Active ion uptake and maintenance of cation-anion balance: A critical examination of their role in regulating rhizosphere pH

Abstract

The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H + or OH-into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion N + or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations ( H 4 + C a 2~ + Mg 2+ + K+ + Na +) and anions (NO( + CI-+ SOl-+ H2PO~) and charge balance is maintained by excretion of an amount of H + or OH which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H + and in particular OH-ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.The contemporary view is that primary active extrusion of H +, catalyzed by a membrane-located ATPase, is the major driving force for secondary transport of cations and anions across the plasma membrane. However, the fact that net OH-extrusion often occurs (since excess anion absorption commonly takes place) implies there is a yebto-be characterized OH ion efflux mechanism at the plasma membrane that is associated with anion uptake. There is, therefore, a need for future studies of the uptake mechanisms and stoichiometry of anion uptake; particularly that of NO~ which is often the predominant anion absorbed. Another related phenomenon which requires detailed study in terms of cation-anion balance is localized rhizosphere acidification which can occur in response to deficiencies of Fe and P.

Atributos químicos de solo e resposta do trigo e da soja ao gesso em sistema semeadura direta

“…A cultura da soja ao fixar N 2 deixa de absorver N-NO 3 -, acidificando o solo, como discutido por Maschner (1995), tal qual a cultura da soja apresenta rizosfera com pH mais baixo. No entanto, Haynes (1990) relatou que na cultura de trigo o balanço cátion-ânion no citoplasma proporciona acúmulo e degradação de ácidos orgânicos que são liberados pelas raízes, complexando o Al 3+ no solo, além da absorção (Pavan et al, 1984) e carbono orgânico (Zambrosi et al, 2007). A maior redução no teor de Al 3+ foi obtida com o uso da dose de 3.000 kg ha -1 nas três camadas avaliadas, tanto aos seis quanto aos 12 meses, condizendo com Souza et al (1996).…”

“…Segundo Haynes (1990), a cultura do trigo pode ter mecanismos de tolerância ao Al, como complexação de Al no citoplasma por ácidos orgânicos, armazenamento de Al no vacúolo e exsudação de quelatos ligantes, que ao ocorrerem nas raízes evitam que o Al atinja a parte aérea da planta. Assim, a cultivar CD 104, com a atuação de um entre esses mecanismos de tolerância ao Al nas raízes, foi suficiente para evitar o direcionamento de Al às folhas.…”

“…Na cultura da soja cultivar CD 214RR ocorreu, entretanto, aumento do teor de Al no tecido foliar em função das doses crescentes de gesso, visto que a aplicação deste proporcionou melhoria nas propriedades químicas das camadas subsuperficiais, fato que pode ter proporcionado aumento de raízes nessas camadas, onde a concentração do Al é maior, absorvendo-o mais intensamente, e o Al 3+ é um cátion e compete com Ca 2+ , Mg 2+ e K + pelos sítios de absorção nas plantas. E mais, a tolerância ao Al 3+ pode ocorrer por diversos mecanismos (Haynes, 1990), que no caso da cultura da soja, cultivar CD 214RR, pode estar relacionada ao armazenamento de Al no vacúolo, fato que evidência o aumento do teor foliar de Al nessa cultivar.…”

Rhizosphere properties of maize genotypes with contrasting phosphorus efficiency

“…The increase in rhizosphere pH in the treatment with high soil P level may be due to a greater anion release (OH -or HCO 3 -) into the plant rhizosphere, to regulate the electrochemical cell equilibrium, due to higher P amounts absorbed by plants from this treatment. In general, a factor that causes major changes in the rhizosphere pH is an imbalance in the ratio of root-absorbed cations/anions (Haynes, 1990).…”

“…Aside from acting as a nutrient and water sink, several other crucial processes of plant mineral nutrition occur in this root region (Marschner, 1995). Plant roots are responsible for considerable changes in rhizosphere pH, caused mainly by the release of H + , OH -or HCO 3 -, to balance the excess of cations or anions absorbed by the roots, respectively (Haynes, 1990). Some studies of calcareous soils have shown R. Bras.…”