A Compartmental Analysis of Abscisic Acid in Roots of Hordeum distichon

An efflux compartmental analysis of abscisic acid (ABA) in barleyroots revealed two compartments of ABA, a fast exchanging one(probably the cytoplasm) containing 3.63 µM ABA and aslowly exchanging compartment (probably the vacuoles) whichcontained 0.39 µM ABA.     

The Conductance of the Plasmalemma for CO2

Permeability coefficients (P_S values) for CO₂ of the plasmamembrane (PM) of the unicellular green algae Eremosphaera viridis,Dunaliella parva, and Dunaliella acidophila, and of mesophyllprotoplasts isolated from Valerianella locusta were determinedfrom ¹⁴CO₂ uptake experiments using the rapid separation ofcells by the silicone oil layer centrifugation technique. Theexperimental P_S values were compared with calculated numbersobtained by interpolation of Collander plots, which are basedon lipid solubility and molecular size, for D. parva cells,mesophyll protoplasts isolated from Spinacia oleracea, mesophyllcells and guard cells of Valerianella, and guard cell protoplastsisolated from Vicia faba. The conductivity of algal plasma membranes for CO₂ varies between0.1 and 9 ? 10^–6 m s^–1, whereas for the plasmalemmaof cells and protoplasts isolated from leaves of higher plantsvalues between 0.3 and 11 ? 10^–6 m s^–1 were measured.By assuming that these measurements are representative for plantsand algae in general, it is concluded that the CO₂ conductivityof algal PM is of the same order of magnitude as that of thehigher plant cell PM. P_s values of plasma membranes for CO₂are lower than those for SO₂, but are in the same order of magnitudeas those measured for H₂O. On the basis of these results itis concluded that theoretical values of about 3000 ? 10^–6m s^–1 believed to be representative for higher plant cells(Nobel, 1983) and which are frequently used for computer-basedmodels of photosynthesis, lack experimental confirmation andrepresent considerable overestimations. However, with severalsystems, including higher plant cells, the conductance of thePM for CO₂ was significantly higher in light than in darkness.This suggests that in light, additional mechanisms for CO₂ uptakesuch as facilitated diffusion or active uptake may operate inparallel with diffusional uptake. Key words: Conductivity, CO₂, permeability coefficient, photosynthesis, plasmalemma     

Tetcyclacis and Abscisic Acid-Sensitive Reduction of Extracellular Ferricyanide by Mesophyll Cells of Valerianella locusta and Lemna gibba

Electron transport across the plasma membrane of Valerianellalocusta mesophyll cells and intact fronds of Lemna gibba, inducedby 10^–3 M ferricyanide, was inhibited by tetcyclacis,an inhibitor regarded to be specific for cytochrome P-450 mono-oxygenases.The effect was dependent on the concentration of tetcyclacisand the duration of preincubation. The apparent rate of trans-membraneelectron transport increased in the presence of catalase, indicatingtetcyclacis-induced H₂O₂-production or additional tetcyclacis-independentH₂O₂ release. The findings suggest an interaction of cytochromeP-450 with the plasma membrane-located electron transport chain.This redox-chain could be involved in the degradation of abscisicacid, being located at the plasma membrane. This assumptionis supported by the finding that ABA inhibits extracellularferricyanide reduction. Key words: Abscisic acid, cytochrome P-450 mono-oxygenase, plasma membrane, tetcyclacis