Light responses of the gas exchange parameters of Campsis radicans (L.) Seem leaves under different soil moisture conditions

Little is known about the optimum soil moisture and light intensity necessary for the growth of Campsis radicans (L.) Seem. Using the CIRAS-2 portable photosynthesis system, light responses of the photosynthetic and physiological parameters of three-year-old C. radicans leaves, such as net photosynthetic rate (P
N
), transpiration rate (E) and water use efficiency (WUE), were measured under different soil moisture conditions. Results showed that PN and WUE of the leaves have critical responses to the levels of soil moisture and the variation of photosynthetically-active radiation (PAR). Non-rectangular hyperbola model was used to simulate a light-response curve with a convexity of approximately 1. With the increase of soil mass water content (Wm) (Wm ranged from 5.5 to 19.4%), light compensation point declined while light saturation point, maximum P
N
, and apparent quantum yield increased. When Wm was about 19.4%, light compensation point was at the minimum (21.61 µmol m
⁻²
s
⁻¹
) and light saturation point was at the maximum (1400 µmol m
⁻²
s
⁻¹
). For the plant to grow normally and have higher P
N
and WUE synchronously, the range of Wm needed to be from 13.5 to 19.4% and the optimum Wm at about 19.4%, with the minimum value of Wm at 7.7%. The higher adaptability of C.radicans to light conditions was observed, and high P
N
and WUE were recorded when PAR ranged from 800 to 1600 µmol m
⁻²
s
⁻¹
.