Studies on mineral ion absorption by plants

Summary

The relative yield response and phosphate uptake ofS. humilis, P. atropurpureus andD. 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 ofS. humilis andP. atropurpureus at P₀ were identical, and slightly greater than that ofD. intortum; the relative response to applied phosphate was similar for the three species, up to the maximum yield attained. The higher relative yield ofD. intortum, at harvest 1, reflected the influence of a higher initial phosphate concentration in the small Desmodium seedling, compared toS. humilis andP. atropurpureus.

The mean absorption rate for phosphate$\overline {\left( {AR} \right)}$ ofD. intortum, and to a lesser degreeS. humilis andP. atropurpureus, showed two distinct maxima: an initial peak at low soil activities (0.3 — 3µM P), and a second at higher soil activities (37–43µM P) when maximum yield had been attained, and luxury accumulation of phosphate appear to occur. The initial peak in$\overline {AR}$ was followed by a decline (significant (P=0.05) forD. 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$\overline {\left( {RGR} \right)}$, averaged over all phosphate levels, fell in the orderD. 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 orderP. atropurpureus >S. humilis >D. intortum, and so counteracted the lower$\overline {AR}$ ofP. 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 toP. atropurpureus, due to a relatively greater shortage of nitrogen in the tops when grown on symbiotically-fixed nitrogen.