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The ATPase activity present in plasmalemma-enriched preparations from maize coleoptiles shows an optimum at pH 6, a strong dependence on Mg2+, and is stimulated by K+ and other monovalent cations, both organic and inorganic. The activation of ATPase by K+ obeys Michaelis Menten kinetics, saturation being reached at 50 mM K+ concentration. K+, Mg2+-stimulated ATPase activity is strongly inhibited by N,N'-dicyclohexylcarbodiimide and by diethylstilbestrol and, to a lesser extent, by octylguanidine.

Ophiobolin B (OPH B), a sesterpene metabolite of Helminthosporium oryzae, inhibits proton extrusion from maize coleoptiles. Moreover OPH B counteracts the biological activity of fusicoccin (FC), another terpenoid toxin produced by Fusicoccum amygdali having a similar basic chemical structure: OPH B inhibits FC-promoted proton extrusion, potassium uptake and cell enlargement.

We studied the recovery of ATP from soil. A soil-water suspension was prepared by two different methods (simple stirring or ballottini mill treatment) at different pH levels and in the presence of different chemicals [Na2SO4, Na3PO4, Na5P3O10, adenosine, ethylenediaminetetra-acetic acid (EDTA), TRIS]. The ATP recovery was evaluated by adding [3H]-8' ATP to the solution and comparing the values obtained by radioactivity measurements with those obtained by an enzymatic assay. Strongly acidic (pH lower than 1.0) or alkaline (pH 10.0) extractions yielded the best ATP recoveries compared with intermediate pH values. At pH 10.0, the addition of Na3PO4 or Na5P3O10 gave a high level of ATP recovery, 68 and 96%, respectively. No ATP hydrolysis occurred under alkaline extraction conditions. Under acidic extraction conditions, the addition of adenosine, EDTA, Na2SO4, or Na5P3O10 improved ATP recovery but it was never higher than 34%. The results were discussed in terms of the effects of different physical and chemical conditions on cell disruption, ATP stability, ATP interactions with soil components, and ATP solubilization.

Chromium ions affect the proton extrusion and the potassium uptake capacity in segments of maize seedling roots. The hypothesis that these effects can explain the chromium ions toxicity in plants is discussed.

Nickel affects the proton extrusion capacity in segments of maize seedling roots; as in the field condition, this effect is enhanced by phosphate and counteracted by Ca++ and Mg++. The effect of nickel on the proton-extruding activity can explain either nickel toxicity and its stimulating effect on plant growth. Such hypothesis is discussed in this paper.

Changes of glycolytic substrates level during ripening of the castor bean seed. — The changes of the concentration of carbohydrates and of the main glycolytic substrates in the castor bean seed during the ripening phase were investigated. The following results were obtained:The level of unphosphorylated sugars and of acid hydrolysable polysaccharides remains almost unchanged, with a tendency to a rise during the ripening phase. The slight increase of these compounds, together with the transition of the R. Q. from high to low values, might be interpreted as an indication of a shift of the seed from the a metabolism of fat synthesis to one of conversion of lipids into sugars, such as is observed in the germinating castor bean seeds.Hexose monophosphate level sharply decreases during the last period of maturation. However, the level of these substrates does not fall so low as to suggest a severe limitation for the pentose-P pathway activity.Fructose diphosphate, DOAP, GAP, 3 PGA, 2 PGA, PEP and pyruvate levels consistently increase during the ripening process. This indicates that the drop of oxygen uptake observed in this phase cannot be due to a lack of glycolytic substrates. On the other hand, the ratios between some substrates are shifted, during ripening, from values close to the theoretical equilibrium constants to quite different values. This finding, when correlated with the one of the strong decrease of the glycolytic flow, strongly suggests a severe inactivation of the glycolyting enzymes during ripening.The increase of pyruvate in tissues showing a decreasing respiratory activity indicates a fall of the oxidative capacity of mitochondria. This might be due to a lack of ADP, or other high energy bond acceptor, following a block of synthetic processes. However, no decrease of ADP level, and an increase of the ADP/ATP ratio during ripening is observed, Among the alternative hypothesis: a) lack or excess of oxalacetate; b) increase of concentration of some Krebs cycle inhibitor; c) inactivation of mitochondrial enzymes, the latter is thought most probable, in view of the finding of a sharp decrease of some other enzyme activities during ripening, of the above mentioned interpretation of the shift of the ratios between glycolytic substrates, and of the very low level of mitochondrial activity in preparation from the mature castor bean seed. These results when correlated with those from parallel investigations on the biochemistry of castor bean seed maturation and germination suggest, as a working hypothesis, that the respiratory metabolic inactivation accompanyng seed repening is due to a general block of the metabolism of ribonucleic acid and thus protein synthesis.