Substrate physicochemical properties and seedling performance of rubber tree seed-grafted seedlings: relative contributions of physical structure and chemical nutrients

Introduction The physical structure and chemical nutrients of substrates jointly regulate seedling growth in container nursery production, but their relative contributions and interactions remain unclear, limiting precise substrate formulation. Rubber tree ( Hevea brasiliensis ) seed-grafted seedlings are important planting materials for the natural rubber industry, yet their lightweight substrate technology needs optimization. Methods Seed-grafted seedlings were grown in an inert control (CK, river sand: perlite = 1:1) and five substrate formulations: T1 (pure red soil), T2 (coco coir:peat = 1:1), T3 (red soil: coco coir: sheep manure = 5:3:2), T4 (sugarcane leaf residue: river sand: sheep manure = 4:4:2), and T5 (peat: coco coir: sheep manure: river sand = 3:3:2:2). Growth, physiological, and nutrient parameters were measured, and membership function, principal component, and variance partitioning analyses (VPA) were used to quantify independent and interactive effects of physical and chemical factors. Results Different substrates significantly affected growth, biomass, leaf nutrients, and photosynthetic pigments, while biomass allocation (stem > root > leaf) remained stable. Comprehensive evaluation ranked T2 highest (membership 0.74–0.79) and T3 lowest (0.15–0.20). T2 exhibited greater root development, which was associated with its low bulk density and high porosity; this may have partially compensated for the low inherent nutrient status of the substrate by increasing the absorptive area. Although T3 had higher leaf nitrogen, its high bulk density and low porosity were associated with inhibited root growth, causing luxury nutrient uptake and photoprotective compensation (carotenoid accumulation with decreased chlorophyll). VPA showed variation in height and diameter was statistically attributable to chemical nutrients alone (21%–22%), root elongation explained by physical structure alone (10%), and root diameter by their shared effect (3%), indicating clear functional differentiation. Discussion Optimizing substrate physical structure is more critical than stacking nutrient types. T2 is compositionally simpler, and recommended for lightweight rubber seed-grafted seedling cultivation in Xishuangbanna.