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.