4.2.8  Nutrient transport through plants

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The membrane transport phenomena discussed above give rise to coordinated transport of nutrients into growing cells where they are used. This transport might be from roots to shoots or from mature leaves to growth zones. In either case, nutrient transfer is a complex process (Sutcliffe 1976) where active and passive processes as well as bidirectional flow combine to optimise delivery of each ionic species. Coordination of these processes is especially evident in long-distance transport, where fluxes of nutrients, water and photoassimilates depend upon membrane properties of vascular tissues (Chapter 5).

The pathway of nutrient transport from root surfaces to shoot meristems illustrates the importance of membranes in ion movement. Ion uptake into root cells (Section 3.6) requires participation of transport proteins in root cell membranes, such as the dual mechanism of K+ import (Section 4.2.6). Selective uptake of calcium is probably achieved by influx through Ca2+ channels. Further adjustment of internal ion levels might ensue through efflux (e.g. of orthophosphate) and modification of the Vmax and Km of carrier proteins.

Ions taken up by cortical cells and transported through the symplasm to xylem parenchyma cells are probably unloaded into mature xylem vessels via ion-specific channels (de Boer and Wegner 1997). Release into immature xylem elements could involve energy-dependent carriers and ion channels. Uptake of ions into shoot cells from dilute xylem sap is catalysed by energy-dependent carrier proteins; how many of these proteins are shoot specific remains to be determined. Ion transfer into growing cells is via the phloem, as transpiration from enclosed meristems is generally small, precluding passive inflow of xylem sap. Once the nutrient- and photoassimilate-rich phloem sap reaches meristems, the final selection of solutes occurs across the membranes of expanding cells. Solute uptake in these cells is regulated by demand (expansion rate) and mediated by many intracellular factors. Turgor-activated channels might play a part in maintaining solute balance of growing cells. Moreover, the tight feedback control on K+ influx identified in roots is probably indicative of similar feedback processes in growing shoot cells. The result is a selective, dynamic process of resource delivery to growing cells, providing solutes for osmotic balance and biosynthesis.