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.