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In this article we will discuss about:- 1. Meaning of Pollen Viability 2. Variations in the Viability of Pollen 3. Causes for the Loss 4. Factors.
Meaning of Pollen Viability:
Pollen viability refers to the ability of the pollen to perform its function of delivering male gametes to the embryo sac. This functional property of the pollen after their release from the anther varies greatly from species to species and its quality is assessed on the basis of its viability. Pollen viability is an index of its quality and vigour.
Pollen viability varies between minutes and years, and which primarily depends on the taxonomic status of the plant and on the abiotic environmental conditions. In order to maintain the viability and fertilizing ability of the pollen for a long period of time special storage conditions are needed.
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Reports on the storages and transportation of date palm pollen were among the earliest concern with pollen viability. The male inflorescence of Phoenix dactylifera was prominently mentioned in trade contracts of the Hammurabi period about 2000 BC when storage of male flower in a dark, dry place was first recognized as prolonging fertilization capacity.
Cryopreservation is the most efficient method for long-term preservation of partly dehydrated pollen grains. In vitro biotechnological techniques like isolation and fusion of reproductive cells, and DNA transformation of artificially produced zygotes and embryos, have opened new prospects for germplasm storage.
Sophisticated methods such as nuclear magnetic resonance (NMR) spectrometry, Fourier transform infrared spectroscopy (FTIR), and different ultra-micro techniques for electron microscopy have helped to carry our precise with molecular changes occurring in membranes during pollen dehydration and rehydration.
Several reasons have been assigned for the loss of viability, like deficiency of respiratory substrate, inability to withstand desiccation and the loss of membrane integrity.
Variations in the Viability of Pollen:
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The life span of pollen is primarily determined by the plant genome but is also influenced by external environmental conditions.
Harrington (1970) on the basis of pollen viability has classified the examined plant taxa into three main groups, viz.:
a) Long Lived Pollen (six months to a year), example, Ginkgoaceae, Pinaceae, Arecaceae, Saxifragaceae, Rosaceae, Fabaceae, Anacardiaceae, Vitaceae and Primulaceae.
b) Pollen with a medium life span (approximately 1-3 months), examples, Liliaceae, Amaryllidaceae, Salicaceae, Ranunculaceae, Brassicaceae, Rutaceae, Scrophulariaceae, and Solanaceae.
c) Short Lived Pollen (from few minutes to a couple of days), examples, Alismataceae, Poaceae, Cyperaceae, Commelinaceae and Juncaceae.
Causes for the Loss of Pollen Viability:
It has been extremely difficult to access the exact reasons behind the loss of viability among pollen grains within a span of short or long period. Studies of Stanley and Linskens (1974) suggest that it is the deficiency of respiratory substrates or/and inactivation of certain specific enzymes or growth hormones that are likely to affect the viability of the pollen.
This idea is however, untenable when it is seen that the pollen of cereals (short lived) inspite of having abundant metabolites quickly lose their viability. Similarly changes in amino acid composition of stored pollen fail to explain the loss of viability. There are variable reasons to explain such inactivity as stated below.
i. Biochemical Alteration in Pollen:
The major biochemical cause for the loss of viability during storage is basically due to the deficiency of respiratory metabolites, which is the result of continuous metabolic activity by the pollen. As a result of long term storage there are reports of considerable changes in the amount of carbohydrate, amino acids and organic acid level in the pollen of different species.
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A higher respiratory rate in the three- celled pollen leads to the scarcity of respiratory substrate that strongly contribute to their rapid loss of pollen viability. It is reported that stored pollen grains require a higher concentration of sugar for germination in vitro than fresh pollen. Higher relative humidity also plays an active role in decreasing germinability by rapidly degrading endogenous substrates essential for germination.
The content of amino acids has also been reported to change in course of storage. An investigation into sixteen amino acids in Zea mays has recorded a consistent increase in aspartic acid, aminobutyric acid, ethanolamine, isoleucine, leucine, lysine, and phenylalanine, while alanine, glycine, glutamic acid, and proline decreased gradually.
A similar correlation in the endogenous level of proline and germinability has been noted in Lilium longiflorum, which however, increased with exogenously supplied proline. The other possibility of decrease in germinability due to deficiency of respiratory substrate might be the inactivation of enzymes like, amylase and phosphatases associated with degradation of reserves stored in pollen grains.
ii. Desiccation and Loss of Membrane Integrity of Pollen:
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The regulation of pollen water content is an important adaptive mechanism for survival after pollen dispersal and accordingly pollen grains that remain viable after dehydration are called desiccation tolerant, and those that lose viability parallel to dehydration are called desiccation sensitive.
The water content of living pollen grains in different families vary between 15% and 35% of fresh weight at the time of shedding, which is however, very high in Poaceae pollen between 35-60 percent.
The original pollen moisture to some extent depends upon temperature, air humidity, and the water supply to the pollen donor plant. This water content is measured accurately with nuclear magnetic resonance (NMR) spectrometry.
An investigation on the membrane state of pollen grains (using fluorescein diacetate test) from different taxa exposed to dry conditions indicated that most of the samples had lost their membrane integrity. It has also been observed that the plasma membrane may undergo gel- phase transition during water loss by increasing van der Waals interaction or free-radical damage.
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Thus water plays an important role in maintaining the structural integrity and the stability of the pollen membrane, by acting through hydrophobic and hydrophilic interactions. A positive correlation has been established between the loss of viability and a reduction in the amount of membrane phospholipids irrespective of the storage conditions.
The studies of Simmon (1974, 1978) on desiccated seeds have shown that when the moisture level of the membrane falls below 20%, the membrane loses its lamellar structure and permeability properties leading to imbibitional leakage, rehydration however, restores membrane integrity.
In most of the desiccated pollen systems, gradual hydration in humid air (ca 95% RH) is favourable for restoration of membrane integrity. This gradual rehydration causes a shift in membrane lipids from, the gel phase to the liquid crystalline phase, which has been explained by Crowe (1989) as phase transition changes in membrane phospholipids following desiccation and hydration (Fig. 7.1).
The pollen grains of grasses are highly sensitive to greater degrees of desiccation and restoration of normal function by controlled rehydration becomes extremely difficult. It has been seen that dehydration after dispersal rapidly disrupts the actin cytoskeleton in wheat pollen and leads to a loss of germination capacity. Thus for a successful preservation of pollen that loses its viability in a short time, special condition should be provided before and during storage.
Factors Affecting Pollen Viability:
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i. Pollen Cytology:
There exists a close relation between the cytology of pollen and its viability. Studies on pollen morphology and physiology have shown that the binucleate and trinucleate pollen grains show differences in their physiological and structural characters at the time of pollen dispersal.
The two celled pollen grains have a longer life span because of their more resistant wall structure, low plasma water content and reduced metabolic activity, whereas the trinucleate pollen grains are short-lived due to their less resistant wall and high moisture content, which can easily be lost by desiccation. This trinucleate pollen has a high rate of metabolism, respiring two to three times more than the binucleate pollen.
ii. Humidity and Temperature:
Environmental factors especially humidity and temperature greatly affects pollen viability. This relationship have been investigated by many authors and it transpired that pollen of majority of the species retained viability best at low relative air humidities (0%-30% RH) and temperature (between 0 and 10 °C) (Table 7.1). In most of the cases it is possible to standardize the conditions (low temperature and/or low humidity) for extending pollen viability of two celled taxa.
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However, on the other hand little progress has been made with the preservation of the three celled pollen taxa under Poaceae, Brassicaceae, Caryophyllaceae, Apiaceae, and Chenopodiaceae families.
The longevity of Poaceae pollen appears to be short under all conditions. Low relative humidities are harmful and pollen stored at 0° – 10°C remains viable only for a couple of days. Under high relative humidity (80% to 100%) also the viability can be prolonged to 1 – 3 weeks at the most.
