EFFECTS OF O2 AND CO2 CONCENTRATION ON THE STEADY-STATE FLUORESCENCE YIELD OF SINGLE GUARD-CELL PAIRS IN INTACT LEAF-DISKS OF TRADESCANTIA-ALBIFLORA - EVIDENCE FOR RUBISCO-MEDIATED CO2 FIXATION AND PHOTORESPIRATION IN GUARD-CELLS

A procedure for following changes in the steady-state yield of chlorophyll a fluorescence (F(s)) from single guard cell pairs in variegated leaves of Tradescantia albiflora is described. As an indicator of photosynthetic electron transport, F(s) is a very sensitive indirect measure of the balance of adenosine 5'-triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), producing reactions with the sink reactions that utilize those light-generated products. We found that F(s) under constant light is sensitive to manipulation of ambient CO2 concentrations, as would be expected if either phosphoenolpyruvate carboxylase or ribulose-1, 5 bisphosphate carboxylase/oxygenase (Rubisco)-dependent CO2 fixation is the sink for photosynthetic ATP and NADPH in guard cells. However, we also found that changing O2 concentration had a strong effect on fluorescence yield, and that O2 sensitivity was only evident when the concentration of CO2 was low. This finding provides evidence that both O2 and CO2 can serve as sinks for ATP and NADPH produced by photosynthetic electron transport in guard cell chloroplasts. Identical responses were observed with mesophyll cell chloroplasts in intact leaves. This finding is difficult to reconcile with the view that guard cell chloroplasts have fundamentally different pathways of photosynthetic metabolism from other chloroplasts in C3 plants. Indeed, Rubisco has been detected at low levels in guard cell chloroplasts, and our studies indicate that it is active in the pathways for photosynthetic carbon reduction and photorespiration in guard cells.