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

The disruption of K⁺ transport and accumulation is symptomatic of NH₄⁺ toxicity in plants. In this study, the influence of K⁺ supply (0.02-40 mM) and nitrogen source (10 mM NH ₄⁺ or NO₃^-) on root plasma membrane K⁺ fluxes and cytosolic K⁺ pools, plant growth, and whole-plant K⁺ distribution in the NH₄⁺- tolerant plant species rice (Oryza sativa L.) was examined. Using the radiotracer ⁴²K⁺, tissue mineral analysis, and growth data, it is shown that rice is affected by NH₄⁺ 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 NH₄⁺. 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 NH₄⁺ influx, as measured by ¹³N radiotracing. In the low-affinity K⁺ transport range, NH₄⁺ stimulated K⁺ influx relative to NO ₃^- controls. It is concluded that rice, despite its well-known tolerance to NH₄⁺, nevertheless displays considerable growth suppression and disruption of K⁺ homeostasis under this N regime at low [K⁺]_ext, but displays efficient recovery from NH₄⁺ inhibition, and indeed a stimulation of K⁺ acquisition, when [K⁺]_ext is increased in the presence of NH₄⁺.