Intercellular strands associated with stomata: Stomatal pectic strands

Summary Pectic strands are shown to connect the lower stomatal ledges and to develop across the posterior chamber of a number of plant species. Similar strands are formed between guard cells and subsidiary cells, and between epidermal cells.     

Mechanical stabilization of guard cell protoplasts of Vicia faba

Guard cell protoplasts of Vicia faba were immobilized in cross-linked Ca-alginate. No visible morphological changes were detected under the light microscope over a period of 14 days. The entrapped cells reacted normally to changes of the external osmolarity by shrinking and swelling. Addition of the calcium complexing agent, citrate, led to dissolution of the matrix. After reequilibration with Ca ions the released cells regained their ability to swell and shrink in response to external stress. The released protoplasts could be stained with the vital dye, neutral which was accumulated in the vacuoles. It should also be noted that the protoplasts can be transported when immobilized.     

Histological examination of spruce needles from a long-term gas exchange experiment in pure and polluted air in the field

Summary At the end of a 4-year period of gas exchange measurements in a natural stand in the Lower Bavarian Forest, needles of an adult spruce [Picea abies (L.) Karst.] were harvested from two chambers, one with pure air and the other with ambient air. The needles were examined as to their histological properties in the stomatal apparatus and in the bundle sheath. In needles from the polluted air UV absorbance at 280 nm was decreased in the walls of the stomatal apparatus. Simultaneously, the deposition of compounds with an absorption maximum at 310 nm increased within the encrusted plate-like thickenings of the subsidiary cells. The contents of the lumina of hypodermal cells and of the bundle sheath exhibited a greater degree of autofluorescence in ambient-air material than in pure-air leaf organs. Differences between needles exposed to pure and polluted air are gradual. The damaged condition is rare in pure air, common in polluted air. The needles from outside the chambers occupied an intermediate position between pure-air and ambient-air needles. This fact is traced to an unnaturally high pollutant load in the liquid phase of the needle surfaces within the ambient-air chamber because in order to compensate pollutant losses within the system, SO₂ and O₃ were added even during periods of irrigation. The reduction of absorption capacity at 280 nm in the walls of the stomatal apparatus is attributed to destruction of lignin due to the high reactivity of the pollutants in the liquid phase on the damp needle surface. The importance of delignification with regard to hydroregulation is discussed.     

Stomata and plasmodesmata

Summary In developing epidermal tissue ofPhaseolus vulgare L. complete plasmodesmatal connections occurred between guard cells and epidermal cells and between sister guard cells of a stoma but they were not seen in fully differentiated tissue. However, incomplete, aborted plasmodesmata were occasionally seen in the common guard/epidermal cell wall, usually connected to the epidermal cell protoplast, in mature tissue. Plasmodesmatal connections between neighbouring epidermal cells were commonly observed in tissue at all stages of development. In all locations, the plasmodesmata were usually unbranched occurring singly or in small pit fields; very rarely branched, incomplete plasmodesmata were also seen in the wall between mature guard and epidermal cells. The significance of these findings were related to stomatal functioning and to the development of plasmodesmata in general.     

Inhibition of stomatal opening during uptake of carbohydrates by guard cells in isolated epidermal tissues

The uptake of glucose and other carbohydrates into the guard cells of Commelina communis L. was found to inhibit the opening of the stomata. The concentration of glucose necessary to achieve about 50% inhibition was of the same order of magnitude as the potassium concentration required for opening; the uptake systems for potassium and glucose appear to be competitive and to exhibit the same degree of affinity. It is suggested that the uptake of glucose occurs via a proton cotransport, which, depolarizing the membrane potential, slows down the electrogenic import of potassium ions. The process of stomatal closure, in contrast, appears not to be affected by carbohydrate uptake. In guard cells of Tulipa gesneriana L. and Vicia faba L., which do not possess subsidiary cells, import of glucose or other carbohydrates did not interfere with the regulation of stomatal movements.     

The effect of Cl- upon the sensitivity of starch-containing and starch-deficient stomata and guard cell protoplasts towards potassium ions,fusicoccin and abscisic acid

Chloride ions are necessary to compensate for the positively charged potassium ions imported into guard cells of Allium cepa L. during stomatal opening. Therefore an external Cl^- supply of intact Allium plants is important. But high levels of chloride have been found to reduce the sensitivity of the starch-lacking stomata and isolated guard cell protoplasts (GCPs) from Allium to potassium ions, fusicoccin and abscisic acid. Furthermore, with high levels of chloride, malate anions disappear from the guard cells of Allium, a finding which contrasts with situation in Vicia where the stomatal sensitivity to K⁺ ions, fusicoccin and ABA is not influenced by Cl^- ions and malate levels are unaffected. It is suggested that the absence of malate as a proton yielding primer inhibits the mechanism of H⁺/K⁺ exchange in Allium.Abbreviations ABA abscisic acid - FC fusicoccin - GCPs guard cell protoplasts     

Control of transpiration in three coffee cultivars: the role of hydraulic and crown architecture

Water use and hydraulic architecture were studied in the coffee (Coffea arabica) cultivars San Ramon, Yellow Caturra and Typica growing in the field under similar environmental conditions. The cultivars differed in growth habit, crown architecture, basal sapwood area and total leaf surface area. Transpiration per unit leaf area (E), stomatal conductance (g _s), crown conductance (g _c), total hydraulic conductance of the soil/leaf pathway (G _t) and the stomatal decoupling coefficient, omega (Ω) (Jarvis and McNaughton 1986) were assessed over a range of soil moisture and during partial defoliation treatments. The relationship between sap flow and sapwood area was linear and appeared to be similar for the three cultivars. Variation in g _c, E, and G _t of intact plants and leaf area-specific hydraulic conductivity (k _l) of excised lateral branches was negatively correlated with variation in the ratio of leaf area to sapwood area. Transpiration, g _c, and g _s were positively correlated with G _t. Transpiration and G _t varied with total leaf area and were greatest at intermediate values (10 m²) of leaf area. Omega was greatest in Yellow Caturra, the cultivar with the greatest leaf area and a dense crown, and was smallest in Typica, the cultivar with an open crown. Differences in omega were attributable primarily to differences in leaf boundary layer conductance among the cultivars. Plants of each cultivar that were 40% defoliated maintained sap flows comparable to pretreatment plants, but expected compensatory increases in g _s were not consistently observed. Despite their contrasting crown morphologies and hydraulic architecture, the three cultivars shared common relationships between water use and hydraulic architectural traits. Received: 17 February 1999 / Accepted: 28 July 1999     

Susceptibility of the guard-cell K+-uptake channel KST1 to Zn2+ requires histidine residues in the S3-S4 linker and in the channel pore

 Potassium channels are inhibited by several mono- and divalent cations. To identify sites involved in the interaction between K⁺ channels and cationic effectors, we expressed the potato (Solanum tuberosum L.) guard-cell K⁺-uptake channel KST1 in Xenopus oocytes. This channel was reversibly blocked by extracellular Zn²⁺ in the micromolar range. In the presence of this heavy metal, steady-state currents were reduced in a pH-dependent but voltage-independent manner. Since Zn²⁺-inhibition was less effective at elevated external proton concentrations, we generated alanine mutants with respect to both extracellular histidines in KST1. Whereas substitution of the pore histidine H271 resulted in a reduced blockade by Zn²⁺, the channel mutant KST1-H160A in the S3-S4 linker lost most of its Zn²⁺ sensitivity. Since both histidines alter the susceptibility of KST1 to Zn²⁺, the block may predominantly result from these two sites. We thus conclude that the S3-S4 linker is involved in the formation of the outer pore. Received: 3 May 1999 / Accepted: 8 July 1999