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 global concern and a severe danger to contemporary agriculture, causing inhabitation and impairing crop growth and development. However, cobalt (Co), a multifunctional non-essential micro-element, has an important role in improving pant growth and development under salinity stress. In the current study, impact of Coon the morphological, biochemical, nutritional, and metabolic changes of African marigold plants at two salinity levels were assessed. Two concentrations of Co (C1; 10mg/L and C2; 20mg/L) were applied as foliar application to the marigold plants under salinity (S1; 300mM and S2; 600mM) stress. The obtained results indicated that salinity substantially reduced the plant growth by negatively affecting the nutritional and metabolic profile. Whereas the C1 application profoundly 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, irrespective of the severity of the salinity. Interestingly, the overall results revealed that C1 as an ideal dose for marigold plants under salinity stress since C2 application showed some toxicity symptoms under both the salinity levels and drastically reduced the plant growth and development. Furthermore, in the current study, two different oxidative stress scavenging pathways are discussed i.e. ROS-scavenging by ascorbate-glutathione (AsA-GSH) cycle and MG-detoxification by glyoxalase. With the limitation to glutathione (GSH) pool size and redox homeostasis, the current findings indicated that GSH played a critical role in both ROS- and MG-detoxification in marigold plants at both salinity levels.