Irradiation and nitrogen regulate growth and physiology in Horsfieldia hainanensis seedlings

Preventing extinction is one of the greatest challenges facing the global community. Nursery stock breeding is an effective means to restore endangered species, such as Horsfieldia hainanensis Merr., with difficulty in natural regeneration period. In this study, we investigated the optimum combination of irradiance and nitrogen for the cultivation of H. hainanensis seedlings by comparing twenty treatments with different combinations of irradiances (100, 67.5, 45.7, 15.6 % of full natural irradiance) and five levels of N supply (0, 1.8, 3.6, 5.4, 7.2 g plant -1 ). We found that the growth and photosynthetic efficiency of seedlings under full irradiance were significantly inhibited compared with shaded seedlings. Under full irradiance, a lack of N resulted in reduced chlorophyll (Chl) synthesis, causing lower photosynthetic efficiency and an imbalance in metabolism. Proper shading (67.5 and 45.7 % of natural irradiance) and N addition (1.8 - 5.4 g plant -1 ) promoted root development, increase Chl content and photosynthesis, and ultimately the accumulation of larger amount of biomass. The biomass of the shaded seedlings was mainly distributed to aboveground tissues, while seedlings exposed to stronger radiation accumulated greater root biomass. Therefore, the best seedling management for this species is a combination of 67.5 % of natural irradiance and moderate N supply (4.6 g plant -1 ).