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(A)sexual reproduction of plants


Are all plants capable of sexual reproduction? Some can do asexual reproduction as well but I haven't found any plants that aren't capable of sexual reproduction. Is it correct to say all plants are capable of sexual reproduction?


No, it's not. There are common plants that can only reproduce asexually.

It's likely you can buy produce from several plants which are unable to do so at your local supermarket. The most famous example is, of course, the banana which is a sterile mutant and all bananas of a particular variety are clones, but some seedless varieties of grapes and oranges would also count (although most seedless fruit you find in the supermarket are actually produced with hormonal treatments rather than being asexual varieties).

These plants are examples of artificial selection but there are also examples of plants that have lost the ability to sexually reproduce in the wild. The evening primrose is a good example, with some 30% of species in the genus Oenothera having evolved to be asexual (Hollister et al, 2015), but this is not the only example. It is likely that, just as in animal species, asexuality has short (on an evolutionary timescale) term advantages but results in lower fitness in the longer term.


Actually, I think you might be referring to apomictic plants.

Apomictic plants are those plants that are unable to complete the normal process of sexual reproduction and have to use another alternative pathway to produce an embryo. One way out is through asexual reproduction, and another is called Agamospermy.

Agamospermy is basically the process of formation of seeds without sexual reproduction. Agamospermy is further of two types, sporophytic agamospermy and gametophytic agamospermy.

In sporophytic agamospermy, the embryo is directly formed by diploid sporophytic cells such as those of the nucellus of the ovule, and this is also known as adventive embryogeny. More often than not, in adventive embryogeny, multiple embryos are formed(this is called adventive polyembryony, but i will not go into that for now).

However in the case of Gametophytic Agamospermy, the nucellus cells first develop into a diploid egg cell( the egg cell is normally haploid, however, in this case meiosis is not possible so egg cell has to be diploid). This diploid egg cell now undergoes parthenogenesis(i.e, formation of embryo without fertilisation or syngamy) to produce an embryo which is a clone of the parent.

Now coming to the examples. Apomixis is really well marked in Citrus plants like oranges, lemons, etc and also in Mango. Also, some varieties of apple are known to show parthenogenesis(DO NOT confuse with parthenocarpy!!)

Even Rubus shows the same.


  • Most plants have roots, stems and leaves. These are called the vegetative parts of a plant.
  • Vegetative Propagation is a type of asexual reproduction in which new plants are produced from roots, stems, leaves and buds. Since reproduction is through the vegetative parts of the plant, it is known as vegetative propagation.
  • Bryophyllum (sprout leaf plant) has buds in the margins of leaves. If a leaf of this plant falls on a moist soil, each bud can give rise to a new plant.

Bryophyllum (sprout leaf plant)

  • The roots of some plants can also give rise to new plants. Sweet potato and dahlia are examples.
  • Plants such as cacti produce new plants when their parts get detached from the main plant body. Each detached part can grow into a new plant.
  • Plants produced by vegetative propagation take less time to grow and bear flowers and fruits earlier than those produced from seeds.
  • The new plants are exact copies of the parent plant, as they are produced from a single parent.

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Define Sexual Reproduction in Flowering Plants.

Events that take place before fertilization are termed as Pre-fertilization events. Following events would occur before actual fertilization takes place in a plant: Gametogenesis-Development of male & female reproductive structures. Formation of male & female gametes. Gamete transfer-Bringing together male & female gamete. Pollination occurs in plants to facilitate gamete transfer.

Know more about this in Sexual Reproduction in Flowering Plants Class 12 Notes pdf.


Contents

The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1 to 1.2 billion years ago. [10]

Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, [11] dealing with competition, DNA repair and masking deleterious mutations. [12] [13] [14] All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes. [15]

Maintenance of sexual reproduction has been explained by theories that work at several levels of selection, though some of these models remain controversial. [ citation needed ] However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms.

Sexual reproduction allows these species to exhibit characteristics that depend on the specific environment that they inhabit, and the particular survival strategies that they employ. [16]

In order to sexually reproduce, both males and females need to find a mate. Generally in animals mate choice is made by females while males compete to be chosen. This can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a Fisherian runaway. Thus sexual reproduction, as a form of natural selection, has an effect on evolution. Sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species. Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits. However, sexual selection is only implied over an extended period of time leading to sexual dimorphism. [17]


Chapter 27 : Sexual Reproduction in Plant Pathogenic Oomycetes: Biology and Impact on Disease

The relative importance of the sexual cycle in each pathosystem can vary depending on factors that include climate, host, cropping practices, patterns of pathogen migration, and the inherent biology of each species. This chapter describes the biology of sexual reproduction in phytopathogenic oomycetes and its role in disease. These infect both mono- and dicotyledonous crops, ornamentals, and native plants, causing foliar blights or root, crown, or fruit rots. Plant pathogens infect both mono- and dicotyledonous crops, ornamentals, and native plants, causing foliar blights or root, crown, or fruit rots. Various factors are reported to stimulate germination including plant extracts, light, carbon dioxide, and alternating temperature and wetness regimes, but the requirements do not seem to have universal effects on different oomycetes. Potatoes, as well as tomatoes and several other Solanum spp., are hosts of the heterothallic species P. infestans . Importantly, this inoculum would typically need to travel from their point of origin to distant potato fields before significant amounts of disease could occur, slowing epidemic progression. Nevertheless, there is evidence that sexual reproduction has occurred based on the appearance of recombinant genotypes. An interesting picture has emerged in which host, pathogen genotype, and environment interact to determine the importance of sexual reproduction. While sexual reproduction challenges efforts to control oomycete pathogens, better knowledge of the mechanisms of oospore formation and germination could lead to new management strategies. Soil populations of oospores might also be reduced by applying compounds stimulating oospore germination near the end of a growing season.


POLLINATION

Pollination refers to the transfer of pollen grains from the anther to the stigma of a flower. It is of two types: self-pollination and cross-pollination.

Agents of pollination

The transfer of pollen grains from plant to plant in cross-pollination takes place through biotic agencies (living agencies) or through abiotic agencies (non-living agencies).

Pollen – Pistil Interaction

Pollen – pistil interaction is a safety measure to ensure that illegitimate crossing does not occur.

It checks the promotion or inhibition of pollen growth. Pollen grains of a number of plants may settle over a stigma but only the right pollen belonging to the same species would germinate while others fail to do so.

The compatibility and incompatibility of the pollen-pistil is determined by special proteins. The compatible pollens are able to absorb water and nutrients from the surface of the stigma. They germinate and produce pollen tubes. Pollen tubes grow into the style. Their growth and path through the style are also determined by specific chemicals.

Significance of pollination

Pollination is a means of taking the male gametophyte for its growth near the female gametophyte.

Pollen-pistil interaction determines the suitability of pollen for carrying out the process of sexual reproduction.

It has freed the seed plants from the dependence on external water during fertilization.

It can be manipulated to produce pure lines as well as desired varieties.


Asexual Reproduction

Vegetative reproduction is a type of asexual reproduction. Other terms that apply are vegetative propagation, clonal growth, or vegetative multiplication. Vegetative growth is enlargement of the individual plant, while vegetative reproduction is any process that results in new plant &ldquoindividuals&rdquo without production of seeds or spores. It is both a natural process in many, many species as well as a process utilized or encouraged by horticulturists and farmers to obtain quantities of economically-valuable plants. In this respect, it is a form of cloning that has been carried out by humanity for thousands of years and by plants for hundreds of millions of years.


Sexual Reproduction in flowering Plants : Chapter Notes - Class 12 Biology

•Four whorls – calyx (sepals), corolla (petals), androecium (Male reproductive organ), gynoecium (Female reproductive organs) •Function of calyx:protects the bud.
•Function of corolla :attracts insects by its colour

Male Reproductive Organ

♦ Androecium consists of Stamens.
♦ Stamen consists of Stamens.
♦ Ather bilobed 4 Microsporangle.

Refer fig. 2.1 of NCERT (L.S. OF A FLOWER WITH DIFFERENT PARTS)

MICROSPOROGENESIS

Pollen grains

have two outer walls
i) Exine
ii) Intine

♦ Exine is mase of spropollenin..(Hardest natural substance). Intine is made of cellulose and pectin.
♦ Mture llen grains have two cells – large vegetative cell & small generative cell.
♦ Generative cell froms to male gameetes by mitotic division.
♦ Pollen grains (Refer fig 2.7 of text book)shed in 2-celled / 3 celled stage)

See Fig 2.5 a and b page 23

Gynoecium / carpel (the female reproduuctive organ)
(Structure of anatropous ovule)Megasporangium

♦ Each Carpel consists of ovary ,style & stigma.
♦ Ovules are attached to ovary by plaxenta.
♦ Funicle – stalk of ovule .
♦ Hilum, a region where funicie is attached.
♦ Micropyle a pore for entry of pollen tube & to imbibe
water.

Anatropous ovule

Megasporogenesis

POLLINATION– transfer of pollen from anther to stigma. Agents of pollination –air, water, insect.bat,bird,man

Double fertilization

♦ Pollen grains geminate on stigma & pollen tube grows through style.
♦ Pllen tube reaches micropyle & releases two male gametes into embryo-sac Fertilisation is the process of fusion of male& female gametes (n+n) to form a diploid (2n) zygote.

Syngamy :Fusion of one male gamete(n) with egg (n)Zygote(2n)Produced First Fusion Fusion of two Polar Nuclei(n+n=2n) Second fusion Male Gamete(n) Fuses with the fusion product of the two polar nuclei(3n) Third Fusion fusion of male gamete with egg cell.

Double fertilzation
i)Fusion of male gamete with egg – First fertilization ,SYNGAMY
ii)Fusion of fusion product of polar nuclei with male gamete – Second fertilization .TRIPLE FUSION Refer fig 2.13 in NCERT

Post fertilisation changes:
Stages of embryo development after fertilization:

1. Zygote divides by mitosis into suspensor & embryo cells
2. Suspensor cell forms a globular basal cell which remains
embedded in the endosperm & a multicellular suspensor bearing the embryo
3.Globular embryo becomes heart-shaped & then mature embryo with radicle, plumule &Cotyledons

♦ Primary endosperm nucleus – divides repeatedly to form
endosperm, food for the embryo.
♦ Mature ovary becomes fruit.
♦ Mature ovule becomes seed.
♦ True Fruit develops only from the ovary, e.g. mango, tomato
False Fruit develops from parts of the flower other than the ovary e.g. apple, peach etc.

Seeds two types:

i) Albuminous (with Endosperm)
ii) Non albuminous(without Endosperm)

Special mechanism of reproduction:-

I) Apomixis- Production of seeds without fertilisation e.g.species of Astereceae and grasses.
ii) Polyembryony- Occurrence of more than one embryo in a
seed e.g.Orange.

Outbreeding devices:

Continued self-pollination result in breeding depression. Flowering plants have developed many devices to discourage self-pollination & encourage cross-pollination such as Bearing unisexual flowers Unisexuality Anther & stigma mature at different times Dichogamy Anther & stigma placed at different positions Heterostyly Pollen grains of a flower do not erminate on the stigma of the same flower Self incompatibility Artificial Hybridisation Heterostyly Types of cross-pollination performed by man for crop improvement. Achieved by
i) Emasculation i.e. removal of anthers from the flower bud of a bisexual flower before the anther dehisces using a pair of forceps and
ii) Bagging i.e. covering the emasculated flowers with a bag of suitable size to protect them from
contamination with unwanted pollen

If flower is unisexual, emasculation is not needed. Flower bud bagged & when the stigma becomes receptive, pollination is done using desired pollen & the flower is rebagged Pollen –pistil InteractionPollen –pistil Interaction
i) All the events from pollen deposition on the stigma until the entry of the pollen
tube into the ovule are together called pollen-pistil interaction.
ii) It is a dynamic process involving pollen recognition by stigma/pistil for compatible pollen by accepting them and if incompatible rejecting them.


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