Agriculture Reference
In-Depth Information
and internal conditions. If the flower bud differentiation does not occur, meristems
follow the vegetative path differentiating vegetative phytomers, each made up of
leaf primordia, axillary meristems and subtending internodes. In young orchards,
the presence of an adult vegetative phase is essential in order to build in the short-
est period the tree woody final structure and made the tree to fill up the assigned
space. Only after this goal has been achieved, the reproductive development should
be encouraged, keeping in mind that a certain degree of vegetative activity should
be maintained throughout the orchard life, since a proper canopy development is
necessary to provide energy for the fruit tree functions. As soon as the tree has
reached the final structure, a balance between vegetative and reproductive activity
should be maintained. In some cases, this could be difficult; in fact, in presence of
vigorous rootstocks and cultivars, an excessive vegetative development could oc-
cur. In this case, the grower should depress vigour by means of growth retardants,
shoot inclination and bending, canopy and root pruning, mineral nutrition, irriga-
tion, etc. On the contrary, when the orchard is made up of dwarf rootstocks and/
or spur or compact habitus varieties, the balance is in favour of the reproductive
development with a limited vegetation. In this case, the vegetative activity should
be stimulated by means of a proper pruning, avoiding the natural acceleration
of senescence affecting these genotypes. In a modern fruit industry, high density
planting is widely adopted in addition to the use of dwarfing rootstocks and spur
or compact habitus varieties.
The Reproductive Phase
Transition Phase
In the deciduous species of temperate areas, reproduction starts with the transi-
tion phase and ends up with seed maturation and fruit ripening. This reproductive
cycle involves two growing seasons. During the first one, transition phase of the
meristem occurs, and inflorescence and flower differentiation proceed up to pol-
len mother cell formation. At this developmental stage, the flower differentiation
stops concurrently with the establishment of endo-dormancy. During the second
season, endo-dormancy is overcome, and reproductive structure differentiation
is resumed up to anthesis. In evergreen species the reproductive cycle progresses
continuously.
Transition phase implies a switch of the genetic program in the shoot meristem
from vegetative to reproductive conditions. Several genetic studies carried out in
model plant species allowed the identification of genes involved in transition phase
leading to definition of LD, SD, autonomous and temperature dependent flower-
ing pathways. GAs may interfere with LD and the temperature dependent path-
ways. The complex of the genetic studies led to the identification of
LEAFY
(
LFY
)
and
APETALA1
(
AP1
) as flower meristem identity genes (Jack
2004
). In plants,
the transition phase appears to be related to the activation of the
FT
gene, whose
