interfering with IAA metabolism. An earlier study of chestnut found physical barriers to root induction as well, with more extensive sclerification of mature cuttings than juvenile (Vieitez and Ballester, 1976).
Phase change is initiated in the shoot apical meristem (Sussex, 1976). Bitonti et al. (2002) examined whether DNA methylation occurs in mature, but not juvenile, shoot apical meristems of peach (Bitonti et al., 2002). DNA methylation changes may cause alterations in gene expression and gene regulation during phase change in plants. These researchers found a much stronger zonation of chromatin condensation, DNA methylation, and distribution of zeatin in adult peach apical meristems than in apical meristems from juvenile shoots. The authors speculated that the zonating patterns in mature tissue could mark change on a cellular level of gene expression that controls shoot identity.
Juvenility is still somewhat of a mysterious concept in horticulture. It is still unknown what causes these phase changes to occur in plants, and what physiological and biochemical changes the plant exhibits when this phase change occurs (Hackett, 1976). Pawpaw for example, loses rooting capability once the plant reaches over 4 months of age (Geneve, unpublished data). At this age, the plant is not sexually mature, not capable of flowering and fruit production, but apparently the plant has undergone some physiological changes that render it incapable of rooting. It seems likely that there is a gradation in maturity as the plant develops. However, since chronological age and biological, or ontogenetic age are not the same in plants, this issue could be overcome by propagating the ontogenetically most juvenile parts of the plant, such as root cuttings or shoots from the base of the plant, such as shoots generated in stooling. Haissig and Riemenschneider (1988) state that in many plants, the ability to root, which is a juvenile trait, and the ability to flower, a mature trait, compete in some plant species to the point that they are mutually exclusive of each other.
The hypotheses for the asexual propagation portion of this thesis research project were that trench layering, mound layering, and pot layering would be viable research propagation methods for pawpaw, that juvenile pawpaw shoots root more readily than more mature pawpaw shoots in a layering system, and that both auxin and wounding