Agriculture Reference
In-Depth Information
of the cell wall middle lamella of the stylar cells. The pollen tube quickly overcomes
the stigma barrier and reaches the central part of the style named transmitting tissue,
which is rich in storages providing C and N sources necessary for pollen tube forma-
tion. The pollen tube growth through the transmitting tissue occurs very rapidly and
is directed toward the ovules by a signal generated by the synergids. Upon reaching
the base of the style, the pollen tube jumps into the ovule through the micropyle (po-
rogamy). In some nut species the pollen tube penetration occurs through the chalaza
at the base of the ovule (aporogamy). Upon ovule penetration the pollen tube ap-
proaches the embryosac at the synergid side and discharges the sperm cells through
the synergids that undergo degeneration. Few minutes (4-7) after the discharge into
the embryosac, one sperm cell fuses with the egg cell to generate the zygote, and the
other fuses with the polar central cell to form the first endosperm cell. The double
fertilization, which is typical of the Angiosperms, concludes the gametophytic phase
of development. An efficient fertilization depends on external factors that strongly
affect gametophyte viability and relies on long effective pollination period (EPP).
EPP, expressed in hours or days, is the difference between the time required to pol-
len tube to reach the ovule after pollination and the lasting of the embryosac vi-
ability since bloom. Temperature plays a major effect on the two parameters. Low
temperature regimes reduce pollen tube growth rate and extend the egg cell viability,
while an opposite effect is produced by high temperature. The EPP is an extremely
important parameter that should be taken into account in choosing the proper pol-
linizers for the varieties that are partially or completely self- sterile.
Mechanisms Preventing Self—Fertilization
The diffuse presence of hermaphrodite flowers in higher plants is a condition that
naturally favours self-fertilization, and progressively erodes the genetic biodiver-
sity of the species and their ability to cope with environmental changes. To avoid
this, species developed different sterility mechanisms preventing self-fertilization.
In fruit tree species, three main mechanisms of sterility are present, namely: mor-
phological (MS) and cytological (CS) sterility, and self-incompatibility (SI).
The MS relies on abnormalities targeting differentiation of flower organs that,
at the end, may affect pollen and ovule viability, causing andro- and gyno-sterility,
respectively. In most cases, andro-sterility is caused by disturbances in the ta-
petum differentiation leading to the formation of no- or non-functional- pollen.
Andro-sterility is present in some old peach varieties (JH Hale, Alamar, Aurora,
June Elberta), in almond (Rof), and in plum (Chabot and Flaming delicious). In
chestnut, andro-sterility is caused by disturbances of stamen elongation and a lack
of anther differentiation, while in some grape varieties (Picolit, Lambrusco di
Sorbara, Laureiro) is due to the formation of abnormal pollen lacking of germina-
tion pores, thus incapable of germination.
The gyno-sterility relies on disturbances of ovary differentiation that may range
from the absence of the organ to a normal ovary but no-viable ovules. The lack of
ovary occurs in some ornamental Prunus species and in Vitis vinifera sylvestris .
Ovule abortion has been reported in Citrus lemon and in Olea oleracea .
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