9 Main Chemical Constituents of Pollen | Palynology

The following points highlight the nine main chemical constituents of pollen. The chemical constituents are: 1. Major Metabolites 2. Carbohydrates 3. Mineral Content 4. Callose 5. Organic Acids 6. Amino Acids 7. Pigments 8. Vitamins 9. Hormones and Steroids.

Chemical Constituent # 1. Major Metabolites:

The metabolites like carbohydrates, proteins and lipids vary considerably in pollen due to (a) the differences in species composition, and (b) the differences in environmental condition during pollen maturation and their subsequent release.

Both carbohydrates and proteins vary considerably among species. Grass pollen, specially maize contains about two-fold more carbohydrates (36.59%) than other angiosperms (17.78%) and gymnosperms pollen (13.15%) (Table 3.1). It is observed that excessive high temperature tend to reduce the pollen carbohydrate during maturation.

Proteins vary broadly with species, accounting for 7.70 to 25.20%. It is observed that short-lived, more rapidly growing pollen like Zea mays, Typlra latifolia, Acacia auriculiformis are generally higher in protein content, while lower amount of protein generally found in the species with slower growing pollen tubes (Table 3.1). Gymnosperm pollen generally contain low amount of protein than angiosperm pollen.

Pollen has a water content about 20% or less at the time of shedding (e.g. Pinus or Typlui), but at dehiscence time pollen desiccation has as high as 50% water content. Proteins are considered to be a component of tolerance. Various types of drought inducible proteins have been identified. Among these dehydrins are reported in mature pollen grains.

Recently a stress-inducible LLA-32 (Mol.wt. 32 KDa) protein is reported from Lily pollen. It is a heterogenous protein located in the cytoplasmic fraction of pollen and this LLA-32 doublet polypeptides differentially increase their levels of accumulation upon dehydration and other stresses.

Generally low to high levels of lipid is present in pollen (1.16 to 25.00%). Pollen grains with oils in their surface contain more lipid (Table 3.1). Low levels of ash (2.35-3.70%) are present in pollen.

Chemical Constituent # 2. Carbohydrates:

Some plants (Typha latifolia, Zea mays) shed their pollen at trinucleated stage and do not keep up their viability in storage. Hence they contain about 30-40% carbohydrates as their dry weight.

Pollen of other plants (e.g. Phoenix dactylifera) are binucleate and remain stable in long time storage, hence they hold low soluble carbohydrates (1-2%). Similarly Zea mays and Typha latifolia contain consistently high amount of starch (Table 3.2).

In gymnosperm pollen more than 93% of free sugar is sucrose, while in angiosperms sucrose represents only 20-50% of the free sugar. The sucrose in pollen plays a vital role in the acquired tolerance to desiccation. Gymnosperm pollen are generally anemophilous and remain suspended in the air for a long time. Hence they contain more sucrose for adaptation in the acquired tolerance to desiccation.

The other soluble sugars are raffinose and stachynose (common in conifers), and rhamnose, arabinose, xylose, galactose. Some rare sugars include turanose, nigerose and lactose are also reported and these sugars are probably the fragments of polysaccharides. Soluble sugars in pollen change markedly with storage. Carbohydrates in pollen occur mainly in the cell walls and also as cytoplasmic polysaccharides.

Thus, there is no seasonal or species disparity in total insoluble polysaccharides compare to the large variation of soluble carbohydrates. Some other carbohydrates related compounds are also reported from pollen. They are cyclitols (pinitol and sequoyitol). Sometimes ribose and deoxyribose sugars are also reported from pollen, probably as hydrolytic products of nucleic acid and myoinositol (both as free and as phosphoinositol).

Chemical Constituent # 3. Mineral Content:

Pinus, one of the first pollen analyzed for minerals, contains four major elements namely, Potasium, Phosphorus, Calcium and Magnesium (Table 3.3).

Many trace elements like Aluminium, Copper, iron, Manganese, Nickel, Tin, Zinc etc. are reported from pollen grains, Iron and Zinc being the most abundant (Table 3.4).

Elements in pollen differ with species and environmental conditions. For example, the desert plant Phoenix dactylifera is usually high in mineral content, because the hard soil where Phoenix is growing accumulates minerals including moderate amount of Silicon. Boron, an important trace element present in various plants, helps in pollen tube growth and elongation. Calcium is generally connected with cell wall formation.

Chemical Constituent # 4. Callose:

Callose, a β-1, 3-glucose polymer, is present in pollen in high amount at maturity which declines and resynthesized during germination. Callose may occur in higher quantity than cellulose and pectin.

Callose is found to be an important and rather common carbohydrate in plants. In Pinus callose occupies substantial cytoplasmic area around the generative cell of pollen. It is reported that callose acts as a storage carbohydrate in pollen and also forms a plug at the tip when pollen tube growth ceases.

Chemical Constituent # 5. Organic Acids:

All TCA cycle acids are found in pollen as metabolized products of carbohydrates, which differ noticeably with stages of development. Various phenolic acids like p-hydroxybenzoic, p-coumaric, vanillic, protocatechunic, gallic and ferrulic are reported from pollen. The role of these acids are not clearly recognized, some of them may lead to lignin-like constituents or are concerned in the production of amino acids and other compounds.

Substantial quantities of certain fatty acids, especially palmitic, linoleic and linolenic acids are reported from pollen of many plants. Pollen of gymnosperms, Pinus in particular, contain high quantity of linolenic acid. The major portion of fatty acids are present in pollen as esters combined with sugars, phosphates and other constituents.

Chemical Constituent # 6. Amino Acids:

It is reported that all the essential amino acids are present in pollen, though most reports on pollen amino acids provide hydrolysis data of the total protein. However, free amino acid pool does not reproduce the proportion of amino acids found in pollen protein.

Different metabolites like fatty acids, organic acids, etc., quickly yield amino acids through transamination process during metabolism. It is reported that proline is necessary for many pollen to germinate and its endogenous level may indicate growth ability.

For example, maize sterile anthers have a lower proline level and a higher alanine level than viable fertile anther. Similarly in apple, a higher ratio of proline to histidine occurs in viable pollen from a diploid than in sterile pollen obtained from triploid.

The study of viable and non-viable pollen in terms of free amino acid cannot be made. It is observed that many pollen have only trace amounts of free amino acid leucine or proline, while histidine may occur at a very higher degree in other pollen.

Chemical Constituent # 7. Pigments:

The chemistry of pollen pigments is considerably investigated. About 80% of investigated species of pollen are yellow in colour and the colour is due to the presence of carotenoids or flavonoids. The carotenoids are primarily composed of a-carotene, and some β- carotene, lycopene, xanthophyll and zeaxanthin are also present. Anthocyanin is occasionally reported, but chlorophyll has so far not been reported from pollen.

Quercetin, kaempferol and isorhamnetin are the most common flavonoid pigments in angiosperm pollen. In gymnosperm pollen substantial amount of naringenin are found, while quercetin and kaempferol occur sporadically.

Isorhamnetin are not reported in gymnosperm pollen, similarly naringenin is rarely found in angiosperm pollen. In many pollen, carotenes are present in surface oil and they are occasionally found in the exine and in the cytoplasm. Some water soluble pigments like flavones are readily diffuse from the pollen.

Several functions are proposed for the pollen pigments. According to one hypothesis carotenoids in pollen tubes are related to sexual compatibility, thus stimulate the sexual process. It is reported that carotenoids are involved in metabolism, possibly as enzyme co-factor leading to increased tube growth.

The hypothesis that pigmented layer on the outer surface of exine (Pollen Kitt) may act as an insect-attractant which assures transmission of pollen by insect vector in entomophilous plants. It is suggested that sometimes pollen carotenoids accomplish the role of vitamin A precursor in bee nutrition.

It is reported that pollen pigments protect the genetic content of the pollen. According to this hypothesis, pigments screen potentially harmful UV radiation. Airborne pollen contains higher amount of UV screening pigments than insect- transmitted pollen.

Chemical Constituent # 8. Vitamins:

A number of vitamins like B-complex, biotin (Vit. H) and Ascorbic acid (Vit. C) are reported from pollen (Table 3.5) which possibly act as enzyme co-factor. Biotin occurs at very low level, while nicotinic acid (Vit.B₃) and vitamin C occur at much higher levels in pollen.

Nielsen and Holmstrom (1957) reported the presence of free folic acid and folic acid conjugates at levels of 0.42 to 2.0 µg/g dry wt. in ten selected pollen. It is reported that vitamins are the essential component for pollen tube growth during germination. The reduction in pollen vitamin pantothenic acid (Vit. B₅) content may occur during storage of pollen.

Chemical Constituent # 9. Hormones and Steroids:

Quite a lot of hormones like auxin, kinins and gibberellins are reported from pollen. Orchid pollinia were one of the first sources of auxin discovered. Several experiments are performed to illustrate the role of hormones in pollen tube growth by using exogenous source of such hormones.

The steroids which considered as or related to hormones in man, are also reported from pollen. A number of steroids like cholesterol, sitosterol, oestrone are reported from many pollen. These steroids often occur in association with fatty acids in pollen, but androgenic activity has rarely been reported in pollen.