Estímulo no crescimento e na hidrólise de ATP em raízes de alface tratadas com humatos de vermicomposto: i - efeito da concentração.

O vermicomposto contém uma concentração elevada de substâncias húmicas e já é bem conhecido o efeito do seu uso sobre as propriedades do solo. No entanto,a ação direta das substâncias húmicas sobre o metabolismo das plantas é menos conhecida. O objetivo deste trabalho foi avaliar o uso de humatos extraídos de vermicomposto de esterco de curral com KOH 0,1 mol L-1 sobre o desenvolvimento e metabolismo de ATP em plântulas de alface. Após a germinação, plântulas de alface foram tratadas com os humatos em concentrações que variaram de 0 a 100 mg L-1 de C, durante quinze dias. Foram avaliados o crescimento da raiz e a atividade das bombas de H+ isoladas da fração microssomal do sistema radicular. Foi observado aumento na matéria fresca e seca do sistema radicular, bem como no número de sítios de mitose, raízes emergidas do eixo principal, na área e no comprimento radiculares, com o uso do humato na concentração de 25 mg L-1 de C. Também foi observado, nessa concentração, aumento significativo na hidrólise de ATP pelas bombas de H+, responsáveis pela geração de energia necessária à absorção de íons e pelo crescimento celular

The plant Ca2+-ATPases repertoire : biochemical features and physiological functions

Ca2+ ATPases are P-type ATPases which use the energy of ATP hydrolysis to pump Ca2+ from the cytoplasm into intracellular compartments or into the apoplast. Plant cells possess two types of Ca2+ pumping ATPases, named respectively ECAs (for ER-type Ca2+ ATPase) and ACAs (for auto-inhibited Ca2+ ATPase). Each type comprehends different isoforms, localised on different membranes. Here we summarise the available knowledge of the biochemical characteristics and of the physiological role of plant Ca2+ ATPases, greatly improved after gene identification which allows both biochemical analysis of single isoforms through heterologous expression in yeast and expression profiling and phenotypic analysis of single isoform knock-out mutants

Acid-base regulation during nitrate assimilation in Hydrodictyon africanum

The acid-base balance during NO3 12 assimilation in Hydrodictyon africanum has been investigated during growth from (1) an analysis of the elemental composition of the cells, (2) the alkalinity of the ash and (3) the net H+ changes in the medium during growth. These investigations agree in showing that some 0.25 excess organic negative charges are generated per N assimilation from No3 12 as N-source and C02 as C-source; the excess OH 12 (0.75 OH 12 per NO3 12 assimilated) appears in the medium. Approximately half of the excess organic negative charge is attributable to cell wall uronates; the remainder is intracellular. All of the excess OH 12 appearing in the medium must have crossed the plasmalemma (as net downhill H+ influx or OH 12 efflux). Previous work has shown that the value of \u3c8co is more negative than \u3c8K+ during NO3 12 assimilation, suggesting that the active electrogenic H+ extrusion pump is still operative despite the net downhill H+ influx. The interpretation of this in terms of H+ 12NO3 12 symport which causes the entry of more H+ than is consumed in NO3 12 metabolism, with extrusion of the excess H+via the active, electrogenic H+ pump, was tested by measuring short-term H+ influx upon addition of NO 123. A net H+ influx occurs before NOa assimilation (as indicated by additional O2 evolution in the light) has commenced, suggesting a mechanistic relation of H+ and NO3 12 influxes. This is consistent with the interpretation suggested above. Determinations of cytoplasmic pH showed no significant effect of NO3 12 assimilation, suggesting that cytoplasmic pH changes sufficient to change the \u2018pH-regulating\u2019 H+ fluxes are smaller than the errors in the determination of cytoplasmic pH

Acid base regulation during ammonium assimilation in Hydrodictyon africanum

The acid-base balance during ammonium (used to mean NH 4+ and/or NH3) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m 123 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H+ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H+ accumulate in the medium per organic N synthesized. Granted a permeability (PNH3) of some 10 123 cm s 121, and a finite [NH3] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH 4+. This means that most of the H + appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H+ accumulates in the medium equivalent to any NH3 entry by requilibration from exogenous NH 4+. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO3+grown cells, suggesting that there is no action of a \u2018biochemical pH stat\u2019 during longterm assimilation of NO3+in H. africanum. Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO3+-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 \u3bcmol dm 123) the net influx was all attributable to NH4+influx via a uniporter, probably partly short-circuited by a passive NH3 efflux due to intrinsic membrane permeability to NH3. The net ammonium influx was in all cases associated with H+ accumulation in the medium. (1.3-1.7 H + per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated. Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a \u2018pH-regulating\u2019 change in H+ fluxes