Fluxes of reserve-derived and currently assimilated carbon and nitrogen inperennial ryegrass recovering from defoliation. The regrowing tiller and its component functionally distinct zones

The quantitative significance of reserves and current assimilates in regrow ing tillers of severely defoliated plants of perennial ryegrass (Lolium per enne L.) was assessed by a new approach, comprising C-13/C-12 and N-15/N-14 steady-state labeling and separation of sink and source zones. The functio nally distinct zones showed large differences in the kinetics of currently assimilated C and N. These are interpreted in terms of "substrate" and "tis sue" flux among zones and C and N turnover within zones. Tillers refoliated rapidly, although C and N supply was initially decreased. Rapid refoliatio n was associated with (a) transient depletion of water-soluble carbohydrate s and dilution of structural biomass in the immature zone of expanding leav es, (b) rapid transition to current assimilation-derived growth, and (c) ra pid reestablishment of a balanced C:N ratio in growth substrate. This balan ce (C:N, approximately 8.9 [w/w] in new biomass) indicated coregulation of growth by C and N supply and resulted from complementary fluxes of reserve- and current assimilation-derived C and N. Reserves were the dominant N sou rce until approximately 3 d after defoliation. Amino-C constituted approxim ately 60% of the net influx of reserve C during the first 2 d. Carbohydrate reserves were an insignificant source of C for tiller growth after d 1. We discuss the physiological mechanisms contributing to defoliation tolerance .