Effect of photorespiratory C2 acids on CO2 assimilation, PSII photochemistry and the xanthophyll cycle in maize

The photorespiration cycle plays an important role in avoiding carbon drainage from the Calvin cycle and in protecting plants from photoinhibition. The role of photorespiration is frequently underestimated in C-4 plants, since these are characterized by low photorespiration rates. The aim of this work was to study the relationship between CO2 assimilation, PS II photochemistry and the xanthophyll cycle when the photorespiratory cycle is disrupted in Zea mays L. To this end, the photorespiration inhibitor phosphinothricin (PPT) was applied individually or together with the photorespiratory C-2 acids, glycolate and glyoxylate to maize leaves. Application of PPT alone led to the inhibition of CO2 assimilation. Moreover, feeding with glycolate or glyoxylate enhanced the effect of PPT on CO2 assimilation. Our results confirm that the avoidance of the accumulation of the photorespiratory metabolites glycolate, glyoxylate or phosphoglycolate, is of vital importance for coordinated functioning between the glycolate pathway and CO2 assimilation. Relatively early changes in PS II photochemistry also took place when the photorespiratory cycle was interrupted. Thus, fluorescence photochemical quenching (qP) was slightly reduced (10%) due to the application of PPT together with glycolate or glyoxylate. A decrease in the efficiency of excitation-energy capture by open PS II reaction centres (F'v/F'm) and an increase in thermal energy dissipation (non-photochemical quenching, NPQ) were also measured. These observations are consistent with a limitation of activity of the Calvin cycle and a subsequent lower demand for reduction equivalents. The increase in NPQ is discussed on the basis of changes in the xanthophyll cycle in maize, which seem to provide a limited protective role to avoid photoinhibition when the glycolate pathway is blocked. We conclude that C-2 photorespiratory acids can act as physiological regulators between the photorespiratory pathway and the Calvin cycle in maize.