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 of the common meadow weed Potentilla reptans show non-uniform ('patchy') chlorophyll fluorescence quenching in the early stages of fluorescence transients. These areas of bright fluorescence can be readily reproduced in detached leaves that are allowed to wilt on the laboratory bench in weak light. The extent and duration of the patchiness increases with increasing water stress (higher relative saturation deficits). Images captured during saturating flashes show that the patches also display slow development of non-photochemical quenching, consistent with the possibility that photosynthetic metabolism is impaired in these areas. Wilted Potentilla leaves readily regain full turgor when petioles are placed in water, and uniform chlorophyll fluorescence is recovered within 30 min. Epidermal impressions reveal closed stomata over areas of both low and high fluorescence in wilted leaves. Because 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 may isolate areas of the lamina, minor veins do not. Relationships to leaf anatomy are discussed.