Restriction of mitochondrial oxidative metabolism leads to suppression of photosynthetic carbon assimilation but not of photochemical electron transport in pea mesophyll protoplasts

Abstract

The present study examines and establishes the primary effect of mitochondrial inhibitors, oligomycin, antimycin A and salicylhydroxamic acid (SHAM), on the photosynthetic carbon assimilation and photochemical electron transport activities, monitored in intact mesophyll protoplasts. These inhibitors caused a marked restriction of malate (+ glutamate)-dependent O2 uptake in mitochondria (53-73%) isolated from pea leaves. When mesophyll protoplasts were illuminated in the presence of mitochondrial inhibitors, there was a significant decrease ( > 45%) in HCO3--dependent O2 evolution, while the decrease in O2 evolution was marginal ( < 10%) in the presence of benzoquinone (BQ) (photosystem PSII-mediated) and NO2--(dependent on PSII + PSI) as electron acceptors. 3-(3.4-dichlorophenyl)-1,1-dimethylurea (DCMU), a typical photosynthetic inhibitor decreased drastically all the three reactions: HCO3- or BQ or NO2--dependent O2 evolution in mesophyll protoplasts. Our results indicate that mitochondrial oxidative metabolism (through both cytochrome and alternative pathways) is essential for maintenance of photosynthetic carbon assimilation, but not for PSI or PSII-dependent photochemical electron transport activities in mesophyll protoplasts of pea.