Internal Phosphorus Flows During Development of Phosphorus Stress in Stylosanthes hamata

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Abstract
Partitioning and net transfer of phosphorus between shoots and roots in the tropical forage legume Stylosanthes hamata cv. Verano during the development of phosphorus deficiency has been studied. Plants were stresssed by either growing them in dilute flowing culture on continuously maintained external phosphorus concentrations that were inadequate for maximal growth, or by transferring plants of varying phosphorus status to phosphorus-free media. An external phosphorus concentration of 1 .mu.M P was found to be just adequate for maximal growth of S. hamata. Phosphorus stress caused rapid and substantial increases in root weight percentage. It is proposed that this represents an important adaptive mechanism for maximising phosphorus uptake by S. hamata growing in phosphorus-deficient soils. Roots contained the minimum proportion of the plant''s phosphorus content when root phosphorus concentrations were 8-10 .mu.mol P g-1 root, and shoot phosphorus concentrations were 16-20 .mu.mol P g-1 shoot. When tissue concentrations were less than these values, plants suffered from phosphorus stress and phosphorus was either preferentially retained by the roots or rapidly tranferred from shoots to roots, reducing the growth rates of shoots, but permitting root growth to continue. Upon reducing the external phosphorus supply to plants whose root phosphorus concentrations exceeded 8 to 10 .mu.mol P g-1 root, excess phosphorus was rapidly transferred from the root to the shoot to maintain shoot growth rates. The mobility of phosphorus within the plant, and the apparent lack of any delay in transferrring phosphorus from shoots to roots as phosphorus stress developed, represent another adaptive feature that is likely to be important to the successful growth of S. hamata in low phosphorus soils. When the phosphorus supply was limited, the plant''s resources were directed toward maintaining root growth. Even extremely phosphorus deficient plants, in which shoot growth had ceased, maintained linear rates of root trowth. These linear rates wererelated to the total phosphorus contents of the plant. In the latter stages of phosphorus deprivation, linear rates of root growth were maintained by remobilisation of phosphorus from the older parts of the root system to sustain the phosphorus supply to the root meristems.