Title : Endogenous allelochemicals as principal regulators of differential germination in hard and soft Sapindus mukorossi seeds
Abstract:
Seed dormancy ensures survival in natural environments but often compromises germination uniformity in cultivation. In Sapindus mukorossi, dormancy is influenced by both physical and chemical factors. Interestingly, hard-coated seeds germinate earlier than soft-coated ones, which become structurally altered upon soaking and exhibit delayed emergence. To understand this dormancy variation, we performed allelopathic bioassays, UPLC–MS/MS metabolomics, and SEM-based seed coat anatomy. Bioassays revealed that soft-seed kernel extracts strongly inhibited Chinese cabbage seed germination. At 100 g/L, soft-kernel extracts reduced germination to just 6%, whereas hard-kernel equivalents allowed up to 100% germination. Root elongation was completely arrested by soft-kernel extracts, while soft seed coats permitted limited root growth (2.9 cm). Metabolomic profiling identified 1,120 metabolites across both kernel types, with 129 showing significant differential abundance. Soft kernels were enriched in 33 flavonoids and 11 alkaloids, including hesperetin-7-O-glucoside, epicatechin, phlorizin derivatives, strictamine, and caffeine candidate allelochemicals responsible for the observed phytotoxicity. SEM showed that soft seeds had rough outer surfaces with porous palisade layers and crystalline inclusions, while hard seeds featured compact palisade layers and interconnected endotesta despite their perforated exteriors. Collectively, these findings indicate that delayed germination in soft S. mukorossi seeds is primarily associated with allelopathic inhibitors accumulated in the kernel rather than physical restrictions imposed by the seed coat. This study underscores the dominant role of chemical factors in regulating dormancy variation in this economically important species.
