Tree development involves maturation, or phase change from a reproductively incompetent juvenile to the competent adult. Phase reversal of the shoot apical meristem of an adult, resulting in emergence of rejuvenated shoots, can also occur. The reversal is particularly important to tissue culturists and nurserymen that are engaged in clonal propagation of elite forest trees, because usually accompanying maturation is a decline of organogenetic competence and vigor. The reversal is achievable variously, e.g., by repeated grafting of the shoot apex onto juvenile rootstocks in vitro. Each successive graft results in shoots that manifest a progressively broader spectrum and greater intensity of juvenile traits. We monitored phase reversal of Sequoia and Citrus by measuring the intensity of restored rooting competence and root and shoot vigor. Rooting frequency and root and shoot vigor comparable to seedling stems were attained after 45 repeated grafts of Sequoia and 710 grafts of Citrus. The restored traits were retained in subcultured shoots at intensities corresponding to the number of repeated grafts. The stability of restored traits implied a self-replicating factor and not phytohormones. Detailed study of Sequoia revealed clear differences in mtDNA restriction fragments between juvenile and adult shoots. Furthermore, rejuvenated shoots contained mtDNA fragments that differed from the original adult, but identical to those of the juvenile. The differences and identities were confirmed by DNA cloning, double restriction analysis, cross hybridization, and sequencing. Ultimately, elucidation of the underlying basis for phase change should lead to simpler and more effective methods of achieving rejuvenation.
Key words: Citrus, Sequoia, rejuvenation.