Comparative Study of the Effects of Shading Treatments on Photosynthetic Capacity, Chlorophyll Fluorescence, Chlorophyll Content, and Antioxidant Activities of <i>Viburnum opulus</i> and <i>V. japonicum</i>

Viburnum opulus is recognized as a medicinal plant, whereas V . japonicum shows promise as an excellent ornamental species. This research aimed to explore their responses by cultivating under four graduated shading treatments (0%, 25%, 50%, and 75% shading). The investigation focused on assessing photosynthetic parameters, chlorophyll fluorescence, chlorophyll content, and enzyme activities. The results revealed that V. japonicum cultivated under 50% shading exhibited the highest photosynthetic rate ( P N ), which was 2.28–3.5 times that of 0% shading treatment. It also showed the highest electron transport rate (ETR) and chlorophyll content. The chla, b, and total chl content were increased by 14%, 6%, and 12%, respectively, compared to 0% shading treatment, alongside the lowest superoxide dismutase (SOD) activity. V. opulus cultivated under 25% shading had the highest chlorophyll content, exceeding the 0% shading treatment by 40%, 43%, and 41% in chl a, chl b, and total chl content, respectively. However, its P N values were the second highest among all treatments, only lower than those under 0% shading. These findings imply that a 50% shading is optimal for V . japonicum cultivation, whereas V . opulus prefers a 25% shading. V . opulus has less tolerance to shade than V . japonicum.