Objective The study aims to identify biostimulants conducive to the cultivation of robust wax gourd seedlings and to determine the optimal concentration for their application.

Method The black wax gourd 'Tiezhu 2' was used as experimental material, the effects of eight biostimulants (seaweed extract, seaweed fertilizer, polypeptide, alginate oligosaccharide, fulvic acid, canthaxanthin, astaxanthin and lactosyl peptide) on the quality of wax gourd seedlings were studied. By measuring the seedling strength index and root vitality of plants under different types and concentrations of biostimulants, the most suitable concentration of biostimulant was selected for subsequent experiments. The physiological indicators including root morphology, leaf area, and chlorophyll content of wax gourd seedlings under different biostimulant treatments were analyzed by principal component analysis, and the capacity of the 8 biostimulants in promoting the growth of wax gourd seedlings was ranked.

Result Biostimulants with suitable concentrations could significantly improve the growth quality of wax gourd seedlings, as manifested by an increase in plant height, stem thickness, root system growth and photosynthetic capacity. Among them, the optimal treatment of 0.2 g/L alginate oligosaccharide significantly increased the stem thickness, plant height, root volume, chlorophyll a, chlorophyll b and leaf area of wax gourd seedlings by 15.03%, 43.42%, 359.41%, 121.62%, 61.11%, and 58.30% respectively compared with those of the control. In addition, the 0.5 g/L fulvic acid and 0.5 g/L lactosyl peptide treatments performed suboptimally, increasing root vitality by 185.16% and 14.34%, respectively, leaf area by 50.04% and 27.79%, respectively, and total chlorophyll a+b content by 54.05% and 51.35%, respectively. The results of the principal component analysis showed that the growth-promoting effects of different treatments on wax gourd seedlings ranked as: alginate oligosaccharide 0.2 g/L (score 0.97) > fulvic acid 0.5 g/L (0.64) > lactosyl peptide 0.5 g/L (0.62), which were significantly better than the control (0.10).

Conclusion The biostimulants with optimal concentrations could significantly improve the seedling strength index and root vitality of wax gourd seedlings, the effect of alginate oligosaccharide was the best at 0.2 g/L, and fulvic acid and lactosyl peptide also showed good growth-promoting effect at a concentration of 0.5 g/L.