Effects of soil drought on photosynthetic traits and antioxidant enzyme activities in <Emphasis Type="Italic">Hippophae rhamnoides</Emphasis> seedlings

Water deficit is one of the major limiting factors in vegetation recovery and reconstruction in the semi-arid area of loess hilly regions. Leaf photosynthesis in Hippophae rhamnoides Linn., a common tree grown in this region, decreases under water stress, but the mechanism responsible is not clear. The objective of this study was to investigate the effects of drought stress on photosynthesis and the relationship between photosynthetic variables and soil water contents to help us better understand the photo-physiological characteristics of H. rhamnoides under water stress and guide cultivation in the loess hilly region. Here, gas exchange, chlorophyll fluorescence and antioxidant enzyme activity in leaves of 3-year-old saplings of H. rhamnoides grown in pots were tested under eight soil water conditions. When soil water content (RWC) was between 38.9 and 70.5 %, stomatal limitation was responsible for the reduced net photosynthetic rate (P _N). When RWC was lower than 38.9 %, nonstomatal limitation was the main factor restricting P _N. Moderate water stress improved the water use efficiency (WUE) of the leaf. Water stress significantly influenced fluorescence variables and the antioxidant enzyme system. When RWC was between 38.9 and 70.5 %, nonphotochemical quenching (NPQ) increased and then decreased, indicating that thermal energy dissipation was a significant photoprotection mechanism. Antioxidant enzymes were activated when RWC ranged from 48.3 to 70.5 %; under severe water stress (RWC < 38.9 %), the antioxidant enzyme system was damaged, the activity of the antioxidant enzymes declined, and membranes were damaged. In the semiarid loess hilly region, RWC between 58.6 and 70.5 % was the economic water threshold value that maintained higher WUE and P _N, and the maximum soil water deficit level that could sustain H. rhamnoides was RWC of 38.9 %.