Metabolic factors and the utilization of phosphorus by plants

The overall process of entry and transport of phosphate by plants has been separated into its component parts. Rapid esterification is involved but a small proportion of the total transport may occur by a non-metabolic route. Mannose alters the metabolism of phosphate in roots of cereals and thereby reduces the transport to the shoot by as much as 99% whereas dicotyledonous species are much less sensitive. The sequestration of phosphate as mannose 6-phosphate is reversible in some species depending on the extent of its conversion into fructose 6-phosphate and it is possible that in vivo controls of this type operate in whole plants. The factors involved in the control of distribution of inorganic phosphate between cellular organelles and throughout the plant are discussed. Enzymic hydrolysis of organic forms by roots and associated microorganisms may also be important in soils where available orthophosphate is limiting. Other inorganic nutrients, particularly boron, play an important role during the transport of phosphate across membranes. Major differences have been found in the capacities of different species to transport absorbed phosphate to the shoot. Peas and field beans absorb efficiently in the early weeks of growth but the rate of transport is about one thirtieth that of oats, barley, mung bean or sunflower. The relevance of the experimental findings to the wider problem of efficiency of fertilizer use and the possible selection of genotypes with high capacities for absorption and transport are discussed.