Exponential Phosphorus Application Enhances Growth, Photosynthesis and Nutrient Accumulation of Homalium ceylanicum...

Phosphorus (P) deficiency is a major nutritional constraint limiting the regeneration and large-scale cultivation of Homalium ceylanicum, a valuable tropical timber species in South and Southeast Asia. To identify an optimal P supply strategy for producing high-quality planting stock, we assessed the growth performance, photosynthetic characteristics, and nutrient status of one-year-old H. ceylanicum seedlings in response to exponential P fertilization at six levels: 0 (control), 25, 50, 100, 200 and 300 mg P per seedling. Seedlings supplied with 50 mg P per seedling performed best, with height, leaf biomass, and leaf area increasing by 57.5%, 152.0%, and 164.7%, respectively, relative to the unfertilized control. These enhancements were paralleled by significant improvements in net photosynthetic rate, chlorophyll content, photosystem II (PSII) photochemical efficiency, and gas exchange, indicating that enhanced photosynthetic carbon assimilation contributed to growth promotion. At the whole-plant level, this coordinated improvement in growth and photosynthesis was accompanied by a 171.04% increase in nitrogen (N) accumulation without inducing excessive P accumulation. In contrast, P supply above 100 mg per seedling reduced seedling height and root collar diameter, suppressed whole-plant biomass accumulation, and disrupted nutrient balance through excessive P accumulation and antagonistic effects on N and K (potassium). Vector analysis further confirmed 50 mg P per seedling as the optimal nutrient-loading dose, achieving effective P loading without inducing N or K excess or nutrient imbalance. In conclusion, exponential P fertilization at 50 mg P per seedling is a targeted nutritional strategy for producing high-quality planting stock, thereby supporting the plantation forestry of H. ceylanicum.