Glyphosate uncouples gas exchange and chlorophyll fluorescence

Abstract

Changes in chlorophyll fluorescence have often been advocated as a sensitive biomarker of plant stress, assuming that any kind of plant stress serious enough to affect plant growth will also affect photosynthesis. Glyphosate affects photosynthetic electron transport indirectly by inhibiting sink processes. The question is how fast this inhibition can be observed on CO(2) assimilation and ultimately on chlorophyll fluorescence? Experiments measuring CO(2) assimilation, conductance and chlorophyll fluorescence using four Kautsky curve parameters on barley (Hordeum vulgare L.) exposed to increasing doses of glyphosate showed a total cessation of CO(2) fixation and conductance without significant changes in chlorophyll fluorescence. The decrease in CO(2) fixation and conductance was significant 1 day after spraying and corresponded well to the decrease in biomass 5-7 days after spraying. A total cessation of CO(2) assimilation can take place without affecting chlorophyll fluorescence. Hypotheses concerning what happens to the energy from the photosynthetic apparatus that is not used for CO(2) assimilation are discussed. The results question the use of chlorophyll fluorescence as a universal indicator of stress on photosynthetic processes. Also, they demonstrate that changes in gas-exchange parameters are more sensitive biomarkers for glyphosate toxicity compared with chlorophyll fluorescence.