Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary

Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7 ± 0.2 mM) or nitrate (5.6 ± 0.2 mM) + ammonium (0.04 ± 0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58 ± 3% when nitrate was the sole N source and by 83 ± 1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less, i.e. ammonium constituted 10 ± 1% of the total N uptake in the P sufficient plants and 30 ± 5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO₃ ⁻ into the proteins, which might cause a negative feedback on NO₃ ⁻ influx and/or stimulate NO₃ ⁻ efflux.