Exchange of oxygen and its role in energy dissipation during drought stress in tomato plants

To elucidate how excess light energy is dissipated during water deficit, net photosynthesis (P N ), stomatal conductance (g s ), intercellular CO 2 concentration (c;) and Chl a fluorescence were investigated in control and drought-stressed tomato plants (Lycopersicon esculentum). Gross O 2 evolution (E o ) and gross O 2 uptake (U o ) were determined by a mass spectrometric 16 O 2 / 18 O 2 isotope technique. Under drought stress P N , g s , c i and U o 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, E o decreases under water deficit. The ratio P N /E o falls under stress while the ratio U o /E o 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 E o as well as P N and U o . Despite reduced charge separation at PSII, the decline of CO 2 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 U o rises relative to E o .