Targets of nitric oxide (NO) during modulation of photosystems in pea mesophyll protoplasts: studies using chlorophyll a fluorescence

Plants adapt to the environmental stresses by using the signalling molecules, such as H2O2 and nitric oxide (NO). We have assessed the NO-induced changes by using sodium nitroprusside (SNP, a NO donor) in photosynthetic components in protoplasts of pea (Pisum sativum) by measuring chlorophyll (Chl) a fluorescence induction kinetics. On exposure to SNP during preillumination, their photochemical activities were severely affected. There was a decrease of 40% in O-2 evolution by 5 min and complete inhibition by 20 min. The patterns of photosynthesis in absence of NO did not vary much over 20 min. Chl a fluorescence transient analysis and the energy pipeline models demonstrated that NO decreased the rates and efficiency of photochemical reactions. Presence of NO lowered the photosynthetic performance index and increased the dissipation per reaction centre with the time. Our results suggest that NO inactivates the PSII by targeting both the donor and acceptor sides of PSII.