Agriculture Reference
In-Depth Information
Fig. 4.2
Diagram representing the different phase transitions occurring during the whole life cycle
of the tree
Life Cycle of a Deciduous Tree
The life cycle of a deciduous fruit tree propagated from seed is classically divided in
three ontogenetic stages, named juvenility, maturity and senescence (Fig.
4.2
). Each
of these may last for several years.
Juvenility
Juvenility starts with seed germination and ends when the physiological maturity
of the shoot apical meristem is attained. The vegetative development of a juve-
nile tree is quite intense as far as the meristem activities are concerned. The tree
displays a great sensitivity to gravity, a monopodial branching and a strong apical
dominances, controlling lateral meristem development. This physiological behav-
iour leads to a basitonic development of laterals, to a cone shaped canopy and to a
main root development controlling the lateral ones. The aerial part is made up by
growth units characterized by a high growth rate and shoots are made up of pre-
formed phytomers in the proximal part, and newly formed in the distal one. Laterals
might be either of proleptic or proleptic and silleptic origin in case of strong or weak
apical dominance, respectively. The morphology of leaf and lateral shoot might be
quite divergent from that of adult tree. Multiple leaves characteristics of the juvenile
phase might evolve to simple leaf at maturity, as well as the leaf margin in which
indentation might change from the juvenile to the adult phase. Juvenile lateral shoot
in the
Rosaceae
family might be thorn-like. Some of the juvenile features are of
horticultural interest, for example the English ivy (
Edera helix
) is used as a ground
covering plants because of its plagiotropic behaviour. An additional useful feature
is represented by the higher rooting ability of the juvenile plants as compared to the
adult ones, which is largely exploited in the nursery industry for rootstock produc-
tion. The juvenile phase represents the complex physiological syndrome not yet
characterized in which growth-controlling internal factors are involved. It is clear
that the shoot apical meristem has the ability to measure the gaining of the ontoge-
netic experience accumulated throughout development. Practically, the ontogenetic
experience may be evaluated by counting the number of phytomers progressively
differentiated. The overcoming of the juvenile phase and the attainment of maturity
