In this article we will discuss about:- 1. Pollen Wall Adaptations 2. Harmomegathic Adaptation 3. Adaptation to Habitat 4. Adaptation to Mode of Pollination
The Sporoderm structure, its chemical constituents and the diverse aperture types of pollen grains, has possibly developed as a need to adaptation in response to their respective habitats and mode of pollination. It has been seen that the size, surface of the pollen grains and the aperture characters are actively considered for transportation by the pollinating agents.
The exine with its unique structural and chemical properties is an adaptation whose corollary is their successful dispersal and accomplishment of fertilization. The pollen apertures which form their identifying hallmarks are associated with protection, harmomegathy (accommodation of volumetric changes), ion exchange and germination.
1. Pollen Wall Adaptations:
The pollen wall including the apertures is primarily associated with protection and this is by virtue of its impermeability and resistance to physico-chemical and biological degradation by virtue of having sporopollenin, the highly resistant biopolymer.
A complex mixture of lipid substance, popularly called pollenkitt present on the surface of mature pollen, provide the necessary shield against the harmful effects of the environment, especially from radiation hazards. This is due to the presence of ultra violet absorbing pigments in pollen kitt. Further its impermeability to water and water vapour protects the pollen from desiccation.
A mucilage coat around the pollen of some aquatic taxa is an adaptation to protect the cytoplasm from the chances of desiccation and also is barrier against the microbial and fungal attack. The apertural areas are more susceptible to fungal mycelia than the non-apertural region. This signifies the presence of a protective surface coat over the aperture in some taxa.
At the level of adaptation the mechanics of biological form and function become more significant. The pollen wall can be viewed as a body subjected to the laws of mathematics and mechanics, like all other structural bodies. A wall built up of continuous layers of exine would be most difficult to bend.
During hydration and dehydration it is the radial and tangential differentiation of exine layers that absorb the bending stresses. During pollen wall development the early delimitation of apertures, that is even before the dissolution of callose pollen mother cell wall, is considered to be the most justified first direct pressure sensitive control. Likewise the triaperturate condition is an expression of mechanical stress mode similarly, periaperturate condition is an equal spacing mode.
The law of equidistance causes proportional alteration of apertural and non- apertural regions. For instance in multiaperturate pollen the law operates either by meridional multiplication of apertures or by periapertury through successiformy or spiralization, and in case of monoaperturate pollen by way of equatorialosulcate, e.g., few Nymphaea species, or meridionosulcate, e.g., Nypa, or spiralized sulcate, e.g., Eriocaulon, conditions.
The stratification of the apertural intine is of special functional significance. The enzymatic inclusion in this layer is sealed above and below by a continuous polysaccharide layer until hydration. Thus following hydration, the outer layer is loosened and gets disrupted releasing the underlying enzymes.
The inner layer then becomes the precursor of the pollen tube. Since the enzymes of the apertural intine are lytic they are associated with the digestion of stigmatic substrates, thus providing the initial nutrition to the pollen tube.
The compatibility reactions are also determined by the enzymes of the apertural intine. Thus the aperture appears to be an obligatory concomitant of siphonogamy. This explains the reasons for the absence of an enzymatic layer in the intine of the spores of Pteridophytes and Bryophytes.
In omniaperturate (a pollen grain in which the exine is very thin or absent and the intine is thick, so that no specific apertural region can be distinguished and thus the whole surface can be considered apertural in nature) pollen the entire sporoderm represents the potential site for pollen tube emergence and clearly the entire intine of the grain is loaded with enzymes.
The multiaperturate grain has the selective value of rapid germination and early fertilization. The increase in aperture number logically reduces the mechanical efficiency of the wall and this lacuna is compensated by increase in exine thickness at the non-apertural areas, e.g., Ipomoea, Hibiscus, etc.
The tapetal proteins present in the chambered exine acts as a recognition substance that is responsible for the acceptance or rejection of the pollen by the stigma. This functional outcome means that the pollen lacking a chambered exine would be incapable of sporophytic incompatibility.
2. Harmomegathic Adaptation:
Wodehouse (1935) has defined harmomegathus as an organ or mechanism which accommodates a semi-rigid exine to change in volume and harmomegathy is volume-change accommodation.
Pollen grains possess certain harmomegathic mechanisms to accommodate volume changes associated with the phases of desiccation and rehydration. Such contrivances help to avoid plasmolysis of the cytoplasm.
The different harmomegathic mechanisms are briefly outlined below:
a. The invagination of the aperture membrane in colpate and colporate grains, this is comparable to the meridional folding.
b. Extensive invagination in synorate endoapertures, comparable to equatorial folding wherein the equatorial belt acts as a hinge zone.
c. Invagination of the polar regions which becomes prominent by the thinning of exine at the polar region or by the fusion of the apertures at the poles (syncolpy).
d. In order to control the direction of folding there is a development of striate ornamentation, linear operculum, conduplicate margins and periapertural thinning.
e. There is an invagination of the aperture membrane of the porate grains. However, in porate grains with distinct thickening the interpore invaginates mainly due to the plasticity of the exine.
f. The flexibility of the pollen grains is enhanced by the discontinuity of the sexine layers and by the absence or reduction of the nexine to a lamellar condition.
g. In some members of Asteraceae with immobilized apertures, the interapertural cavea develops to act as a buffer zone between the environment and the cytoplasm. Further in such cases the movement of the lacunae floor compensates the immobility of apertures.
h. In the turgescent stage due to stretching the pollen grains are susceptible to breakage and desiccation in non-turgescent stage. This is compensated by the presence of band-like sexinal elements that spreads over the aperture membrane, e.g., Crocus, Calectasia, etc.
i. In Morinaceae there is a development of protruding apertures with oncoid plugs to function as harmomegathic processes.
j. The thinning or development of perforations in the proximal part of the monads of pollen dyads, pollen tetrads and polyads enable them to function as a single harmomegathic unit, e.g., Annonaceae, Asclepiadaceae, Burmanniaceae, Mimosaceae and Orchidaceae.
3. Adaptation to Habitat:
i. Adaptation to xeric condition:
The high temperature of the arid zones prefers to select pollen with thicker exine and/or fewer and smaller apertures to check excessive loss of water. This is supported by the pollen morphology of few Apiaceae and Boraginaceae. As a result of thick exine the form and size of pollen remains the same.
In multiaperturate operculate pollen grains the thick exine has given a selective advantage to them by virtue of a mechanism, to resist desiccation and quick germinability thus ensuring rapid fertilization in the arid zones, where the species have a short life span, like members belonging to Cactaceae.
ii. Adaptation to hydrophytic condition:
In the aquatic angiosperms most of the pollen grains are with a less rigid, discontinuous or very thin exine thus permitting volume changes. The aquatic species of Utricularia have large number of apertures with an equatorial harmomegathus, whereas the terrestrial species of Utricularia have 3-5 apertures, which not being synorate (pollen with lalongate ora anastomose latitudinally), lack the equatorial harmomegathus.
In most of the epiphytic species the brevicolpate pollen has restricted apertural areas and same is also seen in few terrestrial species, but never in hydrophytic species.
iii. Adaptation to halophytic condition:
The pollen of several mangrove species inhabiting tidal zones of alternating salinity shows remarkable harmomgathic adaptations, like extension of apertures, multiplication of aperture and transfer of harmomgathic functions to non-apertural regions. Table 8.1 shows the harmomegathic adaptation of few halophytes.
4. Adaptation to Mode of Pollination:
The size and surface of the pollen and apertures are usually selected on the basis of the pollinating agents, like air, water and insects. Such a correlation with the agents is exemplified by the significant association between the structure of viscin threads in Onagraceae and the pollen vector. The beaded viscin threads are associated with moth and bird pollinated taxa, while smooth viscin threads occur in bee pollinated taxa.
Among the tropical woody taxa of Caesalpiniaceae and Fabaceae there is a remarkable similarity between rugulate- verrucate pollen and the large pollinators, like bats and birds. At the same time there is no correlation between exine sculpturing and pollination in Cactaceae and Polemoniaceae.
i. Adaptation in entomophilous pollen:
The pollen grains of entomophilous species are heavier, relatively larger, and are with various types of exine ornamentation among which reticulate pattern predominates. This pattern helps in the adherence of the grains to the appendages of the pollinating agents. The pollen wall protein provides a greater genetic specificity than the exine morphology.
Some of the flavonoids that contribute to the viable colour of the pollen grains provide a fragrance or taste, which either encourages or discourages the pollinating insects.
The pollen transported by insects has rich electron dense homogenous pollenkitt spread all over the exine surface and this adhesive factor has an adaptive value. Further the cohesion of monads into tetrads and polyads has a functional advantage, since they often behave as a single harmomegathic unit.
ii. Adaptation in anemophilous pollen:
The anemophilous flowers are usually small, non-attractive due to reduced perianth, neither produce nectar nor they emit any fragrance, have dangling anther lobes, and feathery stigmas.
Pollen produced by such flowers is small and produced in large quantities, and have a high surface to volume ratio. Further, the exine surface is generally smooth. They have little amount of pollen kitt and that is mostly locked in the exine cavities. Thus such grains are less adhesive and often are without distinct sculptural elements.
Pigments especially carotenoids have the potentiality to screen UV radiation. Air borne pollen contain higher amount of UV screening pigments than entomophilous pollen. Since they have a longer ambience, such pigments make them more adaptive.
iii Adaptation in hydrophilous pollen:
It has been seen that the reduction in thickness of exine is linked to the origin of the species in aquatic e.g., Zosteraceae or highly moist e.g., Heliconiaceae habitats. Plants have unique devices to make their pollen waterproof or to adapt them for an aquatic environment. Such taxa have omniaperturate pollen grains with thin elastic exine that could stretch easily and accommodate the increase in size due to imbibition of water.
However, there are aquatics like Aponogeton and pontederia that overcome this problem by flowering above the water surface. In marine angiosperm, like the filiform pollen of Amphibolis is covered by droplets of lipid and mucilage which is so important that it regulates the cohesion and the water relations of the grains when they are dispersed in sea water for pollination.
The exine reticulum of Ruppia is believed to play an important role in keeping the pollen afloat. The linear tetrads of Halophila and the dyads of Podostemonaceae with elliptic to cylindrical forms may be the product of selection for dispersal by water.
Conclusion:
The structures, forms and apertures of pollen grains are sometimes not properly understood due to limitations of the available techniques. The varied forms of aperture are considered to be the result of either a particular mode of development or evolution or for adaptation to the varied habitats and ecological factors.