Imbalances between photosynthetic electron transport and CO2 assimilation in higher plants and algae

Abstract

The fluorescence parameter (Fq¿/Fm¿, often referred to as the Genty factor and previously defined by (Fm¿-Ft)/Fm¿, (Fm¿-Fs)/Fm¿ or DF/Fm¿) provides a good estimate of the relative quantum yield of PSII electron transport. It exhibits a strong quantitative relationship with CO2 assimilation in healthy leaves. An increasing number of reports suggest that the relationship between Fq¿/Fm¿ and CO2 assimilation breaks down when algae and leaves of higher plants are exposed to environmental stresses. In C4 plants grown at low temperatures, the increases in the ratio of electron transport:CO2 assimilation are associated with increases in the activities of enzymes associated with active O2 metabolism, consistent with an enhanced Mehler reaction rate. Increases in PSI:PSII could also result in an increased Fq¿/Fm¿ that is not associated with an increase in linear electron transport. Overcycling of the C4-acid cycle, operation of a Q cycle and onset of significant photorespiration could also contribute, but have been discounted as major factors. In intertidal microphytobenthic biofilms very large decreases in CO2 assimilation were observed during the afternoon in the absence of any major change in Fq¿/Fm¿. The surprisingly small decreases in Fq¿/Fm¿ observed on four-fold increases in PPFD when sun broke through the clouds were attributable to light-dependent vertical migration of some diatom species into the sediment. Shading of the migrating species maintained a high Fq¿/Fm¿ of the biofilm as PPFD increased, which was not associated with the predicted increase in electron transport. However, electron transport was considerably in excess of the requirements for CO2 assimilation.