Fig. 8. Model depicting the role of methyl jasmonate (MeJA) in melatonin-induced H₂O₂ accumulation and cold tolerance in watermelon plants grafted onto pumpkin or figleaf gourd rootstocks.

When grafted watermelon plants suffer cold stress, rootstock-sourced melatonin may act as a long-distance signal that induces melatonin accumulation in leaves. Melatonin accumulation in leaves induces MeJA accumulation, which in turn increases melatonin accumulation, forming a self-amplifying feedback loop. MeJA further triggers H₂O₂ generation and subsequently enhances cold tolerance