Influences Of Soil Water Stress To Leaf Gas Exchange Of Three Tree Species In Loess Hilly Region

Semi-arid loess hilly region, in which water is the main factor of restricting vegetation restoration and plant restoration and reconstruction, is the typical ecological fragile area in our contry. Aiming at the main character of drought and water shortage in this region, pot experiment of Prunus armeniaca, Hippophae rhamnoides and Pinus tabulaeformis were arranged in Yukou, Fangshan, Shanxi. Based on the monitoring technology and analysis theory of gas exchange and chlorophyll fluorescence, processes and characteristics of continuous variation of different tree species'leaf gas exchange parameters such as photosynthetic rate, transpiration rate and water use efficiency and chlorophyll fluorescence parameters in the range from soil saturated humidity to wilting humidity were elucidated, soil water critical points at which photosynthetic parameters change significantly and its changing machanism were revealed, soil water ranges maintaining high photosynthetic rate and water use efficiency of different tree species were determined. The results of this research can have theoretical value to enrich and develop photosynthetic physiology and water physiology of tree, and have practical value to scientifically direct tree species choice, site allocation and favored trees for suitable land in loess hilly region. The main results are as followes:(1) Plant leaf gas exchange parameters have clear threshold value responses to soil water and light intensity. According to the relationships between phtosynthetic rate and soil water, transpiration rate and soil water, water use efficiency and soil water, suitable soil water range, maximum water deficit and suitable light intensity range of keeping high photosynthetic productivity and water use efficiency of 3 tree species were preliminary determined.â‘ Suitable soil water ranges for Prunus armeniaca, Hippophae rhamnoides and Pinus tabulaeformis growing were respectively 36.39%～80.90%, 38.49%～82.86% and 35.72%～80.83% ; light intensity were repectively 600～1400μmol·m^-2·s^-1, 500～1600μmol·m^-2·s^-1 and 400～1600μmol·m^-2·s^-1. Under mild water stress, the 3 tree species can keep high photosynthetic rate, water use efficiency and photosynthetic productivity.â‘¡AQY,Rd and LCP of the 3 tree species had clear threshold value responses to soil water. The ability of using weak light of Prunus armeniaca was bigger than that of Hippophae rhamnoides, and the ability of Hippophae rhamnoides was bigger than that of Pinus tabulaeformis.The consumption of photosynthetic product of Hippophae rhamnoides was more than that of Prunus armeniaca , and the consumption of Prunus armeniaca was more than that of Pinus tabulaeformis. The shade tolerance of Hippophae rhamnoides was larger than that of Pinus tabulaeformis, and the tolerance of Pinus tabulaeformis was larger than that of Hippophae rhamnoides.(2) Influences of CO₂ concentration and soil water stress to plant leaf gas exchange parameters.â‘ The research of responses of plant leaf gas exchange parameters to CO₂ concentration in different water conditions was benefit to elucidate response characteristics of plant to CO₂ concentration and soil water. In each CO₂ concentration, P_n was largest when the RWC was 65.71%. And in each soil water condition, P_n did not decrease when CO₂ concentration increased. Under heavy water stress, P_n,T_r and G_s all kept low level and did not change significantly, which showed that leaf stomata nearly closed in heavy water stress condition and plant normal physiological activities were influenced badly.â‘¡CE,Γand R_p of the 3 tree species had clear threshold value responses to soil water. Under heavy water stess, CE and R_p of Prunus armeniaca and Hippophae rhamnoides were both at their lowest values, and both of their values incressed when water stress was alleviated. Response process ofГto soil water was opposite to that of CE to soil water.Гreached its lowest value and it increased when soil water increased or decreased.(3) Influences of chlorophyll fluorescence parameters to soil waterWhen the experimental trees were in soil water stress conditions, their Fo increased, F_m and F_v/F_mdecreased. But the changing characteristics of qP and NPQ were different. qP of Prunus armeniaca increased as degree of soil water stress aggravated while that of Hippophae rhamnoides showed the opposite trend; NPQ of Prunus armeniaca and Pinus tabulaeformis reached their highest level under mild water stress. To the contrary, NPQ of Hippophae rhamnoides had the biggest value in the CK condition and the smallest value in mild water stress condition.