Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots

Abstract

Influx of nitrate into the roots of intact barley plants was followed over periods of 1–15 min using nitrogen-13 as a tracer. Based on measurements taken over 15 min from a range of external nitrate concentrations (0·2–250 mmol m⁻³), the kinetic parameters of influx, I_max and K_m, were calculated. Compared with plants grown in the presence of nitrate throughout, plants that had been starved of N for 3 d showed a significantly greater value ofI_max for ¹³N-nitrate influx (by a factor of 1·4–1·8), but a similar value of K_m (12–14 mmol m⁻³). Pre-treating N-starved plants with nitrate for about 5 h further increased the subsequent rate of ¹³N-nitrate influx, but had little effect in the unstarved controls. Allowing for this induction of additional nitrate transport, the difference in rates of nitrate influx in control and N-starved plants was sufficient to account for the previously-observed difference in net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, but with ammonium as N-source, both I_max and K_m for subsequent ¹³N-nitrate influx were significantly decreased (by about one-half) compared with the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulation of net uptake of nitrate is discussed.