Using (14)CO(2) gas exchange and metabolite analyses, stomatal as well as total internal CO(2) uptake and evolution were estimated. Pulse modulated fluorescence was measured during induction and steady state of photosynthesis. Leaf water potential of Digitalis lanata EHRH. plants decreased to -2.5 megapascals after withholding irrigation. By osmotic adjustment, leaves remained turgid and fully exposed to irradiance even at severe water stress. Due to the stress-induced reduction of stomatal conductance, the stomatal CO(2) exchange was drastically reduced, whereas the total CO(2) uptake and evolution were less affected. Stomatal closure induced an increase in the reassimilation of internally evolved CO(2). This ;CO(2) recycling' consumes a significant amount of light energy in the form of ATP and reducing equivalents. As a consequence, the metabolic demand for light energy is only reduced by about 40%, whereas net photosynthesis is diminished by about 70% under severe stress conditions. By CO(2) recycling, carbon flux, enzymatic substrate turnover and consumption of light energy were maintained at high levels, which enabled the plant to recover rapidly after rewatering. In stressed D. lanata plants a variable fluorescence quenching mechanism, termed ;coefficient of actinic light quenching,' was observed. Besides water conservation, light energy dissipation is essential and involves regulated metabolic variations.