Uptake and Transport of Calcium and Phosphorus in Lolium perenne in Response to N Supplied to Halves of a Divided Root System

The uptake and transport of Ca²⁺ and HPO₄^2? from roots of Lolium perenne L. was studied using variable N nutrition supplied to halves of a divided root system. Plants were grown for 4 weeks in solution containing 0.11 mM NO₃^?–N; then one-half of the root system was supplied with either 4.0 mM NO₃^?–N or 0.28 mM NH₄⁺–N while the other half of the root system remained in low-N solution. Uptake and transport of Ca²⁺ increased and uptake of HPO₄^2? declined in root halves supplied with high NO₃^?–N for 16 h. After supply of high NO₃^?–N or NH₄⁺–N to half the root system for 6 days, the roots supplied with high-N exhibited significantly higher rates of uptake and percentage transport to shoots of both Ca²⁺ and HPO₄^2?–. However, in neither the 16-h nor 6-day treatment did Ca²⁺ or HPO₄^2? uptake of the root half supplied with low N differ significantly from the control (low N supplied to both halves of the root). Significantly higher N concentrations were found in low-N supplied roots (compared to the control) as a result of internal translocation of N from high-N supplied roots to low-N supplied roots. Although N concentration in the low-N supplied roots increased, uptake rates of Ca²⁺ or HPO₄^2? did not change implying that external N concentration may be the important factor which influences or governs N mediated uptake responses. This would further suggest that the site of uptake regulation for Ca²⁺ and HPO₄^2? exists on the outer plasma membrane which is in direct contact with the external solution. Transport of Ca²⁺ and HPO₄^2? to the shoot was generally increased in low-N root halves after 6 days of high-N supply to the other half of the root. This implies that plant growth demand may be a major factor in regulating rates of Ca²⁺ and HPO₄^2? transport from roots to the shoot. It also reinforces the hypothesis that uptake and transport of ions out of the root are separately controlled or regulated in the plant.