Simultaneous CO2- and 16O2/18O2-gas exchange and fluorescence measurements indicate differences in light energy dissipation between the wild type and the phytochrome-deficient aurea mutant of tomato during water stress

The CO 2 -, H 2 O- and 16 O 2 / 18 O 2 isotopic-gas exchange and the fluorescence quenching by attached leaves of the wild-type and of the phytochrome-deficient tomato aurea mutant was compared in relation to water stress and the photon fluence rate. The chlorophyll content of aurea leaves was reduced and the ultra-structure of the chloroplasts was altered. Nevertheless, the maximum rate of net CO 2 uptake in air by the yellow-green leaves of the aurea mutant was similar to that by the dark-green wild-type leaves. However, less O 2 was produced by the leaves of the aurea mutant than by leaves of the wild-type. This result indicates a reduced rate of photosynthetic electron flux in aurea mutant leaves. No difference in both photochemical and non-photochemical fluorescence quenching was found between wild-type and aurea mutant leaves. Water stress was correlated with a reversible decrease in the rates of both net CO 2 uptake and transpiration by wild-type and aurea mutant leaves. The rate of gross 16 O 2 evolution by both wild-type and aurea mutant leaves was fairly unaffected by water stress. This result shows that in both wild-type and aurea leaves, the photochemical processes are highly resistant to water stress. The rate of gross 18 O 2 uptake by wild-type leaves increased during water stress when the photon fluence rate was high. Under the same conditions, the rate of gross 18 O 2 uptake by aurea mutant leaves remained unchanged. The physiological significane of this difference with respect to the (presumed) importance of oxygen reduction in photoprotection is discussed.