Reversible, water stress-induced non-uniform chlorophyll fluorescence quenching in wilting leaves of Potentilla reptans may not be due to patchy stomatal responses

After exposure to full sunlight under natural conditions, attached leaves o f the common meadow weed Potentilla reptans show non-uniform ("patchy") chl orophyll fluorescence quenching in the early stages of fluorescence transie nts. These areas of bright fluorescence can be readily reproduced in detach ed leaves that are allowed to wilt on the laboratory bench in weak light. T he extent and duration of the patchiness increases with increasing water st ress (higher relative saturation deficits). Images captured during saturati ng flashes show that the patches also display slow development of non-photo chemical quenching, consistent with the possibility that photosynthetic met abolism is impaired in these areas. Wilted Potentilla leaves readily regain full turgor when petioles are placed in water, and uniform chlorophyll flu orescence is recovered within 30 min. Epidermal impressions reveal closed s tomata over areas of both low and high fluorescence in wilted leaves. Becau se highly fluorescent patches also persist when wilted tissues are exposed to high CO2 (i.e., patchiness is unlikely to be due to local differences in CO2 supply) the data suggest direct effects of water stress on metabolism in wilted leaves. Leaf transverse sections show that although major veins m ay isolate areas of the lamina, minor veins do not. Relationships to leaf a natomy are discussed.