Apparent Km and Vmax for net NO3− uptake and short-term translocation patterns of recently absorbed N were compared in a stay-green mutant and wild-type selection line of Lolium perenne L. by means of a series of depletion studies using 15NO3−, performed over 12 d under conditions of progressively increasing N deprivation. In view of the greater retention of N in senescent leaves of the stay-green phenotype, it was predicted that NO3− uptake would be up-regulated relative to the normal line, and that a proportionally higher fraction of recently absorbed N would be allocated to young leaves. It was shown that the stay-green trait had significant phenotypic consequences for plant N relations, with higher ‘sink strength’ of shoots for recently absorbed N, and higher Vmax for NO3− uptake compared with those of normal plants. The stay-green mutation had no effect on the Km of the nitrate uptake system. Although the N-use efficiency might be expected to be lower in stay-green than in normal plants, there were no differences in rates of dry matter production.
