydrates of leaves.（5）Water stress significantly decreased chlorophyll content of Torreya grandis leaves under 0% and90% shading levels,however,which did not change a lot under 50% and 75% shading levels.Whether well water or water stress,antioxidant enzymes（SOD,POD,CAT）activity and content of active oxygen free radical（O₂·-and H₂O₂）of Torreya grandis leaves under 0% and 90% shading levels were significant higher than those under 50% and 75% shading levels.Under water stress,Torreya grandis leaves antioxidant enzyme activity and active oxygen free radicals content did not change significantly under 50% and 75% shading levels.The results showed that 50% and 75%shading levels can effectively ease oxygen stress of Torreya grandis leaves to water stress.（6）Under water stress,nitrogen content per unit dry weight/area（NM or Na）of Torreya grandis leaves under 0%,50% and 75% shading levels significantly increased,and significantly decreased under90% shading level.Water stress significantly decreased photosynthetic efficiency（PNUE）under allshading levels.Either under well water or water stress,PNUE of Torreya grandis leaves under 75%and 90% shading levels were obviously higher than those under other shading treatment,and PNUE was the lowest under 0% shading level.Therefore,under water stress,75% shading level can be benefit of biomass accumulation of Torreya grandis,and can also effectively promote the nitrogen utilization.