A New Mechanism for the Regulation of Stomatal Aperture Size in Intact Leaves (Accumulation of Mesophyll-Derived Sucrose in the Guard-Cell Wall of Vicia faba)

Abstract

At various times after pulse-labeling broad bean (Vicia faba 1.) leaflets with 14C0,, whole-leaf pieces and rinsed epidermal peels were harvested and subsequently processed for histochemical analysis. Cells dissected from whole leaf retained apoplastic contents, whereas those from rinsed peels contained only symplastic contents. Sucrose (Suc)-specific radioactivity peaked (1 11 CBq mol-') in palisade cells at 20 min. I n contrast, the 14C content and Sucspecific radioactivity were very low in guard cells for 20 min, implying little CO, incorporation; both then peaked at 40 min. The guard-cell apoplast had a high maximum Suc-specific radioactivity (204 CBq mol-') and a high Suc influx rate (0.05 pmol stoma-' min-'). These and other comparisons implied the presence of (a) multiple Suc pools i n mesophyll cells, (b) a localized mesophyllapoplast region that exchanges with phloem and stomata, and (c) mesophyll-derived SUC in guard-cell walls sufficient t o diminish stomatal opening by approximately 3 pm. Factors expected to enhance Suc accumulation in guard-cell walls are (a) high transpiration rate, which closes stomata, and (b) high apoplastic SUC concentration, which i s elevated when mesophyll Suc efflux exceeds translocation. Therefore, multiple physiological factors are integrated in the attenuation of stomatal aperture size by this previously unrecognized mechanism.For many years sugars played a prominent role in the explanations of stomatal movements. The classical theory (starch + sugar during stomatal opening, and vice versa) invoked an osmotic role for sugars within guard cells sufficient to create the requisite turgor for stomatal opening. This theory was based on the usual observation of a reciprocal relationship between guard-cell starch content and stomatal aperture size. For the lack of methods at the time, the theory was tested only semiquantitatively for starch and not at a11 for sugars. Upon the discovery that massive K+ accumulation in guard cells accompanies stomatal opening and that K+ loss from guard cells accompanies stomatal closure, the classical theory was discarded (for history, see Hsiao, 1976;Raschke, 1979; Outlaw, 1983 Poffenroth et al., 1992;Talbott and Zeiger, 1993), also working with V. fubu, reported that red light causes an increase in stomatal aperture size on epidermal peels and a decrease in guardcell qs without either an increase in guard-cell K+ concentration or a decrease in guard-cell starch content. Under other conditions, they found that stomata also open without an increase in guard-cell K+ concentration but with a loss of guard-cell starch content. They indicated that their data are not consistent with K' being the universal guardcell osmoticum, and they suggested, as additional osmotica, interna1 sugars arising from the PCRP or starch breakdown. Important to comparisons made with our work, they noted that stomata induced to open in intact leaves have a substantially higher K+ content than those of epidermal peels discussed above. In summary, the work discussed above a...

Acclimation to Fluctuating Light Impacts the Rapidity of Response and Diurnal Rhythm of Stomatal Conductance

“…This has also been described previously by transpiration (Lu et al, 1997;Outlaw 2003;Kang et al, 2007;Kelly et al, 2013;Lawson et al, 426 2014;Daloso et al, 2016). However, no significant acclimation of the slow decrease in A and 427 g s through the day by growth light intensity and pattern was observed in this experiment.…”

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

“…Initially proposed by Outlaw and coworkers (Outlaw and Manchester, 1979;Lu et al, 1995Lu et al, , 1997Ewert et al, 2000;Outlaw and De Vlieghere-He, 2001;Kang et al, 2007), Suc generated by mesophyll photosynthesis is uploaded to the phloem and transported away from sources to sinks driven by transpiration (Outlaw and De Vlieghere-He, 2001). Excess Suc (when photosynthesis is high) is carried toward the stomata by the apoplast, stimulating stomatal closure either through some signaling mechanism or by acting as an osmoticum (Lu et al, 1997;Outlaw, 2003;Kang et al, 2007;Kelly et al, 2013). Such a process could only occur over longer time scales, as high rates of photosynthesis are not associated with low g s ; however, decreases in g s often are seen toward the end of the diurnal period, despite environmental conditions being similar to morning conditions .…”

“…Over the diurnal period, a number of species display a decrease in g s and A that is not driven by decreases in light intensity or the temporal response of g s (Mott and Parkhurst, 1991;Allen and Pearcy, 2000;Mencuccini et al, 2000;Moriana et al, 2002;Dodd et al, 2006;de Dios et al, 2012), but the exact mechanism for this requires further investigation. As discussed earlier, sugar accumulation due to high A is believed to provide a long-term photosynthetic feedback on g s (Lu et al, 1995(Lu et al, , 1997Outlaw, 2003;Kang et al, 2007;Kelly et al, 2013), which also needs to be taken into account when considering the incorporation of temporal responses into models of stomatal behavior. Noe and Giersch (2004) proposed a model based on the assumption that the pool of sugars, resulting from the difference between the rate of sugar production by photosynthesis and their rate of export, increasingly inhibited A over the diurnal period.…”

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

“…The source of the osmotically active sucrose in guard cells from intact leaves also remains unclear. Possible sources are starch hydrolysis and photosynthetic carbon fixation within the gcc and import from the apoplast (Lu et al , 1997). Indirect evidence for all three sources has been reported (for a review, see .…”