Light Quality and Osmoregulation in Vicia Guard Cells

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Abstract
Osmoregulation in opening stomata of epidermal peels from Vicia faba L. leaves was investigated under a variety of experimental conditions. The K+ content of stomatal guard cells and the starch content of guard cell chloroplasts were examined with cobaltinitrite and iodine-potassium iodide stains, respectively; stomatal apertures were measured microscopically. Red light (50 micromoles per square meter per second) irradiation caused a net increase of 3.1 micrometers in aperture and a decrease of -0.4 megapascals in guard cell osmotic potential over a 5 hour incubation, but histochemical observations showed no increase in guard cell K+ content or starch degradation in guard cell chloroplasts. At 10 micromoles per square meter per second, blue light caused a net 6.8 micrometer increase in aperture over 5 hours and there was a substantial decrease in starch content of chloroplasts but no increase in guard cell K+ content. At 25 micromoles per square meter per second of blue light, apertures increased faster (net gain of 5.7 micrometers after 1 hour) and starch content decreased. About 80% of guard cells had a higher K+ content after 1 hour of incubation but that fraction decreased to 10% after 5 hours. In the absence of KCl in the incubation medium, stomata opened slowly in response to 25 micromoles per square meter per second of blue light, without any K+ gain or starch loss. In dual beam experiments, stomata irradiated with 50 micromoles per square meter per second of red light for 3 hours opened without detectable starch loss or K+ gain; addition of 25 micromoles per square meter per second of blue light caused a further net gain of 4.4 micrometers in aperture accompanied by substantial K+ uptake and starch loss. Comparison of K+ content in guard cells of opened stomata in epidermal peels with those induced to open in leaf discs showed a substantially higher K+ content in the intact tissue than in isolated peels. These results are not consistent with K+ (and its counterions) as the universal osmoticum in guard cells of open stomata under all conditions; rather, the data point to sugars arising from photosynthesis and from starch degradation as additional osmotica. Biochemical confirmation of these findings would indicate that osmoregulation during stomatal opening is the result of three key metabolic processes: ion transport, photosynthesis, and sugar metabolism.