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Apparent K m and V max for net NO S″ 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 18 NO 3 , 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 NO S″ 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 V max for NO 3 uptake compared with those of normal plants. The stay-green mutation had no effect on the K m of the nitrate uptake system. Although the N-use efficiency might he expected to be lower in stay-green than in normal plants, there were no differences in rates of dry matter production.