Zhong, WJ; Schobert, C; Komor, E: Transport of magnesium-ions in the phloem of ricinus-communis l seedlings, Planta, 190, 114-119 (1993)
During growth of Ricinus communis seedlings, magnesium ions are mobilized in the endosperm, taken up by and accumulated to very high levels (150 mumol.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 200 nmol.h-1.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+, NH4+) or anions (Cl-, NO3-,SO42-), 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+, NH4+ and Ca2+ 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.

last modified 2004-04-13