Soil applied glycine betaine with Arbuscular mycorrhizal fungi reduces chromium uptake and ameliorates chromium toxicity by suppressing the oxidative stress in three genetically different Sorghum (Sorghum bicolor L.) cultivars

Background Chromium is the most toxic pollutant that negatively affects a plant's metabolic activities and yield. It reduces plant growth by influencing the antioxidant defence system's activities. In the present study, a completely randomized block design experiment with three plants/pot in three replication was conducted on three varieties of sorghum viz. SSG 59-3, HJ 513 (multi-cut) and HJ 541 (single-cut) for amelioration of chromium toxicity (2 & 4 ppm) by exogenous application of GB (50 & 100 mM) with and without AMF in soil. The ameliorative effects were tested at two growth stages viz. vegetative (35 DAS) and grain filling (95 DAS), in terms of Cr uptake, grain yield, antioxidative defence system parameters (viz. enzymes - SOD, APX, CAT, GR, POX and metabolites - proline, glutathione, ascorbate, beta-carotene) and indices of oxidative stress parameters (viz. PPO, H2O2, and MDA). Results The results delineated that Cr uptake and indices of oxidative stress were increased with increasing concentration of Cr stress in all the varieties (HJ 541, HJ513 & SSG 59-3) at both the growth stages (35 & 95 DAS). At higher concentration (4 ppm), Cr stress decreased the grain yield (45-50%) as compared with controls. Polyphenoloxidase activity, MDA and H2O2 content increased at both growth stages in all the varieties. However, antioxidative enzymes and metabolite activities increased due to Cr stress but this increase was not sufficient to counteract with ROS generated under Cr stress which was enhanced on the application of AMF and GB either individually or in combination (spiked in soil). It decreased the indices of oxidative stress and ameliorated the Cr toxicity and increased grain yield (65-70%) in all the varieties. Conclusions Both GB and AMF improved the antioxidative activities and stress tolerance capacity of the plant. Glycine betaine at both 50 and 100 mM level, significantly ameliorated Cr toxicity. However, AMF concomitantly with GB further boosts up the amelioration behaviour of the plant against Cr toxicity, at both growth stages in all the varieties. The combination of 100 mM GB with 10 g AMF was observed most effective among all the treatments. Among the varieties, SSG 59-3 had the lowest chromium uptake, indices of oxidative stress, and highest antioxidative system's activity as compared to HJ 513 followed by HJ 541 variety. Thus AMF and GB either individually or in combination may be used to maintain plant yield attributes under Cr toxicity.