Potassium Chloride as Stomatal Osmoticum in Allium cepa L., a Species Devoid of Starch in Guard Cells

Abstract

K+ and Clr contents of guard cells and of ordinary epidermal cells were determined in epidermal samples of Alium cepa L. by electron probe microanalysis; malate contents of the same samples were determined by enzymic oxidation. KCI was, in general, the major osmoticum in guard cells, irrespective of whether stomata had opened on leaves or in epidermal strips floating on solutions. lation ofP-enolpyruvate (14). P-enolpyruvate is very likely derived from starch (4). In guard cells of Viciafaba, the balance between potassium chloride and potassium malate was determined by the availability of C1-(17). Guard cells of V. faba contain chloroplasts filled with starch, but guard cells of Allium species possess only poorly developed chloroplasts (1, 10, 22, 23) which do not contain starch (1,2,8,22,24) or only traces ofit (11). Guard cells ofAllium species, therefore, appear to lack substrate for the production of the quantities of malate required for osmotic purposes. Possibly, these cells satisfy their anion requirement solely through import of C1-. Indeed, there are indications for a lack of involvement of malate metabolism in the stomatal movement of onion: epidermal strips of V. faba possess phosphorylase activity, those of Allium cepa do not (13,18,22). Aluminum ions inhibit stomatal opening in V. faba and other species containing starch in guard cells (21) Preparation and Incubation of Epidermal Samples. Leaves (about 10 cm long) were cut in the morning before the beginning of the light period and divided into sections (about 2 cm long) which were floated on distilled H20 in the dark. Samples of the lower epidermis were peeled from these sections in room light (from fluorescent tubes) and rubbed on their mesophyll side with a dissecting needle in distilled H20 in order to remove any adhering mesophyll cells. The strips were then rinsed and collected on deionized H20. For the determination of the initial conditions, some samples were immediately extracted for malate analysis or prepared for electron probe microanalysis after measurement of stomatal apertures. Other epidermal samples were transferred to small plastic dishes containing the incubation media. The dishes were placed under inverted crystallization dishes (150-mm diameter, 75-mm height) in the light (85 w/m2). A stream ofhumidified air was passed through the crystallization dishes at 50 liters/h.The air was either free of CO2 or contained about 320 pl C02/1.Epidermal samples were incubated in the light at 21 C for 4 h. The incubation media were buffered with Mes (10 mm, pH 5.6) and contained 0.09 to 0.11 eq K+/1 and 0 to 0.1 eq Cl-/i. Iminodiacetate (J. T. Baker Chemical Co.) was used to balance K+ not balanced by CF-. Indirect evidence indicates that guard cells do not absorb iminodiacetate (16,17). The solutions were prepared with double distilled H20. The estimated maximal concentration of Cl-resulting from impurities in the chemicals was 10-6 eq/l. No Cl-could be detected in water in contact with plastic dishes for 24 h (17).After incubation,...

Rethinking Guard Cell Metabolism

“…This starch-sugar hypothesis (Lloyd, 1908) was the accepted theory until the 1960s, when it was replaced by the K + -malate theory (Imamura, 1943;Fischer, 1968;Raschke, 1975) correlating stomatal opening with K + uptake, along with the counter ions malate 22 and/or Cl 2 (Allaway, 1973;Schnabl and Raschke, 1980;Outlaw, 1983). As outlined above, this became accepted as the main osmoregulatory pathway and often is still considered the exclusive mechanism for regulating stomatal aperture.…”

“…It seems, however, that the contribution of malate as a counter ion for K + depends on the availability of Cl 2 and can range between 50% and 90% of total cellular osmolytes (Raschke and Schnabl, 1978). In the extreme case of onion (Allium cepa), which lacks starch in guard cells, malate accumulation is not observed and Cl 2 is used as the exclusive counter ion (Schnabl and Raschke, 1980;Amodeo et al, 1996).…”

The Role of Phosphoenolpyruvate Carboxylase during C4 Photosynthetic Isotope Exchange and Stomatal Conductance

“…Studies with epidermal peels have demonstrated strong correlations between stomatal opening and malate accumulation in guard cells (Allaway, 1973;Pearson, 1973;van Kirk and Raschke, 1978) and with the use of a PEPC inhibitor (DCDP) it has also been shown that stomatal opening is restricted when PEPC activity is reduced in epidermal strips (Parvathi and Raghavendra, 1997). In epidermal strips, the importance of malate as a counter ion to K 1 is influenced by the availability of chloride (van Kirk and Raschke, 1978;Schnabl and Raschke, 1980;Willmer and Fricker, 1996). The low g s and epidermal PEPC content in the pp mutants show that maximum stomatal opening is dependent on PEPC activity and presumably malate in the guard cells in vivo.…”

CONTENIDO MINERAL Y DE CLOROFILA DE LA HEMIPARÁSITA Psittacanthus calyculatus (DC) G. Don Y DE CUATRO DE SUS ÁRBOLES HOSPEDEROS

“…Chlorophylls are very important for plants because, they allow plants to perform photosynthesis and stabilize proteins that bind and protect them from degradation. Reaction centers contain chlorophyll a, while light harvesting complex also que es usado como osmolito para atraer el flujo transpiracional hacia sus propias hojas (Schnabl & Rascke, 1980;Kelly & Horning, 1999). El análisis estadístico indica que U. divaricate acumula mayores cantidades de ciertos minerales que podrían ser usados en el estudio de situaciones especí-ficas, como la fitorremediación (Baxter, 2009).…”

“…Moreover, hemiparasitic plants prefer legumes as hosts because of the greater availability of N (Llugany et al, 2009); this could explain the relatively high protein content of leaf of Psittacanthus. According to the results shown in this study, P. calyculatus is a plant that efficiently extracts nutrients from their hosts, accumulating high levels of K, used as osmolyte to attract transpirational flow to their own leaves (Schnabl & Rascke, 1980;Kelly & Horning, 1999). The statistical analytatistical analysis indicates that U. divaricate accumulates higher amounts of certain minerals that could be used in the study of specific situations, such as phytoremediation (Baxter, 2009).…”