Einfluss von Bor auf Membranpotential und Membranwiderstand von Elodea-Blattzellen und Cucurbita-Wurzelzellen

Available from: Hannover Univ. (Germany). Universitaetsbibliothek / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

SHORT-TERM EFFECTS OF BORON, GERMANIUM AND HIGH LIGHT-INTENSITY ON MEMBRANE-PERMEABILITY IN BORON DEFICIENT LEAVES OF SUNFLOWER

WOS: A1995TD52000002The effects of varied boron (B) supply (0.01-50 mu M) and light intensity (100-580 pmol m(-2) s(-1)) on plant growth, leaf symptoms and membrane permeability of leaves were studied in sunflower plants over a 10-day-period of growth in nutrient solution. Membrane permeability was measured by incubating isolated young leaves in aerated distilled water. Additional experiments showed the effect of short-term supply (20 min to 2 h) of B and germanium (Ge) on membrane permeability of B-deficient leaves incubated in distilled water. Low supply of B decreased shoot and root growth and caused brown-purple pigmentation of young leaves, particularly under high light intensities. Leakage of K+ from leaves increased with severity of B deficiency whereas, in B-sufficient leaves, leakage of solutes was low. Also leakage of phenolics, amino acids and sucrose was enhanced by B deficiency. Compared to B-sufficient leaves, the leakage from B-deficient leaves was 35-fold higher for K+, 45-fold higher for sucrose and 7-fold higher for phenolics and amino acids. Increases in solute leakage were marked in leaf areas with brown-purple pigmentation and particularly pronounced by increasing light intensity or by exposure of leaves to continuous light. In severely B-deficient leaves grown under high light intensity, treatment with B and Ge up to 1 000 mu M for 20 min resulted in an immediate decrease in K+ efflux to about the same levels as in B-sufficient leaves. Similar rapid decreases in K+ efflux were also found in B-deficient leaves when exposed to darkness for 16 h or 30 h before the leakage measurement. The results demonstrate a particular role for B in maintaining the integrity of plasma membranes. Boron presumably stabilizes the structure of the plasma membrane by complexing membrane constituents. It also has a protective effect on membrane constituents by complexing phenolics, so that oxidation of phenolics to highly toxic quinones and oxygen free radicals is prevented or limited. In these functions, boron seems to be to a large extent replaceable by germanium

Eliminating ‘life worth living’

This article argues for the elimination of the concept of life worth living from philosophical vocabulary on three complementary grounds. First, the basic components of this concept suffer from multiple ambiguities, which hamper attempts to ground informative evaluative and classificatory judgments about the worth of life. Second, the criteria proposed to track the extension of the concept of life worth living rest on unsupported axiological assumptions and fail to identify precise and plausible referents for this concept. And third, the concept of life worth living is not shown to serve any major evaluative or classificatory purpose besides those served by already available axiological concepts. By eliminating the concept of life worth living, philosophers will free themselves of the task of addressing ill-posed axiological questions and ground reflection about the worth of life on more rigorous conceptual foundations