Vermicompost promotes apple seedling growth by enhancing photosynthetic efficiency and driving chlorophyll biosynthesis.

To evaluate the comprehensive effects of different vermicompost incorporation ratios on the growth performance, nutrient accumulation, and photosynthetic efficiency of apple seedlings (G935, Geneva 935), plants were treated with varying proportions of vermicompost (0%, 50%, 80%, and 100%). Results indicated that increasing vermicompost proportions significantly optimized growing substrate physicochemical properties. Specifically, growing substrate pH decreased from a baseline of 8.02 to 7.78 under 100% vermicompost treatment, a modest reduction that crucially improves micronutrient solubility (especially Fe and Zn) in alkaline conditions, while electrical conductivity (EC), organic carbon, and total nutrients were substantially elevated from their respective control levels. Additionally, vermicompost robustly promoted apple seedling growth and significantly enhanced light energy capture efficiency by increasing the maximum photochemical efficiency of photosystem II (Fv/Fm), actual photochemical quantum yield [Y(II)], electron transport rate (ETR), and non-photochemical quenching (NPQ). The 100% vermicompost treatment maximized these benefits, driving total chlorophyll content and leaf nitrogen concentrations from 4.25 mg g-1 and 7.71 mg g-1 in the control to 9.37 mg g-1 and 12.34 mg g-1, respectively. Mechanistically, this enhanced light utilization was driven by the significant accumulation of chlorophyll biosynthesis precursors-including 5-aminolevulinic acid (ALA), magnesium protoporphyrin IX (Mg-Proto IX), and protochlorophyllide (Pchlide)-relative to their control baselines (e.g., ALA increased from 15.98 ng·g-1 FW to 22.47 ng·g-1 FW). In conclusion, vermicompost improves growing substrate properties, enhances nutrient supply, and specifically stimulates the chlorophyll biosynthetic pathway, thereby increasing light energy utilization efficiency and providing solid theoretical support for its application in fruit tree nursery production.