3.6. SA, Ca2+ and NO rescue Ni2+-induced damage by up-regulating non-enzymatic antioxidants
The non-enzymatic antioxidants: Cys, NPTs and Pro act as barrier against metal toxicity; which were negatively regulated by Ni2+-stress (Fig. 6), while upon SA, CaCl2 and SNP addition, a sharp increment in the these antioxidants was noticed, which might be due to their increased activation after perceiving NO signal or de-novo synthesis (Sun et al., 2018). This increment in antioxidants was arrested after c-PTIO or/ and EGTA treatment which was even less than that of untreated control except Ni+SA+NO+EGTA treatment (Fig. 6), which could be due to their greater demand to (i) stabilize the tertiary structures of protein, (ii) synthesize proteins, (iii) synthesize GSH, (iv) Fe-S cluster of photosynthetic apparatus, and (v) synthesize phytochelatins (PCs), which decreases metal availability by binding with them (Foyer and Noctor, 2005) to counteract the Ni2+-stressed situation as was also reported by Singh et al. (2012) in Cu-stressed cyanobacteria. The increment in Pro accumulation under stress is an adaptive mechanism either to detoxify ROS by chelation or as an antioxidant (Zhang et al, 2008). The arresting of these antioxidants after c-PTIO or/ and EGTA addition, confirmed the role of Ca2+ and NO in regulating the SA induced antioxidant defense system to combat the Ni2+-toxicity.
Figure 7(A) reveals the correlation of growth with that of different physiological and biochemical parameters. DW is positively correlated with the biomass and the biomass had positive correlation with that of parameters related to photosynthesis, nitrogen metabolism (except GDH) and antioxidants, while negatively correlated with the oxidative stress markers, respiration and GDH. Among different extracted components that determined the significant variance among studied parameters, the maximum contribution is of F1 (87.48) followed by F2 (9.72) with cumulative contribution (97.21) of both factors. If we analyze the impact on the biomass with respect to treatments (Fig. 7B), Ni, Ni+SA+Ca+c-PTIO, and Ni+SA+c-PTIO+EGTA were found to be negatively correlated with the biomass, while remaining treatments were positively correlated or showed some varied results.