Abstract:

Drought and high temperature often occur simultaneously in arid and semiarid regions, but few investigators have studied the interactions between these two stresses. Our objective was to compare the effects of soil moisture, temperature and their interactions on the photosynthesis and growth of Leymus chinensis. Plants were subjected to different soil moisture regimes (relative soil water content ranged from 25% to 80%) at temperatures of 20, 23, 26, 29 and 32 ℃ in a controlled environment. Under sufficient soil moisture conditions, both net photosynthetic rates and water use efficiency decreased with increasing temperature whereas both stomatal conductance and transpiration rates increased. There were no significant effects of soil moisture alone on net photosynthetic rates but significant soil moisture-temperature interactive effects were observed; net photosynthetic rates increased under conditions of moderate soil drought at 20-26 ℃ but were significantly reduced under drought conditions at extremely high temperature (32 ℃). Water use efficiency showed a similar response to changes in temperature and soil moisture as net photosynthetic rates. Soil moisture did not significantly affect leaf stomatal conductance or transpiration rates indicating that higher adaptive ability to soil drought may be exhibited under these experimental conditions. However, under all soil moisture conditions, both net photosynthetic rates and water use efficiency always decreased with increasing temperature whereas both stomatal conductance and transpiration rates always increased with increasing temperature. Leaf biomass also responded to changes in soil moisture and temperature. The leaf biomass was greatest under conditions of light to moderate soil drought at temperatures from 20-26 ℃ but decreased with decreasing soil moisture at higher temperatures of 29 ℃ and 32 ℃. Leaf biomass of plants grown at 26 ℃ was greatest under sufficient soil moisture and light drought conditions, but the leaf biomass at 23 ℃ was greatest under moderate to high drought conditions. These results suggest that the optimal growing temperature might be lowered under droughty conditions. Leaf biomass was reduced in plants grown at higher temperatures (29 ℃ and 32 ℃) under all soil moisture regimes. Interactions between water stress and temperature were highly significant for several physiological processes examined. Leaf gas exchange characteristics and growth were more impacted by drought conditions at high temperatures than at low temperatures. Similarly, the productivity of L. chinensis was reduced more by the combined stresses of drought and high temperature than by either stress alone and much of the effect was on photosynthetic processes. Our research suggests that the decreased precipitation and increased temperatures forecasted for this semi-arid region due to global climate change could adversely affect the distribution and abundance of L. chinensis. To conserve this species, future research should focus on ways to enhance the drought tolerance of L. chinensis at high temperatures.