The Response To Water Stress Or Salt Stress Of The Three Shrubs' Photosynthetic Characteristics In Loess Plateau Region

In order to study the plants'physiological and photosynthetic characteristics responding to the water stress and NaCl stress, we used three water-and-soil-conservative shrubs-- Shepherdia argentea, Hippophae rhamnoides Linn and Atraphaxis frutescens-as the research objectives, based on the SPAC theory. During the growth period in 2008, Li-6400 portable photosyntometer was employed to measure the photosynthetic characteristics of the above three shrubs and their corresponding responses to the water stress and NaCl stress were also analyzed. The main study includes:measuring the growth index of the three shrubs under the water-stress condition; the influence of radiant intensity on the three shrubs'photosynthetic characteristics and the water use efficiencies under the water-stress condition; the three shrubs' photosynthetic rates, transpirational rates and their correlations under the water-stress condition; the influence of NaCl stress on the three shrubs'growth and physiological index; the influence of NaCl stress on the three shrubs'photosynthetic characteristics.Our study results show that:(1) With the increase of water-stress levels, the three shrubs'biomass (i.e. the total dry weights and the total fresh weights) and the overall leaf areas per tree both decrease remarkably. And the influence degrees are Shepherdia argentea> Atraphaxis frutescens>Hippophae rhamnoides Linn;(2) The water stress has high influence on the three shrubs'net photosynthesis rates, transpirational rates and water use efficiency. Mediate water-stress level could increase their water use efficency. In particular, we observed that Shepherdia argentea and Atraphaxis frutescens have relatively higher water use efficiency in the soil which has a 7.2%--14.35% water content and Hippophae rhamnoides Linn showed relatively higher water use efficiency even in the soil with less than 7.2% water content;(3) The radiant intensity has high influence on the three shrubs' net photosynthesis rates, transpirational rates and water use efficiency. In particular, Shepherdia argentea, Hippophae rhamnoides Linn and Atraphaxis frutescens showed higher net photosynthesis rates and water use efficiencies on the radiant ranges 800-1800μmol·m-2·s-1,1200-2000μmol·m-2·s-1 and 800-2000μmol·m-2·s-1 respectively.(4) The three shrubs' biomass, the overall leaf areas per tree and chlorophyll content all decreasd remarkably with the increase of NaCl-stress levels. Besides, given the same NaCl-stress level, Hippophae rhamnoides Linn's physiological characteristics dropped by larger amounts compared with the other two plants and it also showed sensitivity to salt;(5) With the increase of NaCl-stress levels and the stress time,the three shrubs'net photosynthesis rates, transpirational rates and stomatal conductance all extremely decreasd. Besides, given the same stress conditions, Hippophae rhamnoides Linn's physiological characteristics dropped by larger amounts than the other two plants. And the three shrubs'Pn decrease converted from stomatal limit to non-stomatal limit with the extension of the NaCl stress time.