Transport of magnesium ions in the phloem of Ricinus communis L. seedlings

Abstract

Abstract. During growth of Ricinus communis seedlings, magnesium ions are mobilized in the endosperm, taken up by and accumulated to very high levels (150 ~tmol.g FW 1) in the cotyledons, and translocated to hypocotyl and roots. The magnesium gain from days 6 to 7 in the cotyledons and the seedling axis necessitates a total uptake rate of 600 nmol.h 1.seedling--1 and the phloem translocation rate must amount to 200nmol.h ~. seedling 1. The phloem loading of magnesium and the regulatory properties of this process were investigated, making specific use of the ability to collect pure phloem sap from the cut hypocotyl of 6-d-old Ricinus seedlings. The concentration of magnesium in sieve-tube sap (5 mM) was fairly constant under many incubation conditions, e.g. incubation in magnesium-free buffer, incubation with different cations (K +, Na +, NH~-) or anions (C1 , NO3-, SO] ), or incubation with sucrose and amino acids. Even addition of magnesium chloride to the cotyledons did not enhance phloem loading of magnesium ions. Therefore the high magnesium content of the cotyledons was sufficient for continuous phloem loading of magnesium, irrespective of external ionic conditions. Also, the flow rate of sieve-tube sap did not influence the magnesium concentration in the sap. Only the incubation with sulfate and phosphate ions increased the magnesium-ion concentration in the phloem. Magnesium sulfate offered to the cotyledons caused a threefold increase of magnesium ions in the sieve-tube sap, which was inhibited by Na +, NH + and Ca 2+ in rising order, but not by K +. Incubation with phosphate for a prolonged period (8 h) led to an increased mobilization of intra-cotyledonary magnesium and an enhanced phloem loading of mobilized magnesium. It is concluded that phosphate availability is a decisive factor for mobilization and translocation of magnesium ions within the plant.

Loading and translocation of various cytokinins in phloem and xylem of the seedlings of Ricinus communis L.

“…Such a phenomenon is so far unprecedented for phloem-borne solutes, because up to now either a constant concentration of solutes has been observed, e.g. for sucrose (Kallarackal et al 1989) or magnesium (Zhong et al 1993) or, where there was a strong decrease in volume flow with time, there was a concomitant steady increase in concentration (Orlich and Komor 1992). When the cotyledons had been incubated in buffer, there was no appreciable time dependence of the cytokinin levels in the phloem (data not shown).…”

Magnesium deficiency in citrus grown in the Gisborne district of New Zealand

“…Zhong et al (1993) showed that the mobilisation and translocation of Mg by Ricinus communis seedlings was enhanced by increased P supply. Most plants (except for those adapted to acidic conditions such as blueberries) supplied with NH 4 + as the sole N source will show reduced uptake of Ca +2 , K + , and Mg +2 owing to competitive inhibition (Magalhaes et al 1987).…”

Relação fósforo e magnésio na fertilidade do solo, no estado nutricional e na produção da alfafa

“…Observou-se também que o incremento obtido no teor foliar de P, à semelhança do teor de Mg, está diretamente relacionado às quantidades utilizadas para obtenção das concentrações de P e de Mg disponíveis no solo, não havendo neste caso interação dos nutrientes na absorção, o que difere dos resultados relatados na literatura, por Skinner & Matthews (1990), em parreira, e por Zhong et al (1993), em mudas mamoneira, de que a disponibilidade do P é um fator decisivo para a mobilização, absorção e a translocação do Mg para dentro da planta. A provável causa desse efeito talvez seja a baixa exigência de Mg pela alfafa (Moreira et al, 2007), sendo a aplicação de 100 mg dm -3 suficiente para suprir toda exigência nutricional, mesmo com baixas concentrações de P no solo.…”

“…De qualquer modo, apesar de existirem resultados que indicam essa R. Bras. Zootec., v.37, n.6, p. [984][985][986][987][988][989]2008 interação (Zhong et al, 1993;Mengel & Kirkby, 2001), ainda não existem evidências diretas do efeito do P e Mg sobre a concentração de Mg ou P na solução do solo, no transporte para superfície das raízes e na absorção ou na translocação desses íons dentro das plantas (Skinner & Matthews, 1990).…”