LU3 Plant Reproduction PDF

LU3: Plant Reproduction  Sexual Reproduction Sexual reproduction in plants requires that flowers form, that pollination and fertilization occur, that seeds develop, and that the seeds grow into new plants. The flower is the sexual reproduction unit that functions to produce and hous...

LU3: Plant Reproduction  Sexual Reproduction Sexual reproduction in plants requires that flowers form, that pollination and fertilization occur, that seeds develop, and that the seeds grow into new plants. The flower is the sexual reproduction unit that functions to produce and house gametes (sex cells) and to attract pollinators.  Sexual Reproduction Meiosis Sexual activities begin within the ovules and anthers of a flower. Diploid (2n) cells in the pollen sacs on the anther go through meiosis to produce haploid cells known as microspores. Each microspore nucleus divides by mitosis to form two haploid nuclei – the tube nucleus and the generative nucleus. The outer wall of the microspore hardens, and the  Sexual Reproduction The formation of a pollen grain. Source: Rick Parker (2004)  Sexual Reproduction In each ovule, a single diploid cell undergoes meiosis to produce 4 haploid cells. Only one survives. This surviving cells is called a megaspore. The megaspore nucleus divides by mitosis to form 2 haploid nuclei. Mitosis continues until a total of 8 haploid nuclei are produced. These nuclei and the cytoplasm around them represent the entire gametophyte generation. Of the 8 nuclei produced, only 3 are important in reproduction – 2 polar nuclei in the center of the ovule and the true egg at one end. The remaining 5 haploid nuclei die. The formation of a an egg. Source: Rick Parker (2004) Significance of sexual reproduction: Sexual reproduction makes it possible to combine genes from two parents into a single hybrid plant. Recombination of these genes produces a large number of genotypes. This is an essential step in creating variation through hybridization. Almost the entire plant breeding is based on sexual reproduction. Pollinat ion When mature pollen grains, released from anther, are carried to stigma of a flower of the same or different species, it is called pollination. Pollination is of two types: Cross-pollination and Self- pollination  Pollinat ion Although self-pollination less energetically demanding since it does not require production of nectar or extra pollen as food for pollinators, self-pollination leads to less genetic diversity in the population since genetic material from the same plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination (or out-crossing) leads to greater genetic diversity because the pollens and ovules are derived from different plants. Importance of Pollination : 1. It results in fertilization and stimulates the ovule to get converted into seed. 2. New varieties of plants are formed through new combination of genes in case of cross pollination. 3. During pollination pollen tube produces growth hormones which stimulate ovary to develop into fruit. Cross pollination is brought about by various external agencies such as, wind, insects, water, birds and other animals.  Pollinat ion not all plants can self-pollinate because: However, The pollen and the ovary mature at different times. The flowers have physical features that prevent self- pollination, such differences in anther and stigma length. The male and female flowers are located on different plants, such that each plant makes only male or only female gametophytes. olution to these problems? CROSS-POLLINATION Cross-Pollination Agents Pollination by wind (Anemophily) : (Anemos : wind, Phile: to love) Pollination by insects (Entomophily) : (Entomo : insect, phile : to love) Pollination by Water (Hydrophily) (Hydros : water) This takes place in aquatic plants. Pollination by Animals (Zoophily) (Zoon : animal)  Fertilizati on Pollen grain bears two male gametes and one tube cell. Each pollen grain forms a small tube-like structure called pollen tube which emerges through the germ pore. The contents of the pollen grain move into the tube and the tube nucleus occupies the tip of the pollen tube. Pollen tube grows through the tissues of the stigma and style and finally enters the ovule through the micropyle. Vegetative nucleus or the tube nucleus degenerates and the two sperms (or male gametes) released into the embryo sac. Fertilization One sperm fuses with the egg and forms a diploid zygote (2n). The other sperm fuses with the secondary nucleus to form the primary endosperm nucleus which is triploid (3n) in nature. This process is termed as double fertilization. After fertilization, a seed develops. The zygote (2n) develops into an embryo. The endosperm nucleus (3n) divides many times to form a mass of tissue called the endosperm. This is the food source for the embryo. Fertilization The process of fertilization. Source: Rick Parker (2004) ruit Development and Maturation Upon fertilization, the ovary begins developing into a fruit and the ovules into seeds. As the fertilized egg within the ovule develops into an embryo, the ovule walls develop into a seed coat, forming the ovule into a seed. The seed serves as the unit of dispersal for the new plant. It also provides some protection from injury and drying and some nourishment for the young plant until it can make its own food. Seeds are living organisms in a dormant or inactive state. Seeds Most seeds have three main parts: (1) seed coat, (2) endosperm and (3) embryo The seed coat protects the seed and may be very thick and hard. The endosperm feed the embryo until the young plant can make its own food. The embryo is made up of three parts: (1) the young shoot (plumule), (2) the stem (hypocotyl) and (3) the root (radicle) Seeds A seed will begin to germinate when conditions inside and outside the seed are suitable. The most critical factors for germination are moisture, oxygen and temperature. Water Water helps soften the seed coat. The seed absorbs moisture and swells, and the embryo begins to grow by cell division. Water is necessary for movement of the stored food in a seed to the growing points of the embryo. The growing embryo breaks through the seed coat that has been softened by water. Temperature The temperature at which germination occurs may range widely. Some seeds, like cabbage, germinate best at low temperatures, whereas others, like okra or rice require warmer temperatures. Oxygen Germinating seeds also need oxygen. Dormant seeds also require oxygen, but not as much. Seeds planted in soil saturated or covered by water may swell because they have absorbed water, but fail to germinate because they do not get enough oxygen. Dormancy Seed dormancy is the temporary failure of a viable seed to germinate after a specific length of time under a particular set of environmental conditions that normally allow germination. Seed dormancy is imposed by abscisic acid hormone during seed development which persists in mature seeds. Therefore, it is not unusual that mature seeds fail to germinate immediately after dispersal or harvest, even under the conditions favorable for germination (eg, water, oxygen, optimal temperatures). Dormancy Seed dormancy lasts from a few weeks to several years, depending on species. This is an important survival strategy of plants in their natural environments. Dormancy allows seeds to eventually germinate under suitable conditions, and different levels of dormancy among individual seeds in a population allows for a spread of germination with time, which is critical for the continuation of a species. Overcoming dormancy There are several ways to overcome dormancy and force seeds to germinate: Seed may be soaked in hot water for several hours. Some seeds are soaked in acid to soften the seed coat. Dormancy caused by hard seed coat can be broken by scarification. The scarification method involves scratching of the seed coat by some mechanical process to allow moisture or oxygen to enter the seed. Dormancy caused by embryo can be overcome by placing the seed o  Asexual Reproduction Within the flowering plants, numerous methods of asexual reproduction have evolved. Many plants are able to propagate themselves using asexual reproduction. This method does not require the investment required to produce a flower, attract pollinators, or find a means of seed dispersal. In asexual reproduction, offsprings are produced from a vegetative unit formed by a parent without any fusion of gametes or sex cells. Asexual reproduction produces plants that are genetically identical to the parent plant because no mixing of male and female gametes takes place.  Asexual Reproduction Asexual reproduction: 1.Vegetative reproduction: new plant develops from a portion of the parent plant 2.Apomixis: a unique mechanism of asexual reproduction in certain plants (e.g. dandelions) which produce seeds without pollination and fertilization. 3.Parthenogenesis: is the development of an embryo directly from an egg cell without fusing to a sperm cell.  Asexual Reproduction Vegetative reproduction It involves formation of new plantlets from vegetative (somatic) cell, buds or organs of the plant. tub er A vegetative part (Root, stem, leaf or bud) gets detached from the parent body and grows into an independent plant. corm It requires only mitotic division, no gametic fusion is involved, and newly-formed plants are genetic clones of the parent plant. rhizom bulb e  Asexual Reproduction Natural Method : In natural methods, a portion of the plant gets detached from the body of stolo the mother plant and grows into an n independent plant. The parts maybe stem, root, leaf or even flower. A bulbil is a small, Sucker: Vegetative young plant that formation of a new is reproduced stem and root vegetatively from system from an axillary buds on adventitious bud of a the parent plant's stem or root, either stem or in place naturally or by of a flower on an sucke human action bul bil inflorescence.  Asexual Reproduction Artificial method: x Cutting Layerin g  Asexual Reproduction Artificial method: Grafting Tissue culture (micropropagation)  Asexual Reproduction Advantages of vegetative reproduction: Rapid means of reproduction and spread (the resulting plant will mature faster). Offsprings identical to parent. The desired varieties can thus be preserved genetically for use. Food storage organs allow survival in adverse conditions. Since the new plant is arising from an adult plant or plant parts, it will also be sturdier than a seedling. Vegetative propagation is easier and a less expensive method of multiplying plants.  Asexual Reproduction Disadvantages of vegetative reproduction: Overcrowding and competition for space unless separated artificially. New varieties cannot be produced by this method except by mutation. As these are not genetically diverse, such plants are prone to diseases.  Asexual Reproduction Apomixis Apomixis is asexual reproduction by seed (agamospermy) without undergoing fertilization. In apomixis, the seed is replaced by a plantlet or bulbils and no gametes are involved in the process. It does not undergo meiosis. The offspring is genetically similar to the mother plant.  Asexual Reproduction Apomixis Thus, apomictic plants have the advantage of seed reproduction (mainly dispersal), but conserve the identical (successful) genotype of the parent. Examples: Dandelion, Blackberry, Meadow grass Dandelion Meadow grass Black berry  Asexual Reproduction Parthenogenesis Parthenogenesis is another type of asexual reproduction in which the development of mostly a female gamete takes place without fertilization. It produces embryos only from the egg cell. Hence, there is no fertilization with sperm cells. In parthenogenesis, as the egg cell develops into an embryo, meiosis occurs in the process. And the egg cell is haploid with half the number of chromosomes as the diploid organism.  Asexual Reproduction Parthenogenesis Parthenogenetic offspring contain haploid number of chromosomes. The fruit resembles a normally produced fruit but is seedless. Varieties of the pineapple, banana, cucumber, grape, orange, grapefruit, persimmon, and breadfruit exemplify naturally occurring parthenocarpy.