SIMULTANEOUS CO2 AND O-16(2) O-18(2) 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 CO2-, H2O- and O-16(2)/O-18(2) isotopic-gas exchange and the fluor escence quenching by attached leaves of the wild-type and of the phyto chrome-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 alt ered, Nevertheless, the maximum rate of net CO2 uptake in air by the y ellow-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 indic ates a reduced rate of photosynthetic electron flux in aurea mutant le aves, No difference in both photochemical and nonphotochemical fluores cence quenching was found between wild-type and aurea mutant leaves, W ater stress was correlated with a reversible decrease in the rates of both net CO2 uptake and transpiration hy wild-type and aurea mutant le aves, The rate of gross O-16(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 O-18(2) uptake by wildtype leaves increased during water stress when the photon flue nce rate was high, Under the same conditions, the rate of gross O-18(2 ) uptake by aurea mutant leaves remained unchanged. The physiological significane of this difference with respect to the (presumed) importan ce of oxygen reduction in photoprotection is discussed.