A leaf cuvette has been adapted for use with a pulse-modulation fluorometer and an open gas exchange system. Leaf water potential (psi) was decreased by withholding watering from Digitalis lanata EHRH. plants. At different stages of water deficiency the photochemical (q(Q)) and nonphotochemical (q(E)) fluorescence quenching was determined during the transition between darkness and light-induced steady state photosynthesis of the attached leaves. In addition, the steady state CO(2) and H(2)O gas exchange was recorded. Following a decrease of leaf water potential with increasing water deficiency, the transition of photochemical quenching was almost unaffected, whereas nonphotochemical quenching increased. This is indicative of an enhanced thylakoid membrane energization during the transition and is interpreted as a partial inhibition of either the ATP generating or the ATP consuming reaction sequences. Complete reversion of the stress induced changes was achieved within 6 hours after rewatering. In contrast to the variations during transition, the final steady state values of q(Q) and q(E) remained unchanged over the entire stress range from -0.7 to -2.5 megapascals. From these results we conclude that, once established, electron transport via photosystem II and the transmembrane proton gradient remain unaffected by water stress. These data are indicative of a protective mechanism against photoinhibition during stress, when net CO(2) uptake is limited.