Effects of drought and shading on growth and photosynthetic characteristics of Pinus massoniana seedlings

To explore the physiological response and adaptation mechanism of Pinus massoniana to drought and shading stress, we investigated the effects of growth and photosynthetic physiological characteristics in control（CK）, drought（DR）, shading（LL）, and the interaction of drought and shading（DRLL） environments on 2-year-old P. massoniana seedlings. The result showed that:（1） under drought, shading and their interaction treatment, the increment of the basal diameter and height of P. massoniana seedlings significantly reduced, and their interaction aggravated the respective negative effect of drought and shading alone. The influence of the interaction conforms to the "Interaction theory".（2） Under drought treatment, the length of needles and specific leaf area decreased. The net photosynthetic rate（Pn）, transpiration rate（Tr）, stomatal conductance（Gs）, and intercellular CO2 concentration（Ci） significantly decreased. The water use efficiency（WUE） significantly increased. However, there was no effect on photosynthetic pigment content.（3） Under shading treatment, the length of needles and specific leaf area increased. The Pn, Tr, and Gs significantly decreased. The photosynthetic pigment content significantly increased, while the WUE and Ci had no variation.（4） Under the interaction of drought and shading treatment, the length of needles and specific leaf area decreased. The Pn, Tr, and Gs significantly decreased and the decreasing degree was higher than drought alone. The photosynthetic pigment content significantly increased and the increasing degree was less than that in shading alone. We found that both drought and shading inhibited the growth of P. massoniana seedlings and their photosynthetic physiological characteristics showed different response mechanisms. The P. massoniana adapted to the drought environment by sacrificing the length of needles and specific leaf area and adjusting the stomatal conductance to achieve the purpose of water retention. By increasing the length of needles, specific leaf area and the photosynthetic pigment content, the P. massoniana adapted to the shading environment. While under the interaction between drought environment and shading environment, the shading environment not only relieved the decrease of needle length, specific leaf area, and stomatal conductance caused by drought-stress, but also weakened the ability to synthesize photosynthetic pigments. Furthermore, the drought combined with shading may generate greater risks on