Effect of water deficit on photosynthetic oxygen exchange measured using 18O2 and mass spectrometry in Solanum tuberosum L. leaf discs

The effect of leaf dehydration on photosynthetic O₂ exchange of potato (Solanum tuberosum L., cv. Haig) leaf discs was examined using ¹⁸O₂ as a tracer and mass spectrometry. In normal air (350 μl·l^?1CO₂) and under an irradiance of 390 μmol photons·m^?2·s¹, a relative water deficit (RWD) of about 30% severely decreased net O₂ evolution and increased O₂ uptake by about 50%, thus indicating an enhancement of photorespiration. Increasing CO₂ concentrations diminished O₂ uptake and stimulated net O₂ evolution both in well-hydrated and in dehydrated (RWD of about 30%) leaves. Much higher CO₂ concentrations (up to 4%) were required to observe a complete effect of CO₂ in dehydrated leaves. The chloroplastic CO₂ concentration at the ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) level (C_c) was calculated from O₂-exchange data in both well-hydrated and dehydrated leaves, assuming that the specificity factor of Rubisco was unaffected by desiccation. When plotting net O₂ photosynthesis as a function of C_c, a similar relationship was obtained for well-hydrated and waterstressed leaf discs, thus showing that the main effect of water deficit is a decrease of the chloroplastic CO₂ concentration. At saturating CO₂ levels, the non-cyclic electron-transport rate, measured either as gross O₂ photosynthesis or as the chlorophyll fluorescence ratio (F_m -F_s)/F_m, was insensitive to water deficit, provided RWD was below 40%. In this range of RWD, the decrease in gross O₂ photosynthesis observed in normal air was attributed to the inability of oxidative processes to sustain the maximal electron-flow rate at low chloroplastic CO₂ concentration. The maximal efficiency of photosystem II, estimated as the chlorophyll fluorescence ratio (F_m -F₀)/F_m measured in dark-adapted leaves, was not affected by water deficits up to 60%.