Alleviating cadmium toxicity in maize plants: role of glycine betaine in enhancing growth, photosynthetic efficiency, water status, and antioxidant defense mechanism

The issue of heavy metals (HMs) contamination poses a significant challenge in the environment, exerting a severe impact on the growth and productivity of crops. Cadmium (Cd) is specifically identified as the seventh heavy metal among the top 20 pollutants, primarily due to its elevated phytotoxicity and its solubility in water. In the current study, foliar application of glycine betaine (GB) (500 µmol) investigated the toxic effects of cadmium in maize plants subjected to two Cd concentrations (50 and 100 µmol) as CdCl2. The maize plants exposed to Cd stress exhibited a massive reduction in growth, biomass, photosynthetic pigments [chlorophyll a (Chl a), chlorophyll b (Chl b), carotenoids, and total pigments], gas exchange parameters [transpiration rate (Tr), net photosynthetic rate (Pn), intracellular CO2 concentration (ci), and stomatal conductance (gs)], relative water content (RWC), and organic osmolytes content [total soluble protein (TSS), and total soluble sugar (TSS)]. These impacts were significant with the 100 µmol CdCl2 treatment. Moreover, Cd led to remarked increase in proline, nonenzymatic antioxidants levels [ascorbic acid (AsA) and glutathione (GSH)] as well as the activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR). On the other hand, GB application efficiently relieved the Cd toxic impacts on maize and maintained higher growth criteria, gas exchange parameters, photosynthetic pigments, RWC, and organic osmolytes. In addition, the exogenous application of GB added more enhancement to the antioxidative system (enzymatic and nonenzymatic). These results imply that GB could significantly preserve maize growth under Cd toxicity conditions by maintaining photosynthetic characteristics, water status, and antioxidant system. This suggests an enhancement in the plant's resilience to stress induced by heavy metals.