POLLINATION AND FERTILIZATION
C Kameswara Rao
Foundation for Biotechnology Awareness
and Education, Bangalore,
the transference of pollen from the anthers (male
structures) of a flower to the stigma (the
receptive part of the female structures) of the same or another flower,
mediated by abiotic or biotic means.
Pollination is the first in a series of crucial events that lead to seed and
Simple physical deposition upon agitation of the
anthers/flowers, wind action and dew or rain constitute the abiotic means of pollination in the majority of wild and
cultivated species of plants. Several different biotic vectors such as
insects (honey bees, bumble bees, butterflies or other insects) and small
animals (bats, hummingbirds) render invaluable service by pollinating certain
species of wild and cultivated plants. Nevertheless, pollination by
biotic vectors is rarely related to the species of the vector or the
plants. Even casual and incidental visitors like thrips,
ants and predatory spiders are known to cause pollen transfer. The pollen
stick to each other and the body parts of insect visitors because of a sticky
coating on the pollen surface, the pollen kit. There
is no biology involved even when biotic vectors carry out pollination. They
merely physically transfer pollen to another flower, which they visit
next, whatever species that might be.
Biotic pollen vectors do not always pollinate the
flowers they visit. They may merely consume nectar, pollen or even some parts
of the flower, without effecting pollination.
Bats were thought to be the pollinators
of the West African scarlet bell (Spathodea campanulata
), now a common avenue tree
in the tropics, but it was found that the bats make a hole at the base of
corolla, suck the nectar without ever touching the anthers or the stigmas,
leaving the species to self-pollinate. In the large cardamom (Amomum
subulatum), honeybees take most of
the pollen without pollinating and in the process deny feed to bumble bees,
the actual pollinators, often seriously affecting crop yield.
In general, pollination, whether by abiotic or biotic vectors, is non-species specific,
incidental or even accidental.
While in a vast number of species pollination can
occur through both abiotic and biotic vectors,
pollen of some species are accessible only to some insect vectors.
Flowers that are very small and those with long narrow tubular corollas are
not accessible to bees, but are to butterflies and thrips.
In some species pollination does not occur in the absence of a specific
species of the vector in the environment, resulting in reproductive
failure. Some famous examples of extreme vector dependence occur among
the orchids such as the bee orchid (Ophrys apifera),
fly orchid (Ophrys
insectifera) and spider orchids (species of Caladenia),
where the flower has evolved to resemble the female of the vector species to
attract the males. However, such cases are rare and occur almost always
among the wild species.
Like wind, many insect pollinators only physically
disturb the anthers, pollen and stigmas. The bumble bees and
hummingbirds agitate the flowers/anthers by a process similar to ‘sonication’
(buzz pollination) which displaces pollen from their
The pollen of many species are
easily carried away by wind or animal vectors when the anthers are exposed
and deposited on the stigma of any species, where stigmas are exposed.
Consequently, the stigma of a flower usually receives pollen of most
similarly oriented species in its environment.
When the pollen of a flower are
deposited on the stigma of the same flower, it is self-pollination. When
pollen are deposited on the stigma of another flower
of the same species, it is cross-pollination.
There are many examples such as the pea where intricate floral structures
have evolved to facilitate cross-pollination, though self-pollination occurs
in such species too. In several species self-pollination occurs even
before the flower opens and the cross-pollination that occurs subsequently
has no consequence. In general, in most species either self- or
cross-pollination can occur, ensuring seed and fruit set by one or the other
Pollen germination and viability
and pollen viability
are different aspects.
The pollen of several different species in the
vicinity of a plant are likely to land up on the
stigmas of its flowers. Pollen become
dehydrated prior to transit and may be further dehydrated during flight,
depending upon the temperature, relative humidity and the time in
transit. Rehydration of pollen upon landing
on the stigma is the first crucial step in pollen germination, the process of
production of long narrow pollen tubes. The pollen kit contains proteins including lectins
which play an active role in pollen-stigma recognition and pollen
germination. The pollen taken into the vector’s mouth do not germinate
because the chemistry of the pollen kit is altered by the regurgitated
contents of the mouth.
Viability is the further rapid growth of
the pollen tubes carrying the male gametes, through the tissues of the
stigmas and the styles, a long way to reach the ovules in the ovary.
This is a physiological process controlled by a number of physico-chemical
factors. Pollen inappropriate to the species/variety may also
germinate, but the pollen tubes would not be capable of growing through the
ovarian tissues due to factors that determine compatibility.
Additionally, there is a time factor that limits pollen viability and/or
Pollen of a very large number of species
contain two nuclei at the time of
dispersal. One of these nuclei divides to form two male gametes
by about the time the pollen tube reaches the ovary. In several other
species such as those of the grasses (cereal crops included) the pollen
contain three nuclei, as the male gametes are already formed by the time of
dispersal. Pollen of such species have
notoriously short viability, less than 10 minutes in rice to about two hours
in some others.
The pollen tubes carry the male gametes to the egg
cells in the ovules. Fertilization,
the fusion of the male and female elements, leads to embryo development and
seed and fruit set.
When the egg cells of a flower are fertilized by the
male cells from the pollen of the same flower, it is self-fertilization and
in other cases it is cross-fertilization. Genetically determined self-incompatibility
is one means of ensuring cross-fertilization which facilitates new gene
combinations paving way for further evolution of the species. However,
this has been an impediment in breeding such crops as mustards.
Pollination and fertilization in field crops
Species are reproductively isolated, with
the identity of species/varieties being maintained through several
genetically controlled reproductive barriers
that operate at one or more stages of pollen germination, viability,
fertilization, embryo development and seed germination. In the absence
of such a natural isolation, there cannot be so many species and varieties of
plants. There is little chance of rampant natural interspecific
Most field crop species are self-pollinated and
self-fertilized, except those such as the cucurbits and corn, where the
flowers are unisexual (contain either the anthers or the ovaries).
Several crop species such as the mustards, though self-incompatible in the
wild ancestral states, are adapted to a high degree of self-fertilization on domestication. In
a number of species like the pulse crops, self-pollination occurs even before
the flower opens. When self-pollination is possible, cross-pollination
is largely inconsequential, as the former has an advantage of time, and even
What is actually important in crop reproductive
biology is not whether there is self- or cross-pollination, but whether
self-fertilization can occur and its genetic consequences. This can
only be determined by an analysis of the progeny for any visible marker
characters or a genetic evaluation.
Most characters are controlled by two states
(alleles) of one single gene, which may be identical (homozygous) or
different (heterozygous) in a given individual. Characters like growth
and yield are simultaneously controlled by several genes each with two
alleles (quantitative characters) where the inheritance is more
Crop plants are selectively bred for beneficial
characters through repeated crossing with one of the parents, which results
in a high degree of homozygosity for the select
characters. Any heterozygosity for other
characters is usually ignored. Whether a crop is self-pollinated or
cross-pollinated, is not an issue of serious consequence in most crops,
because even when the pollen come from plants in
another crop field, they are homozygous for the chosen traits, except when
the traits in question are quantitative.
February 22, 2008