3.5 Activity of NAD(P)H dehydrogenase (NDH)-dependent cyclic
electron flow in both species under salt stress
NDH cyclic pathway activity around PSI was assessed as the
post-illumination rise (PIR) of Fo Chl fluorescence was
monitored after switching off actinic light Essemine et al. (2016). The
magnitudes of PIR for SV and SA under both control and
salt stress conditions were displayed in Figure 6. Under normal
conditions, we observe more than two times higher NDH activity inSA than in SV (Figure 6). The results show as well an
increase in the NDH in leaves of SV plants endured 50 mM NaCl for
12 days by about 2.36 times (Figure 6). However, SA plants
exposed for the same time period (12 days) to 250 mM NaCl exhibited a
significant decrease (about 25%) in the NDH activity (Figure 6). This
is very likely attributable to the activation of PTOX in Spartina under
salt stress. Hence, the activity of PIR declines in SA in favour
of that of PTOX. This reflects the existence of an efficient competition
between these two pathways (PTOX and NDH) for the oxidation/reduction of
the PQ pool, respectively. Eventually, the oxidation of the PQ pool by
PTOX overcomes its re-reduction by NDH cyclic (Figs. 6 and 8). So far,
PTOX may represent an alternative pathway to cyclic and linear routes
for the protection of SA against intersystem over-reduction and
minimize or avoid damages to both photosystems (PSI and PSII). Thereby,
it may function as a safety valve for the photosynthetic transport
chain. In this regard, our findings are in line with that of Ahmad et
al. (2012), where authors have shown a decrease in the PIR in tobacco
overexpressing PTOX from Chlamydomonas reinhardtii (Cr-PTOX)
compared to WT (Ahmad et al., 2012) and they demonstrated that the
decrease in PIR is attributed to the enhanced activity of PQ pool
oxidation by the high level of PTOX protein in the over-expressed line.