CHARACTERISTICS OF JUVENILE PLANTS

There are numerous substantive phenotypic traits associated with juvenility, but they vary onsiderably among species.  Commonly, the leaves on young plants are of a different shape than those on mature parts and may be simple rather than compound (or occasionally the reverse); juvenile leaves may also have a special type of cuticle and be arranged with a distinct phyllotaxy. Compared to their adult counterparts, young plants may have a modified resistance to pests and diseases.  Juvenility in woody plants is often manifested by prolonged vigorous shoot growth.  With Citrus and Gleditsia triacanthos, juvenile forms are thorny, whereas adult forms lack the thorniness and with some tree species, such as those of  Quercus and  Fagus, juvenile forms and older, more juvenile parts of the trees, hold their senesced leaves throughout the winter.

1.1.1.      Vegetative propagation

            To the plant propagator, the most important attribute of juvenile shoots is their ability to rovide cuttings that readily form adventitious roots or explants that respond and grow well  in vitro. Cuttings taken from adult shoots of plants can be rooted, but the frequency of success is often low, especially with woody plants.  Likewise, researchers have had great challenges when attempting to micropropagate adult forms of many woody species. The change from the juvenile to adult phase is the most serious constraint to rooting in shrubs and trees (Howard, 1990).  Most of the difficulty experienced in rooting mature shoots seems to be caused by their altered physiology, but can also be related to greater contamination with microorganisms and viruses.

            Hedera helix has been used widely to study juvenility because it has a distinctively different morphology between the juvenile and adult phases. Juvenile plants have a different growth habit, leaf shape, and an enhanced ability to form adventitious roots.  When petioles from the juvenile form were excised and treated with auxin  in vitro, cortical parenchyma cells adjacent to the vascular bundles divided and formed root primordia (Geneve  et al., 1988).  However, when  petioles from adult leaves were treated in a similar manner, callus formed and some callus cells divided to form root primordia.  The juvenile form had pre-existing competent cells that were able to respond to auxin and become determined to form roots.  However, the adult form appeared to lack cells with pre-existing competence to form roots, but competence was acquired by some callus cells once they had been initiated.

            Explants taken from mature shoots are frequently more liable than juvenile material to uffer necrosis, especially when surface disinfested and placed in culture (Hanus and Rohr, 1987).  For example, shoot tip explant death can occur within a few hours for adult  Juglans nigra whereas healthy growth was evident on seedling explants when both sources were compared in different vessels containing the same medium (Preece and Van Sambeek unpublished).  It was only by changing the medium and culture conditions that adult J. nigra shoot cultures have been maintained for years  (Pearson and Preece un-published). However, adult origin  J. nigra micro-shoots still cannot be rooted.

For tissue culture, juvenile explants are usually more readily established  in vitro and grow and proliferate at a more rapid rate than adult material. This is particularly true with tree species where micropropagation of adult material is often difficult.

1.1.2.      Plant propagation dilemma 

            It is well known that it is easier to propagate vegetatively, juvenile forms of plants than adult forms of plants.  When breeding and selecting new, superior plants for clonal propagation, it is sually necessary to wait until the plant reaches maturity.  This allows for evaluation of important features, such as ultimate form and size, flowering and fruiting characteristics, autumnal coloration, and other traits.  At the point that the mature phenotype is known, the plant is an adult and often becomes difficult to propagate clonally.  Libby and Hood (1976) showed that juvenility can be maintained by hedging radiata pine. They rooted cuttings of many juvenile selections and  by allowing some individuals to grow to maturity for evaluation, other, hedged members of the same clone could be maintained as juvenile plants for propagation.  This technique also has potential for micropropagation.