Ammonium toxicity and the real cost of transport

Recently, it has been proposed that ammonium is toxic to barley because of the energetic cost of pumping ammonium that has leaked into root cells back into the soil. This does not occur in rice because high levels of ammonium reduce the potential difference across the plasma membrane of rice -whereas the potential difference in barley appears to be ammonium insensitive. These results highlight the potentially high costs of membrane transport, and thus the central importance of transport processes in plants.