Low soil moisture improved shading tolerance by regulating leaf functional traits in Tetracentron sinense Oliv. seedlings

Tetracentron sinense Oliv. is an endangered relict tree found in south-central China. A low number of seedlings among the natural populations has resulted in the poor regeneration of this species. However, the mechanisms underlying seedling survival and growth remain poorly understood. Leaf functional traits (LFT) likely characterize the growth and development of T. sinense in different ecological environments, playing a crucial role in its adaptive strategies to adversity. In this study, two-factor orthogonal control experiments with two levels of light intensity (L1: 50 % natural light intensity in forest edge; L2: 10 % natural light intensity in understory) and soil moisture content (W1: 30 % moisture content in valley; W2: 15 % moisture content in mountainside) were conducted to analyze the response of the LFT of 7-month-old T. sinense seedlings to different light and soil moisture conditions. With the light intensity decreased, superoxide dismutase activity decreased, and peroxidase activity increased. With a decrease in soil moisture, the contents of chlorophyll (Chl) a, Chl b, Chl t, and soluble sugar increased, and Chl a/b decreased. A decrease in light and soil moisture decreased leaf thickness, leaf dry weight, leaf dry matter content, leaf volume, chloroplast volume, and starch grains, but increased osmophilic grains, the total nitrogen, and phosphorus contents. Moreover, the chloroplast morphology of L2W2 was more complete than that of other treatments. The results showed that the LFT of T. sinense seedlings acted synergistically to adapt to different light and soil moisture conditions, low soil moisture alleviated the effects of shading in T. sinense seedlings, and T. sinense seedlings adopted the “quick investment-return strategy” adaptation strategy under L2W2. Finally, the soil moisture of the mountainside in the natural habitat was favorable to the survival and growth of T. sinense seedlings.