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Effect of water deficit on photosynthetic oxygen exchange measured using 18 O 2 and mass spectrometry in Solanum tuberosum L. leaf discs

Christophe Tourneux and Gilles Peltier
Planta
Vol. 195, No. 4 (1995), pp. 570-577
Published by: Springer
Stable URL: http://www.jstor.org/stable/23383313
Page Count: 8
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Effect of water deficit on photosynthetic oxygen exchange measured using
          18
          O
          2
          and mass spectrometry in Solanum tuberosum L. leaf discs
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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-1 CO2) and under an irradiance of 390 μmol photons·m-2·s-1, 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 water-stressed 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%.

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