Function and contribution of the root tip in the induction of NO3−uptake along the barley root axis

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Abstract
The seminal roots of N-free-grown barley seedlings were able to take up NO3− immediately upon initial exposure; the uptake rate in the tip was 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) indicating a regulation of NO3− induction on a narrow local scale. In NO3− grown plants, NO3− uptake as well as NO3− content increased basipetally along the root axis concomitantly with increasing vacuolization of the cells. Although NO3− uptake into the tip was only half of that into the 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% of 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 20nmolg−1 root fr. wt. translocated from the tip were sufficient for acceleration of NO3− induction in the root base of N-free-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.

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