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 -wherea
s the potential difference in barley appears to be ammonium insensitive. Th
ese results highlight the potentially high costs of membrane transport, and
thus the central importance of transport processes in plants.