Agriculture Reference
In-Depth Information
(some stone-fruits), the rapid development of the fruit can cause apparently normal
seeds not able to germinate. The causes of this phenomenon can be attributed to a
discrepancy between the development of mesocarp and seed (Bonghi et al.
2011
).
The fruit is a metabolic sink stronger than the seed when the embryogenetic process
has not been completed yet. In other cases (such as seedless grapes), competition
between seed and fruit leads to an early abortion.
As previously reported, in some cases embryo development does not rely on
fertilization. The process is called apomixis and the embryogenetic program is
started in nucellar cells, different from the normal embryo sac (Bicknell and Kol-
tunow
2004
). There are three types of apomictic development, named “diplospo-
ry”, “apospory” and “adventitious embryony”. In the case of “diplospory”, the
megaspore mother cell switches from sexual to an apomictic pathway to produce
an unreduced embryosac. This can happens because of aberrant meiosis (“meiotic
diplospory”) or from the failure of the megaspore mother cell to enter meiosis
(“mitotic diplospory”). In both cases, the unreduced cell continues development
similarly to a normal megaspore. In contrast to diplospory, aposporous embryo
sacs form from additional cells that differentiate from the nucellus forming mega-
spore mother cell. Adventitious embryony develops from cells in tissue external
to a sexual embryo sac. The nucellar form of adventitious embryony is the most
common and is well described especially in
Citrus
. Somatic embryos originate
from nucellar tissues within the same embryo sac. Apomixis is recognized as one
of the most valuable biological process in improving agricultural important crops.
In fruit species, apomixis is present in apple and, as above mentioned, in
Citrus
where the phenomenon is exploited in the nursery industry for rootstock propaga-
tion. Obviously, apomictic seeds cannot be used to propagate cultivars since they
develop through a juvenile phase before reaching maturity. Thus, the orchard un-
productive phase would last for several years, negatively affecting the economic
return of the orchard.
Fruit Development and Ripening
Fruit Set and Physiological Drop
The fruit set defines the relationship between the number of fruit and the number
of flowers initially present in the tree. The initial fruit set is determined by genetic
characteristics of the species and by the climatic conditions occurring during anthe-
sis; it can vary from 2 to 10 % in the case of pome-fruit and 15 to 30 % in stone-fruit,
reaching the highest values (70-100 %) in species with little or no physiological
fruit drop (e.g. kiwifruit).
These phenomena are depending upon correlative inhibitions between fruits and
shoots and, subsequently, among fruits, allowing the tree to adjust the fruit load, in
order to be able to properly support fruit growth and complete seed development.
The fruit drop is therefore the most important strategy of self-regulation that the tree
