Abstract:

Diurnal course and light response curves of photosynthesis in the Black locust Robinia pseudoacocia in Weibei region were measured using a LI-6400 portable photosynthesis system under sunny and cloudy days in June and August 2009. Regardless in June or in August, the net photosynthetic rate (P_n) and stomatal conductance (G_s) of Black locust under the sunny days were higher than those under the cloudy days before 10:00, while both of them were lower between 12:00 and 14:00. The intercellular CO₂ concentration (C_i), ambient CO₂ concentration (C_a), relative humidity (RH), light use efficiency (LUE) across most times of the day were significantly lower under the sunny days than those under the cloudy days. In contrast, the transpiration rate (Tr), vapor pressure deficit (VPD), photosynthetic active radiation (PAR), air temperature (T_a) in the whole day were markedly higher under the sunny days compared to those under the cloudy days. Under the sunny day conditions, although Black locusts had lower daily mean values of G_s, C_i, C_a, RH, LUE, they showed higher daily mean values of P_n, Tr, VPD, T_a and could in turn accumulate more average daily photosynthetic carbon fixation amounts. Under the cloudy day conditions, Black Locusts presented a greater maximum photosynthetic rate (P_max) and a pronouncedly lower dark respiration rate (R_d) and light compensation point (LCP), which indicated that Black locust had a strong photosynthetic acclimation capacity. This study would have great implications for the evaluation of stand productivities and for the production and management of Black locust plantations in Weibei region.

Key words: Robinia pseudoacacia, photosynthesis, sunny day, cloudy day, light use efficiency