Light Quality and Osmoregulation in Vicia Guard Cells

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 Recent characterization of a blue light-dependent proton pump at the guard cell plasmalemma has linked light perception with the activation of ion transport mechanisms providing guard cells with the capacity for a quantitative transduction of prevailing light fluences into adequate ionic uptake for osmoregulation (2, 37). In addition to the blue light photosystem, guard cells also possess Chl in their chloroplasts, which acts as a separate photoreceptor to induce stomatal opening (9,12,33). Blue lightinduced stomatal opening is the result of combined responses of the two photoreceptor systems. Blue light-dependent opening has been correlated with proton pumping (2, 37), K' uptake (12), starch degradation (22), and malate biosynthesis (23). In Vicia faba, blue light-dependent opening in epidermal peels saturates at 25 umol m-2 s-', is inhibited by KCN but not by DCMU at intensities 0Iusmol m-2 s-', and is partially sensitive to DCMU at intensities 225 umol m-2 s-' (33). Red light-induced stomatal opening relies on sensory transduction mediated by Chl. In Vicia, red light-dependent stomatal opening in peels saturates at 50 ,umol m-2 s-, is insensitive to KCN, and is completely inhibited by DCMU (33). Red light-induced stomatal opening has been correlated with oxygen evolution and photophosphorylation (36,38) and with synthesis of intermediates of the PCRP3 (11).The interconversion of photosynthetically produced sugars and starch was a fundamental hypothesis of stomatal movements that dominated plant physiology p6rior to its abandonment in favor of the K+ hypothesis (16,21

The Arabidopsis outward K + channel GORK is involved in regulation of stomatal movements and plant transpiration

“…The available information indicates that the dominant solutes involved in guard cell osmoregulation and control of stomatal aperture are K ϩ salts and sugars (mainly sucrose), depending on the environmental conditions and the time of the day (4,29). Genetic tools allowing the assessment of the relative contribution of these solutes and of the mechanisms responsible for their transport and accumulation are highly needed.…”

Rubisco Activity in Guard Cells Compared with the Solute Requirement for Stomatal Opening

“…Using the data of Gotow et al (4), we arrived at a similar conclusion for guard cells of V. faba (Appendix). There exists no evidence to support the conjecture that sugars produced by guard-cell photosynthesis contribute significantly to the modulation of osmotic pressure in the stomata of P. sativum or V. faba as was recently put forth again by Tallman and Zeiger (25). There is also no quantitative basis for claiming that "PGA produced by guard cell protoplasts could supply all the carbon needs for starch synthesis and obviate a dependency on imported sugars" (4).…”

The guard cell chloroplast: a perspective for the twenty-first century

“…Guard cell photosynthesis is DCMUsensitive while the Potassium Cyanide (KCN)-sensitivity of the blue light response implicates oxidative phosphorylation as the energy source (Schwartz & Zeiger, 1984). As discussed earlier, guard cell photosynthesis and the blue light response are also associated with different osmoregulatory pathways: Red light stimulates sucrose accumulation in the absence of potassium uptake or starch degradation, whereas blue light stimulates potassium and chloride uptake, malate synthesis and starch hydrolysis (Tallman & Zeiger, 1988;Talbott & Zeiger, 1993).…”

“…These shifts in photosynthetic activity parallel metabolic shifts of guard cells elicited under blue and red light (Tallman & Zeiger, 1988;Poffenroth Review 417 et al , 1992;Talbott & Zeiger, 1993). Guard cells in peels irradiated with blue light, take up potassium and chloride, hydrolyze starch, and synthesize malate (photosynthesis-independent, osmoregulatory pathway).…”