Effects of light radiation intensity on photosynthetic characteristics and water use efficiency of Platycladus orientalis and Pinus tabulaeformis seedlings

Abstract

In order to offer a scientific basis for cultivation and management of forests, effects of light radiation intensity on photosynthetic characteristics and water use efficiency of Platycladus orientalis and Pinus tabulaeformis were studied under different soil moisture contents. By adopting artificial control methods to soil moisture, and under simulated photosynthetic radiation (SPR), the net photosynthetic rate (P_N), transpiration rate (T_r), water use efficiency (WUE) and intercellular CO₂ concentration (C_i) of Platycladus orientalis and Pinus tabulaeformis in the semi-arid region of the Loess Plateau, were studied. Results are as follows: within the photon range of 0 2,200 µmol/(m² °s), PN, Tr and WUE were enhanced with an increase in SPR in both species. PN and WUE of Platycladus orientalis and Pinus tabulaeformis, however, declined with continued increase in SPR. PN, Tr, WUE and light saturation point (LSP) of Platycladus orientalis were higher than those of Pinus tabulaeformis, while light compensation point (LCP) of Platycladus orientalis was lower than that of Pinus tabulaeformis at the same soil moisture content. The efficiency of light energy utilization of Platycladus orientalis was higher than that of Pinus tabulaeformis; P_N, T_r and C_i of Platycladus orientalis and Pinus tabulaeformis were enhanced by increasing soil moisture content, whereas WUE declined. At soil moistures of 7.90%, 13.00% and 19.99%, LSP of Platycladus orientalis was 1,275, 1,450 and 1,675 µmol/(m² °s) respectively, and LCP was 42, 25 and 13 µmol/(m² °s) respectively, with corresponding maximal net CO2 photosynthetic rates (P_max) of 3.04, 4.06 and 5.53 µmol/(m² °s). At soil moistures of 7.83%, 13.04% and 20.15%, the LSP of Pinus tabulaeformis was 1,100, 1,325 and 1,500 µmol/(m² °s) respectively, and LCP was 60, 30 and 23 µmol/(m² °s), with P_{max of 1.08, 3.35 and 4.36 µmol/(m² °s) respectively.}