The Effects of Shading on the Photosynthetic Performance of Endangered Plant Horsfieldia hainanensis Seedlings

Shading is one of the management practices for preventing the damage or injury of plant seedlings during extreme weather and climate events, such as very high temperatures and heat stress. In this study, we investigated the effects of different shading conditions on the photosynthetic characteristics of the endangered plant Horsfieldia hainanensis in Guangxi, China. The H. hainanensis seedlings in this study underwent five shading treatments, including 20% (L1), 40% (L2), 60% (L3), 80% (L4), and 100% (control) of full sunlight. The net growth of their diameter and height, and photosynthetic gas exchange parameters including their photosynthesis rate (Pn), transpiration rate (Tr), intercellular CO₂ concentration (Ci), and water use efficiency (WUE) were measured for the examined seedlings. The OJIP curve and 820 nm light absorption curve, and the osmotic substances and products of membrane lipid peroxidation were employed to assess photosynthetic capacity, identify the factors constraining photosynthetic carbon assimilation, and investigate the mechanisms influencing photosystem II (PSII) and photosystem I (PSI) in the seedlings under shade stress. The results showed that the seedlings in the L2 treatments had the highest net growth and Pn, the best photosynthetic performance, and the best coordination between PSII and PSI. The net photosynthesis (Pn) levels exhibited a declining trend in the following order: L2 > L3 > L4 > L1. In the L1 treatment, non-stomatal factors emerged as the primary determinant affecting the Pn of the seedlings. The performance index (potential) of PSII, representing the conservation of absorbed photon energy to intersystem electron acceptor reduction (PI_ABS and ΔI/I₀) of the seedlings, decreased in the order of L2 > L3 > L4 > L1. The photosystem performance and the coordination between PSII and PSI (Φ_(PSI/PSII)) of the seedlings decreased in the order of L2 > L1 > L3 > L4. Under the low and moderate shading stresses (L1–L3), more serious damages occurred in PSII than in PSI, including on the donor side of PSII and in the electron transfer from Q_B to the acceptor side of PSI. In contrast, more considerable injury occurred in PSI than in PSII under the stress of the heavy shading treatment (L4). Considering the alterations in their leaf osmotic regulatory substances and membrane lipid peroxidation products, our findings indicate that the L2 treatment was the most conducive to the growth of the H. hainanensis seedlings. In contrast, the L1 treatment subjected H. hainanensis seedlings to the most significant stress, resulting in substantial damage to their growth and photosynthetic mechanisms. Our research provides a scientific insight into and a practical guide for the selection of an appropriate light intensity for the conservation and cultivation of endangered plant species, such as H. hainanensis.