Genotype-specific electrolyte leakage and secondary metabolite production in Andrographis paniculata under heavy-metal stress

Abstract

Toxic metal stress disrupts membrane integrity, causing electrolyte leakage, but genotype-specific differences in leakage composition and metabolic sensitivity under distinct metal stress remain unexplored. The experiment was performed with two genotypes of Andrographis paniculata (T1 and T2) under individual and combined metal (As, Cd, and Pb) stresses. The concentration of different osmolytes (Cl$^{-}$, NO$_3^{-}$, SO$_4^{2-}$, PO$_4^{3-}$, Na$^{+}$, K$^{+}$, Ca$^{2+}$, Mg$^{2+}$, and NH$_4^{+}$), sugars (inositol, sorbitol, sucrose, glucose, and xylose), and secondary metabolites were analysed. Results indicated that both genotypes showed reduced biomass, protein, and chlorophyll under metal stress, with T2 exhibiting greater declines, especially under Pb and combined metal exposure. Andrographolide and neo-andrographolide levels decrease under all metal treatments in both genotypes, with a strong reduction in T2. Basal electrolyte leakage and antioxidant enzyme levels were inherently higher in T2, reflecting greater physiological sensitivity to metal stress compared to T1. Metal stress caused significant increases in both anion and cation leakage with Cl$^{-}$ showing the highest anion rise, and Mg$^{2+}$ in T1 or Ca$^{2+}$ and Na$^{+}$ in T2 showing the highest cation increases. Combined metal exposure induced the greatest overall ion loss, while specific metals triggered distinct leakage patterns, such as Pb-driven nitrate release observed in T1 and Cd-driven nitrate release in T2. Sugar leakage was consistently higher in T2, with xylose showing the largest increase under stress. Inositol, sucrose, sorbitol, and glucose exhibited genotype and metal specific responses, with mixed metals generally causing the strongest release. These contrasting profiles reflect genotype-specific metabolic sensitivity, membrane damage and osmotic responses to metal stress highlighting distinct stress tolerance mechanism and regulation of diterpenoid biosynthesis in two genotypes.