NITROGEN SUPPLY AFFECTS THE REMOBILIZATION OF NITROGEN FOR THE REGROWTH OF DEFOLIATED LOLIUM-PERENNE L

Nitrogen remobilization from roots and pseudostems during regrowth of Lolium perenne L. was studied in miniswards grown with contrasting levels of (NH4)2SO4 in solution culture. Growth with a high N supply (5.0 mol NH4+ m-3) increased the weight of leaf laminae recovered at each of five weekly clippings, and decreased the proportion of photosynthate used for root growth. Clipped plants growing in a steady-state were supplied with 15N for 48 h and the recovery of labelled N in laminae measured after five weekly cuts. Recovery of labelled N in the laminae from the second clipping onwards was derived only from remobilization of N from roots and pseudostem. Miniswards grown with low N (0.5 mol NH4+ m-3) relied more upon remobilization of N for lamina growth than did high N plants. Thus after 14 d 20% of lamina N was labelled in low N plants but only 3% was labelled in the high N treatment. Thereafter, N remobilization declined until at the final clipping after 35 d, labelled N represented only 4% and 1 % of the lamina N in the low and high N plants. When plants were not clipped before the labelling period, they took up more 15N if grown with high N than cut plants. Thereafter, the remobilization of N followed a similar pattern as in the cut plants.Exponential models were used to calculate the rate of N transfer from roots and pseudostem to laminae. When grown with low N, the half-life of remobilization was 1.56 weeks. High N miniswards had an initial rapid remobilization with a half-life of 0.66 weeks, and a slower phase with a half-life of 2.98 weeks. © 1990 Oxford University Press.