Nitrogen Supply Affects the Remobilization of Nitrogen for the Regrowth of DefoliatedLolium perenneL.

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.