Regulation of electron transport in maize mesophyll chloroplasts: The relationship between chlorophyll a fluorescence quenching and O2 evolution.

The relationship between the redox state of the primary electron acceptor of photosystem II (QA) and the rate of O2 evolution in isolated mesophyll chloroplasts from Zea mays L. is examined using pulse-modulated chlorophyll a fluorescence techniques. A linear relationship between photochemical quenching of chlorophyll fluorescence (qQ) and the rate of O2 evolution is evident under most conditions with either glycerate 3-phosphate or oxaloacetate as substrates. There appears to be no effect of the transthylakoid pH gradient on the rate of electron transfer from photosystem II into QA in these chloroplasts. However, the proportion of electron transport occurring through cyclic-pseudocyclic pathways relative to the non-cyclic pathway appears to be regulated by metabolic demand for ATP. The majority of non-photochemical quenching in these chloroplasts at moderate irradiances appeared to be "energy"-dependent quenching.