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

Abstract

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 (Cc) 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 Cc, 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 (Fm -Fs)/Fm, 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 (Fm -F0)/Fm measured in dark-adapted leaves, was not affected by water deficits up to 60%.