Unit 3 Pollination and Fertilization
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UNIT 3 POLLINATION AND FERTILIZATION Structure Page No. Introduction Objectives Pollination 3.2.1 Types of Pollination 3.2.2 Self- Vs Cmss-Pollination Fertilization 3.3.1 Pollen-Stigma Interaction 3.3.2 Pollen Gemination-Events on Stigma and in Style 3.3.3 Pollen Tube Growth in vitro 3.3.4 Syngamy and Triple Fusion Incompatibility 3.4.1 Intraspecific Incompatibility 3.4.2 Interspecific Incompatibility 3.4.3 Biological Significance of Incompatibility 3.4.4 Methods to Overcome Incompatibility Apomixis 3.5.1. The Recurrent Type 3.5.2 Non-recurrent Type 3.5.3 Endosperm Development in Apomicts 3.5.4 Anthers of Apomicts 3.5.5 Causes of Apomixis 3.5.6 Parthenogenesis 3.5.7 Significance of Apomivis summary Terminal Questions Answers - - - 3.1 INTRODUCTION Thi :eproductive smchlre of angiosperms including the details of formation of male and l~malegametophytes were considered in the earlier units. You my recall @at as a result of meiosis, haploid pollen grains and egg cell are formed. After the formation of the gametophytes, the next two essential steps of sexual reproduction, i.e., pollination and fertilisation take place. As a result zygote is formed, which eventually develops into the embryo. Before fertilisation can occur, the pollen must be transferred from the stamen to the stigma of the carpel. The transfer of pollen is known as pollination and it may be accomplished by a number of agencies such as wind, water or animals. Double fertilisation, a process unique to flowering plants, follows pollination. In this unit you will become familiar with the types of pollination, some of the important adaptations exhibited by plants for successful pollination, details of the structural features of the pistil, pollen-pistil interaction, double fertilisation, incompatibility and apomixis. Objectives After studying this unit you should be able to: explain the process of sexual reproduction in flowering plants; describe the methods that the plants adapt to disperse the pollen grains for effective pollinatiow, interpret as to why in certaiq- ases hybridization fails; defrne the methods to overcome incompatibility; analyse how apomixis operates to ensure survival in certain plants; e relate the control mechanism that plants have developed to avoid indiscriminate Polllnatlan and FertIUzatlon I sexwl reproduction. 3.2 POLLINATION Pollination refers to the transfer of pollen from dehiscing anthers to the pistil. Unlike animals, plants cannot move to their mates fur sexual reproduction. Hence, they need some external &vice or agency for the transfer of pollen grains from the male parent to the stigma of the female parent. Exceptionally, in Vallisnena, an aquatic plant, complete male flower may be transported to the female flowers, The physical (wind and water) and biological (insects, birds and bats) agencies promote cross-pollination. The &hiscene of anthers and the transfer of pollen are the prime requirements of pollination. Anther dehiscence: Anther dehiscence simply means the release of pollen graims from dry and mature anthers. It involves the rupturing of anther wall due to the mechanical pressure &veloped by the fibrous thickenings of endothecial cells along the stomium (the area where mechanical layer does not differentiate). If endothecium is lacking, the L mechanical role is passed on to epidermal cells. In most of the angiosperms the stomium . is a narrow strip along the entire length of the anther lobe. It may, however, also be restricted to a lid or valve (Berberidaceae) or pores (Solanurn, Cassia, Polygala). Pollen Transfer: The transfer of pollen can be autogamous (self-pollination) in which -the pollen grains of an anther reach the stigma of the same flower. In cross-pollination, the pollen of one plant reach the pistil of some other plant of the same species. If the pollination occurs between two flowers of the same plant it is termed geitonogamy and , if it is between two flowers on different plants it is xenogamy. 3.2,l Types of Pollination Self-Pollination I 1 Self-pollination refers to the transfer of pollen grains from the anther to the stigma of I the same flower. In chasmogamous flowers the mature anthers and stigma are exposed to pollinating agents. In cleistogamous flowers fertilization is accomplished without I exposing the sex organs (cleistogamy) to the abnosphere. I I Cotnmelina benghalensis produces both chasmogamous (aerial) and cleistogamous I (underground) flowers. Conversion of chasmogamous flowers to cleistogamous type depends on environmentd conditions, such as temperature. Cross-pollination In this kind of pollination the pollen &om anther of one individual is transferred to the stigma of another individual of the same species. The process is mediated by physical or biological agents thdnclu& wind, water, insect, birds or mammals. Whereas, cross- pollination is obligatory in unisexual flowers, the bisexual flowers may have adaptations that prevent self-pollination. These adaptations include: self sterility, dichogarny, herkogamy and heterostyly. These have been discussed in Subsection 3.2.2. We shall now take up some of the common agents that mediate cross-pollination. (a) Anemophily: It is also commonly referred to as wind pollination, i.e., the pollen grains are carried through wind currents. To ensure good pollination the anemophilous plants produce astronomical number of small, dry, light and smooth pollen grains that are released preferably on warm and dry days. Flowers in such plants are unisexual with reduced sepals and petals so as to effectively position the t long and feathery stigma for pollen interception. Stamens have long filaments and I are exposed to facilitate convenient pollen dispersal, Palms, grasses, millets, t I bamboos are common examples. (b) Hydrophily: All hydrophytes are not necessarily pollinated hy water. In fact most of the aquatic plant. are anemophilous, e.g., Myrioph~~llr~rn,k"--t 9,geton or entomophilous, e.g.. Alisma, Nymphaea. Like arre~nophilot~stz- tb floral envelops i Plant Development-I are highly reduced or absent in hydrophilous plants. Hydrophily may involve underwater pollination referred to as hyphydrophily, e.g., Ceratophyllum, Majus, Zostera. A unique example of this type is Zostera mariana (a submerged marine perennial) in which pollen grains are long (up to 250pm) and needle-like resembling pollen tubes. Because of the specific gravity of these pollen they freely float at any depth, and when they come in contact with the stigma they coil around it. In some taxa ephydrophily operates. In these plants pollination is brought about at the surface of water. The classical example is the submerged dioecious plant, Vallisneria. The male and female flowers are produced under water but on maturity the males get detached from the stalk and float on the surface while the female flowers attached to thin, spirally coiled, long, slender stalks are brought to the surface at the time of pollination. Pollination is achieved through water currents when male flowers come in contact with pistillate flowers. After pollination the flowers are dragged down to the bottom by the recoiling of the stalk. The fruits thus develop under water. (c) Entomophily: It involves insects to carry the pollen to achieve pollination. Salvia exhibits a specialised 'tumapipe' floral mechanism that signifies classic adaptation for bee pollination. In Salvia the corolla is bilipped and the stamens are attached to corolla tube. Only one-half of each anther is fertile, the other half being sterile joins together to form a sterile plate of tissue placed above the lower lip at the mouth of the flower. However, the fertile part lies under the hood of the upper lip of the corolla. When a bee visits the flower for nec'tar it pushes against the sterile plate which consequently brings down the fertile anthers on its back dusting it with pollen. When the bee visits another flower, the forked stigmas picks up the pollen from the back of the insect. Plants that are pollinated by insects often have predominently yellow or blue petals. Among insects bees and butterflies do not perceive colour in the same manner as the humans. They are able to see in the ultraviolet range of the electromagnetic spectrum, an area that is invisible to human eye. They see blue and yellow flowers differently than humans. Red appears black to them. Consequently, flowers that are pollinated by insects are not usually red. Many insect-pollinated flowers have dramatic ultra-violet markings, that are invisible to us but direct the insect to the flower, where pollen or nectar may be located. Insects have a well developed sense of smell. Some flowers have developed "fly-trap mechanism" for their pollination, by emitting unpleasant odours, e.g., Raflesia (rotten meat), Arum (human excreta), and Aristolochia (decaying tobacco and humus). In the orchid, Ophrys speculum a highly specialized type of pollination has evolved. It is pollinated by :hairy wasp, Colpa amea. As the appearance and smell of female wasp matches with that of orchid flowers, the male wasp mistakes t'he flower as its female partner and carries our pseudo-copulation. In this process the transfer of pollinia from one flower to another takes place. An obligate symbiotic relationship has been observed between a moth Tageticula and the plant Yucca. The moth cannot complete its life cycle without the association of Yucca flower and in turn Yucca has no other pollinator. The female moth lays her eggs in the ovary. Neither would be able to reproduce successfully without the other. In case, one species were to become extinct, the other would also become extinct eventually. (d) Omithophily: In tropical areas, the birds dominate over insects as important pollinators. The most common among them are humming-birds, sun-birds and honey-eaters. Flowers pollinated by birds are usually red, orange or yellow. Birds see well in this region of visible light. Birds do not have a strong sense of smell; conseiguently, bird-pollinated flowers usually lack much scent.