Kinetics and relative significance of remobilized and current C and N incorporation in leaf and root growth zones of Lolium perenne after defoliation: assessment by ¹³C and ¹⁵N steady‐state labelling

The contribution of pre‐defoliation reserves and current assimilates to leaf and root growth was examined in Lolium perenne L. during regrowth after defoliation. Differential steady‐state labelling with 13 C (CO 2 with δ 13 C = ‐0.0281 and ‐0.0088) and 15 N (NO 3 − with 1.0 and 0.368 atom percentage, i.e. δ 15 N = 1.742 and 0.0052, respectively) was applied for 2 weeks after defoliation. Rapidly growing tissues were isolated, i.e. the basal elongation and maturation zones of the most rapidly expanding leaves and young root tips, with a biomass turnover rate > 1 d −1 . C and N weights of the elongation zone showed a transient decline. The dry matter and C concentration in fresh biomass of leaf growth zones transiently decreased by up to 25% 2 d after defoliation, while the N concentration remained constant. This ‘dilution’ of growth zone C indicates a decreased net influx of carbohydrates relative to growth‐related influx of water and N in expanding cells, immediately after defoliation. Recovery of the total C and N weights of the leaf elongation zone coincided with net incorporation of currently absorbed C and N, as shown by the kinetics of δ 13 C and atom percentage 15 N in the growth zones after defoliation. C isotope discrimination (Δ 13 C) in leaf growth zones was about 23‰, 1–2‰ higher than the Δ in root tips. Δ 15 N in the leaf and root growth zones was 10±3‰. The leaf elongation zones (at 0–0.03 m from the tiller base) and the distant root tips (about 0.2 m from the base) exhibited similar kinetics of current C and N incorporation. The amount of pre‐defoliation C and N in the growth zones, expressed as a fraction of total C and N, decreased from 1.0 to 0.5 at 3 (C) and 5 (N) d after defoliation, and to 0.1 at 5 (C) and 14 (N) d after defoliation. Thus, the dependence of growth zones on current assimilate supply was significant, and stronger for C than for N. The important roles of current assimilates (as compared to pre‐defoliation reserves) and ‘dilution’ of dry matter in regrowth after defoliation are discussed in relation to the method of labelling and the functional and morphological heterogeneity of shoot tissues.