Carbon and nitrogen mobilization in Larix x eurolepis leafy stem cuttings assessed by dual C-13 and N-15 labeling: relationships with rooting

Changes in use of both stored and newly synthesized sources of carbon (C) a nd nitrogen (N) were investigated during rooting of leafy cuttings of Larix x eurolepis A. Henry. We used dual C-13 and N-15 long-term labeling of res erves of stock plants and followed isotope dilution of the labels in the cu ttings to determine the respective proportions of C and N derived from stoc k plant reserves (Q(C,old). Q(N,old)) and from newly synthesized sources (Q (C,new), Q(N,new)). We also assessed their partitioning into the growing ne edles and roots. Because rooting development occurred over several months, destructive harvests were made during three periods (t(0): April 9; t(1): M ay 15 to June 5; t(2): June 12 to June 26). Total C content (Q(C)) of cutti ngs did not change with time, whereas total N content (Q(N)) decreased sign ificantly between t(0) and t(2). In rooted cuttings, unlike unrooted cuttin gs, total N concentration decreased less markedly in needles and in the low er stem component between t(1) and t(2) than between t(0) and t(2). The low er stem of rooted cuttings contained more Q(N,old) than that of unrooted cu ttings and showed lower C:N and C-13:N-15 ratios. The increase in Q(N,new) in the lower stem was positively correlated with rooting status. At t(1), n ew roots appeared and grew mainly at the expense of Q(C,new) and Q(N,old). By t(2), root growth was mainly dependent on new assimilates. We conclude t hat in leafy cuttings the initial amount of N reserves, rather than C reser ves, may constitute a limiting factor for rooting.