Regulation of Mineral Redistribution in Pod-Bearing Soybean Explants

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Abstract
On the way from the roots to the seeds during reproductive development in soybean (Glycine max), a large proportion of the minerals pass through the leaves rather than travelling directly via the xylem. This direct and indirect movement of mineral nutrients has important implications for mineral redistribution, seed development and leaf senescence. Therefore, we have studied the role of cytokinin and mineral flux from the roots in regulating mineral redistribution from the leaves to the seeds using explants, i.e. a leaf, a pod and a subtending stem segment, with their bases immersed in treatment solutions. Thus, defined solutions containing cytokinin and/or minerals can be substituted for the roots. When explants (excised at early-mid podfill) are supplied H2O only, leaf N, P, K, Mo, Mg, Zn, Fe, B, Cu, Ca, and Mn decline, ranging from 93% for Mo to 38% for Fe. In explants on H2O, N, P, K, Mo, Mg, Zn, and Fe appear to be redistributed from the leaves to the seeds, while the B, Cu, Ca, and Mn lost from the leaves do not seem to move to the seeds. Although a mixture of minerals resembling xylem sap can delay net loss of these elements from the leaves, it does not prevent the decreases. The cytokinin zeatin (4.6 μM) inhibits the loss of N, IC, Mo, Mg, Zn, Fe, B, Cu, Ca, and Mn from the leaves, but not that of P. When combined with minerals, zeatin not only prevents the loss of the minerals from the leaves but may even greatly increase them with the possible exception of Zn, Fe, and Cu. Supplying the mineral nutrient mixture increases the quantities of N, P, K, Mg, Cu, and B in the seeds but not Zn, Fe, Mn, Ca, and Mo. For those minerals, especially N, where zeatin inhibits efflux from the leaves, it may reduce the amounts in the seeds, but it does not change P, K, Mg, and Ca. The accumulation and redistribution patterns of the different mineral nutrients show many dissimilarities thereby suggesting differences in the control of their distribution.

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