POLLINATION AND FERTILIZATION

C Kameswara Rao
Foundation for Biotechnology Awareness and Education, Bangalore, India

Pollination

Pollination is 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 fruit formation.

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 anthers. 

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 means. 

Pollen germination and viability

Pollen germination 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 stigma receptivity.

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.

Fertilization

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 hybridization.

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 physiological competence. 

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 complex. 

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