Dose-dependent regulation of morphological, physio-biochemical, nutritional, and metabolic responses by cobalt in Tagestes erecta L. plants exposed to salinity stress

Salinity is a major concern globally and causing reduction in crop growth and development thereby lowering food production. Interestingly, cobalt (Co), a multifunctional non-essential micro-element, has an important role in improving growth and development under salinity stress. In the current study, the effects of Co on the morphological, biochemical, nutritional, and metabolic changes of African marigold plants at two salinity levels were assessed. Two concentrations of Co (C1; 10 mg/L and C2; 20 mg/L) were applied as foliar application to the marigold plants under salinity (S1; 300 mM and S2; 600 mM) stress. The results indicated that salinity substantially reduced the growth by negatively affecting the nutritional and metabolic profile. Interestingly, the C1 application mitigated the salinity effects and improved all the studied parameters including vegetative, nutritional, biochemical and metabolites (amino acids, fatty acids, organic acids, sugars, carotenoids, flavonoids, and terpenoids) by reducing the reactive oxygen species (ROS) and methylglyoxal (MG)-induced oxidative stress, at two salinity levels. The overall results revealed that C1was an ideal dose for marigold plants undergoing salinity stress since C2 application showed some toxicity symptoms under both the salinity levels; with reduced growth and impaired development. Based on the observations, the two different oxidative stress scavenging pathways in marigold are discussed i.e. ROS-scavenging by ascorbate-glutathione (AsA-GSH) cycle and the MG-detoxification by glyoxalase. With the limitations imposed upon the glutathione (GSH) pool size and redox homeostasis, the study indicated that GSH played a critical role in both ROS- and MG-detoxification in marigold plants at both salinity levels.