Genotypic differences in photosynthetic performance, antioxidant capacity, ultrastructure and nutrients in response to combined stress of salinity and Cd in cotton

Combined stress of salinity and heavy metal is a serious problem for crop production; however, physiological mechanisms of tolerance to such condition remain elusive in cotton. Here, we used two cotton genotypes differing in salt tolerance, to understand their response to salinity (NaCl) and cadmium (Cd) either alone or in combination (Cd + Na) via hydroponics. Results showed that salinity and/or Cd drastically reduced plant growth, chlorophyll content and photosynthesis, with greater effect observed in Zhongmian 41 (sensitive) than Zhong 9806 (tolerant). Although salinity and/or Cd induced malondialdehyde (MDA) accumulation in Zhongmian 41 at 5 and 10 days after treatment, MDA content remained unchanged in Zhong 9806, implying that Zhongmian 41 but not Zhong 9806 faced oxidative stress following exposure to salinity and/or Cd. Differential responses of antioxidant enzymes such as superoxide dismutase, guaiacol peroxidase, catalase and ascorbate peroxidase to Cd, NaCl and Cd + Na indicate genotype-and time course- dependent variations. In both genotypes, Cd content was decreased while Na concentration was increased under combined stress compared with Cd alone. Importantly, NaCl addition in Cd-containing medium caused remarkable reduction in Cd concentration, with the extent of reduction being also dependent on genotypes. The salt-tolerant genotypes had lower Na concentration than sensitive ones. Furthermore, obvious changes in leaf and root ultrastructure was observed under Cd, Na and Cd + Na stress, however Zhong 9806 was less affected compared with Zhongmian 41. These results may provide novel insight into the physiological mechanisms of Cd + Na stress tolerance in various cotton genotypes.