Importance of oxidative electron transport over oxidative phosphorylation in optimizing photosynthesis in mesophyll protoplasts of pea (Pisum sativum L.)

Abstract

The role of mitochondrial respiration in optimizing photosynthesis was assessed in mesophyll protoplasts of pea (Pisum sativum L., cv. Arkel) by using low concentrations of oligomycin (an inhibitor of oxidative phosphorylation), antimycin A (inhibits cytochrome pathway of electron transport) and salicylhydroxamic acid (SHAM, an inhibitor of alternative oxidase). All three compounds decreased the rate of photosynthetic O2 evolution in mesophyll protoplasts, but did not affect chloroplast photosynthesis. The inhibition of photosynthesis by these mitochondrial inhibitors was stronger at optimal CO2 (1.0 mM NaHCO3) than that at limiting CO2 (0.1 mM NaHCO3). We conclude that mitochondrial metabolism through both cytochrome and alternative pathways is essential for optimizing photosynthesis at limiting as well as at optimal CO2. The ratios of ATP to ADP in whole protoplast extracts were hardly affected, despite the marked decrease in their photosynthetic rates by SHAM. Similarly, the decrease in the ATP/ADP ratio by oligomycin or antimycin A was more pronounced at limiting CO2 than at optimal CO2. The mitochondrial oxidative electron transport, through both cytochrome and alternative pathways, therefore appears to be more important than oxidative phosphorylation in optimizing photosynthesis, particularly at limiting CO2 (when ATP demand is expected to be low). Our results also confirm that the alternative pathway has a significant role in contributing to the cellular ATP, when the cytochrome pathway is limited.