Ammonium uptake kinetics in root and leaf cells of Zostera marina L.

Ammonium uptake rates and the mechanism for ammonium transport into the cells have been analysed in Zostera marina L. In the cells of this species, a proton pump is present in the plasmalemma, which maintains the membrane potential. However, this seagrass shows a high-affinity transport mechanism both for nitrate and phosphate which is dependent on sodium and is unique among angiosperms. We have then analysed if the transport of another N form, ammonium, is also dependent of sodium. First, we have studied ammonium transport at the cellular level by measurements of membrane potentials, both in epidermal root cells and mesophyll cells. And second, we have monitored uptake rates in whole leaves and roots by depletion experiments. The results showed that ammonium is taken up by a high-affinity transport system both in root and leaf cells, although two different of kinetics could be discerned in mesophyll cells (with affinity constants of 2.2 ± 1.1 μM NH4+, in the range 0.01–10 μM NH4+, and 23.2 ± 7.1 μM NH4+, at concentrations between 10 and 500 μM NH4+). However, only one kinetic could be observed in epidermal root cells, which showed a Km = 11.2 ± 1.0 μM NH4+, considering the whole ammonium concentration range assayed (0.01–500 μM NH4+). The higher affinity of leaf cells for ammonium was consistent with the higher uptake rates observed in leaves, with respect to roots, in depletion experiments at 10 μM NH4+ initial concentration. However, when an initial concentration of 100 μM was assayed, the difference between uptake rates was reduced, but still being higher in leaves. Variations in proton or sodium-electrochemical gradient did not affect ammonium uptake, suggesting that the transport of this nutrient is not driven by these ions and that the ammonium transport mechanism could be different to the transport of nitrate and phosphate in this species.