We developed a new method using 13 CO 2 and mass spectrometry to elucidate the role of photorespiration as an alternative electron dissipating pathway under drought stress. This was achieved by experimentally distinguishing between the CO 2 fluxes into and out of the leaf. The method allows us to determine the rates of gross CO 2 assimilation and gross CO 2 evolution in addition to net CO 2 uptake by attached leaves during steady-state photosynthesis. Furthermore, a comparison between measurements under photorespiratory and non-photorespiratory conditions may give information about the contribution of photorespiration and mitochondrial respiration to the rate of gross CO 2 evolution at photosynthetic steady state. In tomato (Lycopersicon esculentum Mill. cv Moneymaker) leaves, drought stress decreases the rates of net and gross CO 2 uptake as well as CO 2 release from photorespiration and mitochondrial respiration in the light. However, the ratio of photorespiratory CO 2 evolution to gross CO 2 assimilation rises with water deficit. Also the contribution of re-assimilation of (photo) respiratory CO 2 to gross CO 2 assimilation increases under drought.Water deficit limits plant growth and productivity because it decreases net CO 2 assimilation due to reduced stomatal conductance for CO 2 and/or because of non-stomatal effects like inhibition of enzymatic processes by changes in ionic or osmotic conditions (Lawlor, 1995). At high light intensities the lowered consumption of redox equivalents in the Calvin cycle makes it necessary to degrade photosynthetic electrons in processes other than CO 2 fixation to avoid photo-inhibition. Sharkey et al. (1988) showed that the activity of photosystem II can be regulated in a way that the rate of electron transport matches the capacity of the electron consuming reactions and that linear electron transport depends not only on light intensity and CO 2 concentration but also on the O 2 concentration. Oxygen can function as alternative electron acceptor directly in the Mehler reaction or indirectly in photorespiration (Badger, 1985).By combined measurements of O 2 and CO 2 gas exchange it should be possible to investigate the distribution of photosynthetic electrons between the electron consuming reactions CO 2 assimilation, photorespiration, and Mehler reaction (Haupt-Herting, 2000). The influence of drought stress on photosystem II activity, gross O 2 evolution, and gross O 2 uptake in tomato (Lycopersicon esculentum Mill. cv Moneymaker) plants has been published elsewhere (Haupt-Herting and Fock, 2000). This paper deals with the corresponding carbon fluxes determined by a new CO 2 gas exchange method.

In a study on metabolic consumption of photosynthetic electrons and dissipation of excess light energy under water stress, O2 and CO2 gas exchange was measured by mass spectrometry in tomato plants using 18O2 and 13CO2. Under water stress, gross O2 evolution (E(O)), gross O2 uptake (U(O)), net CO2 uptake (PN), gross CO2 uptake (TPS), and gross CO2 evolution (Ec) declined. The ratio P(N)/E(O) fell during stress, while the ratios U(O)/E(O) and E(C)/TPS rose. Mitochondrial respiration in the light, which can be measured directly by 12CO2 evolution during 13CO2 uptake at 3000 microl l(-1) 13CO2, is small in relation to gross CO2 evolution and CO2 release from the glycolate pathway. It is concluded that PSII, the Calvin cycle and mitochondrial respiration are down-regulated under water stress. The percentages of photosynthetic electrons dissipated by CO2 assimilation, photorespiration and the Mehler reaction were calculated: in control leaves more than 50% of the electrons were consumed in CO2 assimilation, 23% in photorespiration and 13% in the Mehler reaction. Under severe stress the percentages of electrons dissipated by CO2 assimilation and the Mehler reaction declined while the percentage of electrons used in photorespiration doubled. The consumption of electrons in photorespiration may reduce the likelihood of damage during water deficit.

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.

under water deficit. The ratio P N /E o falls under stress while the To elucidate how excess light energy is dissipated during water deficit, net photosynthesis (P N ), stomatal conductance (g s ), ratio U o /E o increases. We conclude that tomato plants follow intercellular CO 2 concentration (c i ) and Chl a fluorescence a double strategy to avoid photodamage under drought stress conditions: (1) a substantial portion of light energy is emitted were investigated in control and drought-stressed tomato plants (Lycopersicon esculentum). Gross O 2 evolution (E o ) and as heat and PSII activity is downregulated. This results in a gross O 2 uptake (U o ) were determined by a mass spectrometric 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 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 lowered stomatal conductance and metabolic changes results in the need of degrading excessive photosynthetic electrons. (2) decreases under water deficit, non-photochemical quenching rises. The maximal efficiency of PSII measured in the dark is Oxygen is used as an alternative electron acceptor in photoresnot affected by drought; however, in the light, E o decreases piration or Mehler reaction and U o rises relative to E o .electron transport chain and damage to the photosynthetic apparatus (Aro et al. 1993). There are several mechanisms known to protect plants against photodamage, including the emission of surplus radiant energy as heat in parallel to decreased PSII activity (Horton et al. 1996) and the use of electron acceptors instead of CO 2 , such as oxygen in photorespiration or Mehler reaction (Park et al. 1996). Thermal dissipation of absorbed light energy, which can be determined by non-photochemical fluorescence quenching in relation to CO 2 gas exchange, is well established in studies with excessive irradiance and reduced CO 2 (Dietz et al. 1985, Krause andBehrend 1986). Drought also leads to an increase in non-photochemical quenching in leaves (Stuhlfauth et al. 1988, Scheuermann et al. 1991, Biehler et al. 1997. One factor contributing to non-photochemical quenching is DpH-dependent high energy quenching, which is correlated with a downregulation of PSII activity (Havaux et al. 1991, Horton et al. 1996. In some experiments,