Effects of Pb²⁺ and Cd²⁺ combined stress on photosynthesis of Morus alba

Objective Heavy metals in soil become more serious and the effects of heavy metals on plant are high profile. At present, most studies pay attention to the effects of single heavy metal stress on plants. Compound contamination of heavy metals is one of the important soil contamination types. Therefore, it is critical to investigate the effects of combined heavy metal contaminants on soil.

MethodMorus alba seedlings were treated with different concentration gradients of Pb²⁺ and Cd²⁺ separately and in combination, to investigate the effects of heavy metals on growth indices, the photosynthetic pigment content and its photosynthesis index in leaves of Morus alba.

ResultPhotosynthetic pigment of Morus alba leaves decreased significantly under Pb²⁺ single stress. Chlorophyll contents increased under low concentration and decreased under high concentration while carotenoids content decreased significantly under Cd²⁺ single stress. Inhibition exists in the effect of compound heavy stress on photosynthetic pigment content and is stronger than one of single stress. Light saturation point (LSP), light compensation point (LCP), maximum net photosynthetic rate (P_nmax), dark respiration rate (R_d), apparent quantum yield (φ) decreased as concentration of heavy metals increasing. Heavy metal stress all suppressed on net photosynthetic rate (P_n), intercellular CO₂ concentration (C_i), stomatal conductance (G_s) and transpiration rate (T_r) of mulberry leaves and inhibited leaf biomass and plant height.

Conclusion Both single and compound heavy metal stress all suppressed height growth, photosynthetic pigment content and photosynthesis index in leaves of Morus alba. The higher the concentration of heavy metals is, the stronger the inhibition on mulberry is. It shows a synergistic effect of Pb²⁺ and Cd²⁺ compound stress. Mulberry shows stronger resistance to heavy metal stress.