NH4+-stimulated and -inhibited components of K+ transport in rice (Oryza sativa L.)

The disruption of K + transport and accumulation is symptomatic of NH + 4 toxicity in plants. In this study, the influence of K + supply (0.02-40 mM) and nitrogen source (10 mM NH + 4 or NO - 3 ) on root plasma membrane K + fluxes and cytosolic K + pools, plant growth, and whole-plant K + distribution in the NH + 4 -tolerant plant species rice (Oryza sativa L.) was examined. Using the radiotracer 42 K + , tissue mineral analysis, and growth data, it is shown that rice is affected by NH + 4 toxicity under high-affinity K + transport conditions. Substantial recovery of growth was seen as [K + ] ext was increased from 0.02 mM to 0.1 mM, and, at 1.5 mM, growth was superior on NH + 4 . Growth recovery at these concentrations was accompanied by greater influx of K + into root cells, translocation of K + to the shoot, and tissue K + . Elevating the K + supply also resulted in a significant reduction of NH + 4 influx, as measured by 13 N radiotracing. In the low-affinity K + transport range, NH + 4 stimulated K + influx relative to NO - 3 controls. It is concluded that rice, despite its well-known tolerance to NH + 4 , nevertheless displays considerable growth suppression and disruption of K + homeostasis under this N regime at low [K + ] ext , but displays efficient recovery from NH + 4 inhibition, and indeed a stimulation of K + acquisition, when [K + ] ext is increased in the presence of NH + 4 .