Phosphate Uptake and Plant Requirements

In many habitats, phosphate is a limiting factor for plants because of its low solubility in alkaline conditions (Ca phosphates, apatite) and acidic conditions (Al phosphates, goethite). In the sea, phosphorus accumulates in the trophic levels. Sea birds bring this phosphorus to land: Bird excrement (guano) as the primary source for P fertiliser (for discussion of import nutrients from the sea to islands, see Erskine et al. 1998). Phosphate does not dissolve easily in soils and leaching hardly occurs, in contrast to nitrate which dissolves easily. Root hairs provide good contact with soil particles and the phosphate is dissolved because root hairs secrete protons and organic acids. Mycorrhiza play an important role in dissolving phosphate from minerals and are additionally very important in primary phosphate uptake. After the uptake of phosphoric acid this is directly bound to organic molecules and thus "protected" from transport back into

Chloroplast Cytoplasm | Fig. 2.3.14. The involvement of phosphate in the synthesis of starch and sucrose, and regulation of metabolism by phosphate: ADP-glucose-pyrophos-phorylase regulates starch synthesis: inhibition by P, and stimulation by PGA (/). Phosphate translocation regulates the export of photosynthetically produced substrates from the chloroplast: stimulation by P, and TP (triosephosphate) (2). F6P Fructose-6-phosphate; G6P glucose-6-phosphate. (After Marschner 1995)

Box 2.3.12

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