FUNCTION AND CONTRIBUTION OF THE ROOT-TIP IN THE INDUCTION OF NO3-, UPTAKE ALONG THE BARLEY ROOT AXIS

The seminal roots of N-free-grown barley seedlings were able to take u p NO3- immediately upon initial exposure; the uptake rate in the tip w as half of that in the older root zones (middle and base). A lag of 60 min was required in all root zones before the uptake rates started to increase during induction with external NO3-. This increase could be prevented by the addition of pFPA; we thus assume that additional NO3- transport proteins were synthesized during NO3- induction. During the time-course of NO3- induction different uptake rates were measured in morphologically different regions of the tip (1 mm segments) indicati ng a regulation of NO3- induction on a narrow local scale. In NO3--gro wn plants, NO3- uptake as well as NO3- content increased basipetally a long the root axis concomitantly with increasing vacuolization of the cells. Although NO3- uptake into the tip was only half of that into th e older root zones, this NO3- uptake was very important for the entire root, Firstly, it provided the substrate for protein biosynthesis in the meristematic region: nitrate reductase activity and total soluble protein were highest in the first apical mm of the tip. Secondly, 3% o f the NO3- taken up by the tip was found in the base where it induced NO3- uptake: NO3- was translocated almost exclusively basipetally and as little as 20 nmol g(-1) root fr. wt. translocated from the tip were sufficient for acceleration of NO3- induction in the root base of N-f ree-grown plants. This clearly shows that the induction of NO3- uptake does not depend exclusively on the availability of external NO3- but can be mediated also with internally translocated NO3-. The root tip, therefore, may be considered the NO3- sensing region of the root.