STUDIES OF THE UPTAKE OF NITRATE IN BARLEY .1. KINETICS OF NO-13(3)-INFLUX

13NO3- was used to investigate patterns of NO3- influx into roots of barley plants (Hordeum vulgare L. cv Klondike) previously grown with ('induced') or without ('uninduced') a source of external NO3- ([NO3-]0). In both induced and uninduced plants, 13NO3- influx was biphasic in the range from 0.005 to 50 moles per cubic meter [NO3-]0. In the low concentration range (<1 mole per cubic meter for induced plants and <0.3 mole per cubic meter for uninduced plants), influx was saturable and Vmax and Km, values for influx either increased or decreased according to N03- pretreatment. By contrast, 13NO3- influx in the high concentration range revealed a strictly linear concentration dependence. These fluxes appeared to be mediated by a constitutive, rather than an inducible, transport system.