Abstract
To elucidate how excess light energy is dissipated during water deficit, net photosynthesis (PN), stomatal conductance (gs), intercellular CO2 concentration (ci) and Chl a fluorescence were investigated in control and drought-stressed tomato plants (Lycopersicon esculentum). Gross O2 evolution (Eo) and gross O2 uptake (Uo) were determined by a mass spectrometric 16O/18O2 isotope technique. Under drought stress PN, gs, ci and Uo decline. While photochemical fluorescence quenching decreases under water deficit, non-photochemical quenching rises. The maximal efficiency of PSII measured in the dark is not affected by drought; however, in the light, Eo decreases under water deficit. The ratio PN/Eo falls under stress while the ratio Uo/Eo increases. We conclude that tomato plants follow a double strategy to avoid photodamage under drought stress conditions: (1) a substantial portion of light energy is emitted as heat and PSII activity is downregulated. This results in a decrease in Eo as well as PN and Uo. Despite reduced charge separation at PSII, the decline of CO2 assimilation because of lowered stomatal conductance and metabolic changes results in the need of degrading excessive photosynthetic electrons. (2) Oxygen is used as an alternative electron acceptor in photorespiration or Mehler reaction and Uo rises relative to Eo.
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