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

Abstract

The CO2/-, H2O- and 16O2/18O2 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 CO2 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 O2 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 CO2 uptake and transpiration by wild-type and aurea mutant leaves. The rate of gross 16O2 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 18O2 uptake by wild-type leaves increased during water stress when the photon fluence rate was high. Under the same conditions, the rate of gross 18O2 uptake by aurea mutant leaves remained unchanged. The physiological significance of this difference with respect to the (presumed) importance of oxygen reduction in photoprotection is discussed.