Effects of the in vitro behavior of micropropagated plants on the stability of variegation in Yucca gloriosa, Phormium tenax, and Cordyline australis cultivars

Abstract

Cultivars with variegated foliage could be difficult to obtain in large-scale production of true-to-type plants. To determine the micropropagation potentialities of ornamental variegated monocotyledons in an industrial and commercial context, to define the stability of their variegated phenotypes, and to understand how and when off-type plants appear, the shoot tips of a variegated cultivar of three different species : Yucca gloriosa ‘Variegata’, Phormium tenax ‘Jessie’ and Cordyline australis ‘Pink Passion’ were introduced in vitro for propagation. The propagation rates and the variegation stability were estimated by separately monitoring each plant obtained during the several successive subcultures. During the in vitro process, samples were periodically fixed and studied by staining histological sections of the shoot tips. Leaf tissues were studied by light and confocal microscopy.After three-four subcultures, the multiplication rates obtained were stabilized at around 2.5–3 for Yucca, 1.4–1.7 for Phormium, and 1.2–1.7 for Cordyline. The number of off-type plants obtained after six multiplication cycles was around 10 % for Yucca, 25 % for Phormium and 60 % for Cordyline. The histology study showed that the axillary bud meristems (AxM) are totally (Yucca and Cordyline) or partially (Phormium) repressed by shoot apical meristem (SAM) in PGR-free medium. In the presence of BAP or BAP and NPA, only AxM were observed for Yucca, Phormium mainly developed AxM and some adventitious bud meristems (AdM), and Cordyline developed both AxM and AdM. The leaf section observations revealed that the three variegated cultivars turned out to be three periclinal chimeras.This histological study carried out on these cultivars highlights major differences in the development and in vitro multiplication behavior of three different genera belonging to the order Asparagales (clade monocotyledons). The variegation stability of these chimeric cultivars depends on their propensity to propagate by adventitious meristems.