Agriculture Reference
In-Depth Information
Pollination, Pollen Germination and Fertilization
Pollination is the result of pollen transfer from anthers to the stigma. The event
can occur within the same flower (self pollination) or between different flowers of
the same or different trees. The terms pollinator and pollinizer are often confused.
A pollinator is an agent of pollen transfer (e.g., bees, insects, people, wind, water,
etc.). A pollinizer is the plant variety that produces the pollen. Pollination is a pre-
requisite to fruit set in most fruit crops. The presence of viable seeds in peaches,
plums, cherries, almonds and apricots is necessary for normal fruit development,
while in few apple and pear varieties seedless fruits can develop (partenocarpy). In
some
Malus
and
Citrus
species, as well as in other plants, seed development can
occur without fertilization (i.e., apomixis). Pathenocarpy can occur either naturally
or be stimulated by growth bioregulators. Good fruit set and size in multiseeded
species are usually related to the number of seeds per fruit underlying the role of
fertilization in yield production.
The main systems of pollen transfer are wind and insects, depending on the spe-
cies. Wind pollinated species bloom copiously and the pollen content per anthers
may be higher than 20,000 grains, while in apple and pear, two insect pollinated
species, the content ranges between 1,000 and 3,000. Chestnut, hazelnut, mulberry,
olive, pistachio and walnut are wind pollinated. Nut species have male and female
flowers. The pollen grains are very small and light, and can travel hundreds of me-
ters. Stigmas are large, increasing the chance a pollen grain will land on the stigma
and germinate. The number of pollen grains produced by male flowers is extremely
high. For example, it is estimated that in hazelnut 250,000 pollen grains are pro-
duced by each egg cell.
Some species, such as strawberry, kiwifruit, pear and Italian prune, are pollinated
both by wind and insects, the latter being dominant.
All plant species with showy flowers are insect pollinated. Pome fruits, stone
fruits, and small fruits are all pollinated by insects, mostly bees. Honey bees do
most of the pollination in commercial orchard. Most of the insect- pollinated
temperate fruits have hermaphrodite flowers whose petals and odour attract in-
sects. Anthers shed their pollen as the insect brush against them. The main factors
affecting pollination are compatibility, pollen viability, pollinizer placement and
pollinator activity.
Under normal conditions, a compatible pollen that has reached the stigma sur-
face undergoes hydration and germination within 1 h (Edlund et al.
2004
). These
processes along with the pollen tube formation are carried out and controlled by the
vegetative cell, are insensitive to transcription inhibitors but dramatically affected
by protein synthesis inhibitors. Calcium plays a pivotal role in the pollen tube elon-
gation. Besides other factors, pollen germination is positively related to the number
of pollen grains present on the stigma, and the phenomenon is ascribed to the pollen
density and named population effect. The pollen tube emerges from a germination
pore and elongates within the stigma covered by exudates rich in stygmasterol, bore
and phenolic compounds. Pollen tube grows inter-cellularly upon the production and
extrusion of cell wall hydrolases (cutinases and pectinases) directed to the digestion
