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.