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# STUDIES ON MINERAL ION ABSORPTION BY PLANTS: I. THE ABSORTION AND UTILIZATION OF PHOSPHATE BY STYLOSANTHES HUMILIS, PHASEOLUS ATROPURPUREUS AND DESMODIUM INTORTUM

R. E. WHITE
Plant and Soil
Vol. 36, No. 2 (APRIL 1972), pp. 427-447
Stable URL: http://www.jstor.org/stable/42946811
Page Count: 21
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## Abstract

The relative yield response and phosphate uptake of S. humilis, P. atropurpureus and D. intortum, grown on a very deficient soil, enriched with phosphate, were measured at four successive harvests during 42 days growth under controlled environmental conditions. Except at the first harvest (19-21 days), the relative yields of S. humilis and P. atropurpureus at P₀ were identical, and slightly greater than that of D. intortum; the relative response to applied phosphate was similar for the three species, up to the maximum yield attained. The higher relative yield of D. intortum, at harvest 1, reflected the influence of a higher initial phosphate concentration in the small Desmodium seedling, compared to S. humilis and P. atropurpureus. The mean absorption rate for phosphate $\left({\overline {AR} } \right)$of D. intortum, and to a lesser degree S. humilis and P. atropurpureus, showed two distinct maxima: an initial peak at low soil activities (0.3 — 3 µM P), and a second at higher soil activities (37 – 43 µM P) when maximum yield had been attained, and luxury accumulation of phosphate appear to occur. The initial peak in $\left({\overline {AR} } \right)$ was followed by a decline (significant (P = 0.05) for D. intortum) at soil phosphate activities corresponding to maximum yield, suggesting that the rate of absorption by the roots was influenced by the demand for phosphate created within the growing plant. Mean absorption rates and relative growth rates $\left( {\overline {RGR} } \right)$, averaged over all phosphate levels, fell in the order D. intortum > S. humilis > P. atropurpureus. Conversely, the efficiency of phosphate utilization by the plant, which may be expected to be greater in plants of low RGR, fell in the order P. atropurpureus > S. humilis > D. intortum, and so counteracted the lower $\left( {\overline {AR} } \right)$ of P. atropurpureus, and to a lesser extent, S. humilis. However, S. humilis had the advantage of a lower retention of phosphate in the root system, compared to P. atropurpureus, due to a relatively greater shortage of nitrogen in the tops when grown on symbiotically-fixed nitrogen.

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