Exogenous application of glycine betaine alleviates cadmium toxicity in super black waxy maize by improving photosynthesis, the antioxidant system and glutathione-ascorbic acid cycle metabolites

This study was conducted to examine how glycine betaine (GB) ameliorates the toxic effects of cadmium (Cd) in super black waxy maize. In a Cd toxicity test, biomass accumulation in maize seedlings decreased in comparison with those in the control group (CK). The maize plants exposed to Cd stress exhibited reductions in photosynthetic parameters [net photosynthetic rate (Pn), stomatal conductance (Gs), intracellular CO2 concentration (Ci) and transpiration rate (E)], chlorophyll content and maximum photosynthetic efficiency (Fv/Fm). However, the exogenous application of GB alleviated the negative effects of Cd stress and increased the biomass, photosynthetic parameters (Pn, Gs, Ci, E), chlorophyll content and Fv/Fm of the seedlings. In addition, Cd can decrease the activity of antioxidant enzymes and ascorbic acid (AsA)-glutathione (GSH) cycle enzymes, such as superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase and glutathione reductase, but the activity of these enzymes increased following GB addition. Cd also increased markedly the content of reactive oxygen species and malondialdehyde, but supplementation with GB significantly decreased their content. Furthermore, the levels of AsA and GSH decreased under treatment with Cd, but exogenous GB application increased the content of these compounds. This study shows that GB can alleviate Cd toxicity and provides a theoretical basis for super black maize resistance to heavy metal stress.