Histology of Adventitious Root Formation in Four Woody Species and Effect of a Synthetic Auxin1

Cutting propagation provides uniform plants that retain the original characteristics of the donor. This method requires forming adventitious roots. Our previous studies indicate that semi-lignified stem cuttings of American chestnut [ Castanea dentata (Marshall) Borkh.] and yellow camellia ( Camellia nitidissima Chi.) exhibit low levels of the rooting-promoting hormone indole-3-acetic acid and high levels of other hormones that inhibit rooting induction and development. Additionally, their hormone distribution between leaves and stems differs from that of easy-to-root poplar. This unfavorable endogenous hormone profile may contribute to the recalcitrance of rooting in American chestnut and yellow camellia cuttings. In this study, we found that the vascular cambium region in American chestnut cuttings was the least active and most condensed, with clusters of active cell division not presenting until 48 days after excision and auxin induction. American chestnut stems possess a closed sclerenchyma ring, while sclerenchymata in yellow camellia, poplar hybrids ( Populus spp.), and weeping willow ( Salix babylonica L.) cuttings are dispersed. Promoter analysis of an adventitious root-promoting gene, AINTEGUMENTA , showed that the number of auxin-responsive cis -elements in poplar is at least three times greater than in American chestnut, likely contributing to the contrasting rooting response with auxin between the two species. Compared to 1-naphthaleneacetic acid, a slow-release synthetic auxin, 4-chlorophenoxyacetic acid-l-tryptophan-OMe, induced more adventitious roots in American chestnut cuttings. Species used in this study: American chestnut [ Castanea dentata (Marshall) Borkh.], poplar hybrids ‘Ogy’ [ Populus × euramericana (Dode) Guinier] and 717-1B4 ( P. tremula L. × P. alba L.) , weeping willow ( Salix babylonica L. ), yellow camellia ( Camellia nitidissima Chi.). Chemicals used in this study: 1-Naphthaleneacetic acid (NAA), 4-chlorophenoxyacetic acid–l-tryptophan-OMe.