Critical responses of photosynthetic efficiency in Campsis radicans (L.) Seem to soil water and light intensities

Campsis radicans (L.) Seem is one of the main forestation tree species in semi-arid loess hilly region. Using the CIRAS-2 portable photosynthesis system, the light-response of the photosynthetic efficiency parameters of three-year-old C .radicans leaves, such as net photosynthetic rate (P N ), transpiration rate (Tr), water use efficiency (WUE), and light use efficiency (LUE), were studied under different soil water conditions in order to explore the effects of soil water stress on photosynthesis and the suitable soil water content for water-saving irrigation of this liana. Soil water and light intensity needed by the growth and development of C .radicans were analyzed. The results show that P N , Tr, WUE, and LUE of the leaves had threshold value to soil water and photosynthetically-active radiation (PAR). The nonrectangular hyperbola model was used to simulate light-response curve and the convexity was about 1. With the increase of soil relative water content of field capacity (Wr, ranged from 20.1% to 71.1%), the light compensation point declined while light saturation point, the maximum P N and apparent quantum yield increased. When Wr was about 71.7%, the light compensation point was at the minimum (21.61μmol/m 2 /s) and the light saturation point was at the maximum (1400 μmol/m 2 /s). In order to maintain the normal plant growth and have higher P N , LUE, and WUE synchronously, the range of Wr was from 49.5 to 71.1%. The optimum Wr was 71.1%, and the minimum Wr was 28.2% for the normal growth of C .radicans . The high P N and WUE were recorded when PAR ranged from 800 to 1600 μmol/m 2 /s and the light saturation points ranged from 800 to 1400 μmol/m 2 /s. The peak value of LUE was found when PAR ranged from 100 to 300 μmol/m 2 /s, indicating that the C. radicans had high adaptability to light conditions. Key words : Campsis radicans, soil water content, photosynthetically-active radiation, photosynthetic efficiency, water use efficiency.