Sulfur dioxide promotes germination and plays an antioxidant role in cadmium-stressed wheat seeds

Sulfur dioxide (SO2) is a widespread air pollutant and is toxic to both plants and animals. Here, we studied the physiological role of SO2 on wheat (Triticum aestivum L.) seed germination under cadmium (Cd) stress. Pretreatment with the SO2 donor, Na2SO3/NaHSO3 (3:1), could alleviate the inhibitory effect of Cd (CdCl2) stress on germination in wheat in a dose-dependent manner, with the optimal concentration between 1 and 2 mM. Pretreatment with the SO2 donor moderately increased the activities of amylase and esterase and caused higher levels of reducing sugars and soluble protein in germinating wheat seeds under Cd stress. The overproduction of malondialdehyde, hydrogen peroxide and the superoxide anion, and damages on plasma membrane integrity of the radicle tips of wheat seedlings under Cd stress were reduced by pretreatment with the SO2 donor. Further investigation showed that pretreatment with SO2 donor led to significantly higher activities of guaiacol peroxidase, ascorbate peroxidase, superoxide dismutase, catalase and lower level of lipoxygenase in germinating wheat seeds. A higher level of H2S was also observed in SO2 donor-pretreated wheat seeds compared with water-pretreated controls. Our data indicate that SO2 plays a role in the antioxidant mechanism that protects germinating wheat seeds against Cd stress.