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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

Visser, R. de, Vianden, H., Schnyder, H.
Plant, cell and environment 1997 v.20 no.1 pp. 37-46
Lolium perenne, defoliation, leaves, regrowth, root tips, nitrogen, carbon, nitrogen metabolism, stable isotopes, isotope labeling, nitrates, carbon dioxide, biochemical pathways
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 13C (CO2 with delta 13C = -0.0281 and -0.0088) and 15N (NO3- with 1.0 and 0.368 atom percentage, delta 15N = 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 delta 13C and atom percentage 15N in the growth zones after defoliation. C isotope discrimination (delta 13C) in leaf growth zones was about 23 per thousand, 1-2 per thousand higher than the delta in root tips. delta 15N in the leaf and root growth zones was 10 +/- 3 per thousand. 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.