Enhancing cadmium tolerance and photosynthesis in Cosmos bipinnatus Cav. By hormone methyl jasmonate

Cosmos bipinnatus Cav. (C. bipinnatus), a highly cadmium (Cd)-tolerant landscape plant, shows promise for Cd pollution remediation. This study investigates the role of methyl jasmonate (MeJA) in enhancing Cd tolerance under high Cd stress (40, 120 μM) by regulating endogenous hormones. Cd stress severely impacts C. bipinnatus, reducing fresh/dry weight by 44.49%–83.81%, alongside declines in chlorophyll content and photosynthetic efficiency (F_v/F_m). However, low-concentration MeJA (0.1 μM) improves Cd tolerance by reducing water content by 3.09%–22.43%, increasing biomass by 12.82%–52.73%, decreasing Cd uptake by 4.56%–5.64%, and restoring chlorophyll synthesis and F_v/F_m. MeJA also modulates non-protein thiols (NPTs), especially decreasing NPTs in leaves under Cd stress and enhances glutathione conversion to phytochelatins, critical for detoxification. Endogenously, MeJA triggers rapid accumulation of jasmonic acid, abscisic acid (ABA), and indole-3-acetic acid while reducing gibberellin, which can be harmful to photosynthesis under Cd stress. MeJA also prevents prolonged ABA accumulation, mitigating its levels, thus protecting the photosynthetic system. These findings highlight MeJA's regulatory effects on hormone balance and suggest new approaches for effective soil Cd pollution remediation through enhanced plant tolerance mechanisms.