Pollination an amazing mechanism

Pollination an amazing mechanism 

 

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The floral biology, pollination pattern, fruit and seed dispersal mechanism are closely related to reproductive fitness of a tree.  Phenological and reproductive biology studies are necessary for planning conservation strategies as well as formulating measures for cultivation of species at a large scale.

Pollination is transfer of pollen from an anther onto a stigma. 

•Pollination can never occur in isolation. It takes place in an intricate habitat of rather amazing complexity. Pollination is still amazing to naturalists and biologists.

•There is an incredible variety of flowers ; they differ in size, shape, color, smell and some in their function. Such variations are the basis of diversity.
Such diversity also reflects itself in the large number of pollination mechanisms. 

•Types of pollination

Directpollination: Pollen falls on the ovules directly as in Gymnosperms. 

Indirect pollination: Pollen from the stamens falls on to the stigma through an agent. 

•Cleistogamy:Pollination takes place in unopened flowers.

•Chasmogamy: Pollination takes place in opened flower.

•Autogamy or self pollination: Stigma & stamen of same flower involved.

•Allogamy or cross pollination: Stigma & stamen ofdifferent flowers involved.

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•Geitonogamy: Flowers of the same plant involved in cross pollination.  

•Xenogamy: Flowers of different plant involved. It is true cross pollination.

Self pollination:

•Self pollination and fertilization is generally a strategy used by plants with short life span and easily affected by environmental change. Selfing is to minimize the risk of dependence on external pollinators. Pollination may occur in absolute purity of surroundings within one flower. It is self pollination.When pollinators are not available selfing occurs; because plants and Nature leave nothing to chance.

 Viola produces chasmogamous flowers in normal conditions, but when the environmental conditions are adverse (in snow fall)the plant is unable to produce large ‘normal’ flowers.

•The flowers never opens. Self pollination takes place in these cleistogamous flowers.It is an adaptation to seed set even in harsh climatic conditions. In Commelina bengalensis and in Arachis hypogea some flowers are produced beneath the soil surface.They are cleistogamous. Selfing is the only way for seed set in these flowers. 

The members of Asteraceae adopt another strategy, when crossing fails in protandrous flowers the two stigmatic lobes bend backward and receives what ever pollen remain attached to the staminal tube( the anthers are syngenecious in these members). It is known as safety mechanism.

Safety mechanism in Tridax

But Selfing is disliked by Nature because selfing leads to inferior quality of progeny.

Pollination is a flexible phenomenon which can occur in diverse range of physical conditions

Flower of Fabaceae with papilionaceous corolla.

Although self pollinated, Fabaceae  flowers require a visit of an insect. Insect tripping – mechanically forcing pollen on to the stigma.It is known as Piston mechanism.The large petal on the posterior side is known as standard or flag or vexillum -it attracts the insects, the two laterals provide seating place to the insect, the anterior two encloses the essential organs. A visit of the insect trips the essential organs for pollination. Crossing also occurs  through this mechanism.

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•Cross-pollination: Pollen from one flower has to reach the stigma of another flower.Plants are fixed to the soil and cannot move from place to place, flowers have to develop  mechanisms that will draw insects and birds to them for a successful reproduction. Cross pollination is a must in Uni sexual plants such as Papaya, Borassus(Toddy palm). It is known as dicliny. Maize and Cucurbitaceae members produce unisexual flowers(male and female flowers are produced on different branches. Geitnogamy and true cross pollination occurs in these plants. Although a large number of species have bisexual flowers, yet they opt for cross pollination, because Cross Pollination is  favoured by Nature. It is a mechanism of production of varieties even species.Cross breeding results in heterogenous character of in offspring.A mixture of characters make these offspring more fit than selfed.Immunity of the race towards diseases  and yield of plant is usually maintained.Struggle for existence and survival is due to introduction of variations.Crossing is not easy unless it achieves high degree of adaptation. Here begins the wondrous stories of synchrony and harmonization in Nature  between the flower and the pollinator.

Pollinators can be classified into abiotic and biotic agents. Air/wind, water, and rain are abiotic agents. Insects, birds, reptiles and mammals are living(biotic) agents.

Nearly 2000 more types of pollinators participate in this amazing activity.One species may be pollinated by more than one pollinator depending on the conditions . They are known as non specific mechanisms.In nature many  species follow nonspecific methods.

Pollination involves high level bio-energy transfer, co-ordination, chemistry of floral rewards. 

Agency involved for cross-pollination:

•Anemophily--Air; Hydrophily ---Water;Zoophily--- Animals.

 In nature most of the flowers are bisexual, yet they dislike selfing, and develop some structural and functional strategies or mechanisms to avoid selfing or for promoting cross pollination. 
Structural strategies:1. In the members of Malvaceae stigmas are superior in position than stamens, thus avoid falling of pollen from the same flower. It is known as herkogamy; and functionally they are protandrous. 

Herkogamy in Hibiscus

 The members of Solanaceae are protogynous, and herkogamy is also seen. Gloriosa superba also ehibits herkogamy.

Flower of Solanum

In Abutilon if the pollen from the same flower falls onto the stigma, it will not germinate, some times the flower may wilt. It is known as pollen prepotency.

Abutilon flower.

In order to avoid selfing the essential organs mature at different times.It is dichogamy. If the anthers mature prior to gynoecium it is protoandry. If gynoecium matures first it is protogyny. It is a functional strategy.
A flower may be pollinated by wind or insects depending on the prevailing conditions.The mechanism is  Nonspecific.  

The bees are potent pollinators in nature.The bee expends fair amount of energy in bringing out pollen from distant sources. In return the flower rewards the bee with high energetic nectar rich in amino acids and sugars. The collected nectar is stored in the beehives. Production of honey is an indication of pollination and yield of the crop. Bees visit a variety of flowers. 

Honeybee on Strychnos potatorum

Butterflies are another group of potent beautiful pollinators. The members of Asteraceae are commonly pollinated by Butterflies. The symbiotic relation between plants and butterflies are photographed and for the images

Please visit : https://www.flickr.com/photos/140231536@N03/albums/72157683937912440

A Danus butterfly on Tridax.

A large number of insects have pollen sacs on the bodies.Pollen also serves as a nutrient for the insects. 
Flies tend to be important pollinators in high-altitude and high-latitude systems, where they are numerous and other insect groups may be lacking.
Night opened flowers are white and produce scent or nectar to attract insects and helps the insect to locate the flower. They are pollinated by moths and other night visiting insects.

Jasmine a night opened flower.

Beetle pollination is primitive and flowers have strong scent and colour. The flowers are yellow or orange in colour. If they open during night they are white.Dalechampia indica an Euphorbiaceae member offer resin to the beetles as an unusual reward for pollinating the flowers. The styles connate into a thick column treminating in to a 3 lobed stigma to facilitate the pollination. 

Beetle on a Momardica flower.

Pollination by beetles in Cissus

Bumble bee inGmelina asiatica flower

Carpentor bees pollinate the Cactus flower.

Myophilous plants do not tend to have a strong scent, and flowers tend to be purple, violet, blue, and white,and the shape may be as open dishes, or tubes.They are pollinated by beetles.

Sapromyophile insects, on the other hand, normally visit dead animals or dung. They are attracted to flowers that mimic these odoriferous items! They obtain no reward and would quickly leave, but the plant may have traps to slow them down. In this struggle  pollination occurs. These plants have a strong, unpleasant odor, and the flowers are brown or orange in color.Eg. Aristolochia.

Aristolochia flower

 Besides honey bees and butterflies; ants, dragonflies, and several other insects also helps in pollination. In the members of Asclepiadaceae Eg. Calotropis the pollination mechanism is translator mechanism. The anthers in this group are reduced to pollinium. The entire pollen of the anther lobe is packed into a pollinium. Two adjacent pollinia are attached to a sticky caudicle by retinacula -the stalks. Wen an insect visits the gynostegium part of the flower the pollinium pair (Translator) gets attached to the legs or the sucking mouth parts of the insect. When this insect visits another flower the pollinia falls on the stigma and pollination takes place. 

Pollination by ants in Calotropis

 In Amarphophallus-the yam, inflorescence is a spadix. It provides breeding ground for the insects. The spathe is large and the spadix is included. When the insects enter the inflorescence they stay in for one to two days. In the night the insects stay in and they moves all along the inflorescence and mate with each other and then moves to come out of the spadix. In this process pollination takes place. It is known as pit fall mechanism. The flowers in the inflorescence are uni sexual. 

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Pollination in Orchids is by sexual deceptive methods.
“sexual deception”/ “pseudocopulation

Orchids produce sexual pheromones of a wasp that serves as its pollinator, besides adapting structural changes.Orchids do not  produce nectar or much pollen.They offer no rewards.

A honey bee in Cymbidium aloefolium

Ophrys flowers mimic virgin females of their pollinators, and male insects are lured to the orchid by volatile chemicals and visual cues. At close range, chemical signals from the flowers elicit sexual behavior in males. In this movements the pollen gets attached to the insect body, thus pollinates.

Some plants require specific pollinators:The obligate pollination mutualism between figs and the pollinator(Blastophaga quadraticeps).
Fig wasps is widely known as one of the most elaborate one-to-one mutualisms found in nature. Yucca- moth(Tegeticula yuccasella)mutualism is also an example of this kind.

By the time the plant begin to flower, the moth emerges from the cacoon. Male and female moths copulate.The pregnant moth enters the flower and pollinates. The process is interesting.Yucca produces pollinia instead of loose pollen grains. The female moth collects the pollinia and forms them into golden masses with its maxillary palpi. She carries the pollinia to another flower. The male lack this palpi.

At this time the insect is ready for egg laying. She deposits a single egg in the ovary with the device –ovipostor. After the egg deposition the moth uncurls the palpi and presse the pollinia in to the stigma and insures pollination and food for the larva.

•The egg develops into a larva in the ovary and feed on the seeds. Only a few seeds are eaten. Since the larva develops into a moth that pollinates the yucca plant, the relationship is clearly beneficial to both partners. The risk is if the specified pollinator is not available there is no seed set.

The insect pollinated flowers are with attractive colors, and shapes, produce nectar, and offer pollen, or scented. The pollen may be spiny, so that it can adhere to the insect body.

Many plants pollinated by birds produce red, orange, and bright pink, colored flowers to reduce nectar robbing.  The flowers are relatively large and thick.The nectar is rich in sucrose, where as insect pollinated flowers have glucose or fructose in the nectar.

A parrot pollinating Butea monosperma flower.

Kigelia africana pollinated by house crow

Pollination by Snails in slugs is known as melacophily, Eg.Ariods.

Pollination by bats is known as Chiropterophily. They are characterized by large size, night opened flowers with strong scent. Eg. Adansonia digitata, Anthocephalous.

Adansonia -bat pollinated flower.

Therophily: Pollination by squirrels.

Pollination by squirrels.

Commelina benghalensis

Besides these animals and insects several other insects and animals pollinate many plants either by chance or by specifications.

Commelina benghalensis  the day flower-a worst weed, is andromonoecious; the plant also produces clestogamous flowers under the soil surface; it exhibits autogamy, cleistogamy, geitno-allelogamy, hence it uses all types of sexual systems of reproduction, and flexibility to fix variation. Thus it has become  the worst weed found in arable lands.

Abiotic pollination:

Abiotic pollination, the entire process of pollination occurs without the help of organisms; the agents are wind, water

These pollinators physically carry or transfer the pollen from the anther to the carpel or pistil to bring about pollination in plants.

Pollen presentation: The time of the day of potential pollen presentation appears to be a constant specific character, and it varies in the different species. The actual time of pollen presentation depends on the weather conditions. Secondly, the 'pollination potential' of a species varies with the time of day of its pollen presentation.
Secondary pollen presentation is the developmental relocation of pollen from the anthers onto another floral organ which then functions as the pollen presenting organ for pollination.Nine different types have been identified in sixteen angiosperm families.Pollen is presented on (1) Spathe (Bract the envelop the inflorescens (Araceae); (2) Perianth (Epacridaceae); (3) Androecium (Santalaceae);passive pollen present on terminus of style  (4)-stigmas concealed (Rubiaceae and Proteaceae); (5)sub-terminal stigmas (Marantaceae and Polygalaceae);(6) Active pollen presenters -Terminus of style(Asteraceae, Calyceraceae and Lobeliaceae); (7)) Sub-terminal stylar presenters (Campanulaceae, Cannaceae, Fabaceae and Myrtaceae); (8) Exposed stigma (Rubiaceae); (9) Indusium of stigma (Goodeniaceae and Brunoniaceae). secondary pollen presentation is a character with a selective advantage.They may display reduced herkogamy , promote xenogamy or autogamy to facilitate pollination. Plants displaying secondary pollen presentation exhibit enhanced male function, and almost always protandrous.   

Anemophily -pollination by wind; the wind pollinated flowers exhibit certain modifications in the floral structure;and their functions. The plants flower in dry season ie. during leaf fall in tropical areas, and the flowers are produced in clusters or in drooping inflorescence like catkin. This facilitates easy spread of pollen. The flowers may be naked or the non essential organs may be reduced to scales. The stamens are much exerted, many, the anthers are versatile, the pollen is produced abundantly, which is dry and light, or some times with wings. The stigmas are elongated and  feathery as in grasses or sticky. If the plant produces both male and female flowers the male flowers are on the apical part of the plant, and the female in the lower part, this is because to capture the falling pollen by the stigmas. The wind pollinated flowers can be pollinated by insects also. Nature is not very rigid.  

Spikelet of grass with feathery stigmas and drooping stamens

Spike of a grass with exerted stamens and feather like stigmas.

Pennisetum a wind pollinated crop

Cocos nucifera-a wind pollinated tree

Male flowers of Vallisnaria on the water surface

Hydrophily-pollination by water. All aquatic plants do not pollinate by water; they may be pollinated by wind or insects or animals. Vallisnaria is  a fresh water submerged plant, where both male and female plants are separate. The male releases spongy male flowers (not pollen)on to the water surface and the long pedicellate female flowers grows up to the water surface and comes into contact with the floating male flowers and after the contact the female goes down below the water surface with the attached male flowers  by spirally coiling the pedicel and then the males releases the pollen on the stigma which facilitates fertilization of the flower. It is described as epihydrophily. The members of Hydrocharitaceae almost pollinate by hydrophily only.    

Vallisnaria -epihydrophily.

 In sea grass like Zostera the pollen grains are filiform and the density of pollen grains is equal to the density of water.
Plants have pains takingly evolved pollination mechanisms to keep the gene pool of each species undiluted disallowing any foreign pollen to germinate on a stigma.

Monoculture reduces the yield due to non availability pollinators in sufficient numbers.

Spraying of insecticides affects insect pollinators, thus a decline in the crop yield.

 Due to industrialization urbanization and deforestation there is a habitat loss.During non flowering seasons of the crop the insects thrive on weeds, forest trees. If the forest  and other plants are removed from the vicinity the insects die due to nonavailability of food and shelter. It is practically proven in many areas. When there are variety of plants the honey bees produce honey in large amounts. It may be taken as an indication of rich phytodiversity.   

Wide hybridization ( pollinating with distant relative) contaminates gene pool.

Specialized mechanisms of pollination contribute to the delicate nature of rainforests. (Balance of ecosystem)

The fragmentation of such forest disrupts the finely tuned mechanism.

Disruption of micro-environment to which the pollinator is adapted leads to the decay of highly evolved communities.

There is a greatest diversity in pollination mechanism in a forest. Many tree species adopt more than one type of pollination. Many tree species are cross pollinated, if pollination doesn’t occur they opt for self pollination as an alternative device. Approximately 65% of flowering plants are insect-pollinated. Species pollinated by various groups of pollinators were distributed non‐randomly, hence long-distance pollen flow is necessary in a forest. Flowering periods, time of flowering, floral arrangement, and rewards offered by the flower, mate availability, frequency of visits, intra-floral behaviour of foraging pollinators, fecundity and mate limitation are important factors of fertility in a forest ecosystem. Hence a tree may be pollinated by more than one pollinator or pollinating mechanism; one pollinator pollinates more than one type of plant in one day.

   Flowering time of day was significantly related with pollination systems. Bird and many insect pollinated plants, anemophilous trees open during day.  Most plants pollinated by bats and moths open at night. Beetle-pollinated plants flowered both in daytime and night time.

 Over 80% of the emergent and the canopy tree species bloom in short periods of 3–4 months at irregular intervals of 1–10 yr. Canopy trees are pollinated by bees and small diverse group of insects; but there is great variation in sub-canopy trees. In tropical forests a large number of species bloom in so short a period that pollinator shortages might occur unless there are pollinators that can quickly respond to the general flowering. Many insects maintain low density during non-flowering seasons. They have a short generation time and high fecundity, so as soon as a general flowering starts, they can increase in numbers quickly using the massive floral resources.  Some birds and insects pollinate more than one type of plants at a time. But few species are pollinator specific. 

Financial aspects of Pollination: 

By understanding pollination mechanism of each crop we can improve the productivity of the crop. For example coconut is basically wind pollinated, but it is also pollinated by bees. if honey bees are grown near coconut plants we can harvest honey and more nuts from coconut trees.

Tridax like many weeds serve as survivor/ secondary hosts for many butterflies and bees when the trees are not in flowering. So do not remove these weeds in the name of clean-green. If a pollinator is lost the yield is reduced.

Conclusion: Conservation of pollination mutualisms, provides an important means to conserve ecosystems and biodiversity. If the insects were given a chance to speak they would say "all organisms are created to live together! who gave you the power to kill every one for your short term benifits or comforts? You are also one of the creatures created to live in the universe. But with your disobedience to Nature, and wicked thoughts you have been creating lots of problems to other organisms including you." Is there any one- who dislikes honey? Who hates the sight of a beautiful butterfly on a flower in a pleasant morning?Who dislikes a fruit? Live! let others live; together makes life happy."

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