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

Changes in use of both stored and newly synthesized sources of carbon (C) and nitrogen (N) were investigated during rooting of leafy cuttings of Larix x eurolepis A. Henry. We used dual (13)C and (15)N long-term labeling of reserves of stock plants and followed isotope dilution of the labels in the cuttings to determine the respective proportions of C and N derived from stock 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 needles and roots. Because rooting development occurred over several months, destructive harvests were made during three periods (t(0): April 9; t(1): May 15 to June 5; t(2): June 12 to June 26). Total C content (Q(C)) of cuttings did not change with time, whereas total N content (Q(N)) decreased significantly between t(0) and t(2). In rooted cuttings, unlike unrooted cuttings, total N concentration decreased less markedly in needles and in the lower stem component between t(1) and t(2) than between t(0) and t(2). The lower stem of rooted cuttings contained more Q(N,old) than that of unrooted cuttings and showed lower C:N and (13)C:(15)N ratios. The increase in Q(N,new) in the lower stem was positively correlated with rooting status. At t(1), new 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 that in leafy cuttings the initial amount of N reserves, rather than C reserves, may constitute a limiting factor for rooting.