Let us make an in-depth study of the physiology of reproduction in plants. After reading this article you will learn about: 1. Vegetative Propagation 2. Sexual Reproduction 3. Asexual Reproduction and 4. Physiological Aspects of Sex Determination in Plants.
Ultimately the end of all living organism is death, either natural or accidental. In order to perpetuate a species, it must reproduce and have descendants of its own kind.
It is certainly an essential criterion of life. The main feature of reproduction consists in a portion of an individual continuing after its death and possessing the power of developing into new individuals.
The ways in which reproduction in vegetable kingdom is carried out may be divided broadly into two categories: vegetative propagation and sexual reproduction. While in unicellular and simple multicellular plants, every cell can serve for reproduction, in complex highly organised higher plants a division of labour is sometimes apparent between vegetative organs and organs of reproduction.
This division of labour culminates in the formation of highly specialised forms—the flowers—which are exclusively set apart for reproduction. The subdivisions of plants into classes, orders and families are sometimes based on the diversity of structure and position of the organs of reproduction.
The formation of reproductive bodies may be considered as a stage or phase in a plant’s development. Unlike animals, plants do not bear differentiated reproductive organs at all stages of their life cycle. Rather, they produce their reproductive organs at appropriate seasons in response to primarily environmental stimuli and then consummate the reproductive process fairly promptly. In the century plant, Agave, there is vegetative growth for twenty to even hundred years or so before flowers form. The plant immediately dies after the fruits are produced.
In many plants with bulbs, vegetative growth is terminated by the formation of both sexual and vegetative structures of reproduction, flowers and bulbs or corms. In other species after a period of vegetative growth, both this and production of flowers may continue together as in some perennials and also in some annuals.
In other perennials, however, there appears to be more or less definite vegetative and reproductive phases in each year’s growth as in many trees which produce flowers before their vegetative growth is renewed. They usually shed their reproductive organs and produce them again in the subsequent year.
Reproduction commences as a rule when vegetative growth is slackening. It has even been said that vegetative growth and reproduction are antagonistic to each other and in general it is true that all plants must make a certain minimum amount of vegetative growth before they can make preparation for reproducing their kind.
Contents
Vegetative Propagation:
It is commonly seen in many familiar instances among the horticultural important plants. Among higher plants, which no longer produce descendants sexually, plants like cultivated bananas, some forms of vines, oranges, strawberries, etc. may be mentioned. With ornamental season flowers, e.g., rose, carnation, chrysanthemum, etc., the commonly prevalent practice is to remove shoots from the parent plant and to place the excised shoots in soil under conditions suitable for the formation of new roots.
The modern practice of treating excised shoot ends with artificial hormones to facilitate rooting before putting them into soil has been of great economic importance particularly in plants where such rooting does not take place under normal conditions. The rooted cuttings constitute new individuals which are identical with the parent in genetic constitution. Many kinds of fruit trees, including mangoes, apples, litchies and various citrus species are propagated by grafting.
In grafting, a bud or short piece of stem containing a bud is removed from the parent plant and its tissue is brought into contact with the exposed meristematic cambial cells of a plant of the same or related species. When the graft-union is established, a new individual is produced whose upper and lower portions both retain their individual genetic constitution of the parent from which they are derived. Trichosanthes dioica (Palwal) is propagated by root cuttings. Adventitious buds formed in Bryophyllum leaf-margins can grow into new individuals.
In some plants such as Globba bulbifera, garlic etc., flower-buds are modified into bulbils which grow into new plants when separated from the parent.
Vegetative propagation by fragmentation is of common occurrence in lower cryptogams. Formation of new individuals by budding in yeast, by simple division of the parent body in many algae and fungi, by formation of adventitious branches and by gemmae in liverworts and mosses, are well known. Vegetative propagation after perennation through unfavourable environmental conditions is a characteristic of underground stems and tuberous roots.
Thus, it is evident that plant species can be perpetuated by simple vegetative propagation and it must be noted here that in many cases no degeneration associated with purely vegetative multiplication has really been observed.
Thus, if the monogenic reproduction—where the progeny is identical in genetic constitution with the parent—is sufficient to maintain species, why then the more uncertain and at the same time more complex sexual reproduction has evolved in higher plants? We do not know the precise answer to that yet as we do not know fully the chemical processes associated with the complex control mechanism of sexual reproduction.
Sexual Reproduction:
If we consider the case of the lower cryptogams such as algae and fungi, it might be supposed that sexual reproduction led to the formation of specially resistant zygotes which could endure a longer period of rest under unfavourable conditions compared to spores produced asexually. But even this is reversed in higher cryptogams such as pteridophytes where the fertilised egg cell must develop immediately after union with the sperm or they perish; here the asexual spores are adapted to endure unfavourable habitat conditions.
It is the rule in higher plants that sexual cells are individually incapable of further development unless they unite in pairs. Why are the individual sexual cells arrested in their growth if no union with its opposite number takes place? Can we say that sexual union removes the arrest of growth and the independent development of sexual cells which was evident before they united, for the united zygote is certainly potentially capital of developing and growing into a new individual?
It is true that in the gradual loss of the capacity of independent development of sexual cells, the probability of fusion between pairs was increased? And this union certainly makes the new individual produced from it, different from either of the sex cells for the progeny is formed by the fusion and mingling of characters (genes) of both.
Sexual reproduction certainly brings about a qualitative change in the growth of the plant, a change from the vegetative to the reproductive state.
In higher plants, flowering is controlled chiefly by two environmental factors, e.g., length of the day and the temperature. In the short- day and long-day flowering plants, the effects of temperature seem to be closely related with photoperiodism.
In plants which flower in short-day lengths, the response to flowering is primarily to the length of dark period. During the course of dark periods the initiation of the qualitative change from the vegetative to reproductive phase is set into motion—ultimately the vegetative buds are converted into flower primordia.
As we have seen before, this alteration in habit is theoretically possible through the production and export of one or more hormonal substance, which brings about this conversion. Plants which flower in long daylight periods of summer do not flower in short days; in such plants, long dark periods appear to produce some substance or substances, which being antagonistic to flowering, inactivate the hormone essential for the production of flowers. Flowering response of the plant to the long dark periods is essentially conditioned by the prevailing temperature of long nights.
Thus flowering of long-day plants is favoured by short nights. Still other species of plants, e.g., writer cereal varieties, flower only after their seeds or the vegetative buds have received an exposure to low temperature (a range of 0-5°C) and the appropriate length of day necessary for their flowering (when the temperature treatment is given artificially it is called, vernalisation). Tomatoes appear to flower over a wide range of day-lengths. They do not seem to have a critical photoperiod.
The initiation of the flower primordia is but the first stage in the sequence of reproductive development. As is well known, this is followed by the growth of the flower, pollination, fertilisation, growth of the fruit and the development of seed. If any one of the sequence of events fails to take place in strict order, the sexual reproduction is totally ineffective. Sometimes, even if all the steps follow one another in the right order, sexual union may not take place. This may be due to incompatibility and incompatibility is determined by some internal factors, e.g., Rh factor of blood in higher animals.
In flowering plants sometimes, although pollination may be fully effective in securing the deposition of pollen grains on the stigma, the sexual union between male and female cells does not take place due to the arrest of growth of pollen tube which fails to reach the embryo sac cell.
Other cases of incompatibility are known, such as withering away of the pollen grains after their deposition on the stigma, which may be due to the absence of certain substance secreted by the stigma and which is essential for the germination of the pollen grain. Incompatibility may also be due to length of the style. Shortening and successful grafting of style has long been accomplished as a measure against such incompatibility.
The nutritional aspect of sexual reproduction which gained a certain amount of importance a few decades ago may be briefly mentioned here. In lower plants, particularly in fungi and algae, it has been known for long time that they tend to reproduce vegetatively when there is a plentiful supply of assimilable food materials and water in the medium. A familiar example is the vegetative reproduction by budding in yeast which generally takes place when the sugar content is high in the medium.
The sexual union in yeast seems only to take place under unfavourable environmental conditions, particularly scarcity in carbohydrate food supply. It is also interesting to note that in hydrophytes which are adapted to an aquatic medium, vegetative propagation is the general rule rather than an exception.
For higher plants, Klebs proposed a sort of nutritional theory which envisaged the initiation of flowering related with nitrogen and carbohydrate contents of the plant— the so-called carbon/nitrogen ratio. A high carbon-nitrogen ratio is supposed to promote flowering by bringing about the conditions favouring the qualitative change of the vegetative buds to flower primordia, whereas a low carbon-nitrogen ratio stimulates vegetative growth and propagation. This nutritional theory, however, fails to explain the flowering in short-day plants.
Asexual Reproduction:
The formation of asexual reproductive cells, the spores, is an intermediate stage in the life histories of higher plants. In many fungi and in algae, however, spores may be the only reproductive units.
The asexual reproductive spores are really analogous to gametes or sex cells as regards their genetic constitution (genotype). The spores are, however, physiologically very different from the sexual units for spores are capable of further growth (forming the gametophyte) whereas the gametes are totally arrested in their further development unless they unite in pairs. The united diploid cell is only then capable of growing. Thus, physiologically the asexual spores can be compared with this product of union of sex cells, the zygote nucleus, which by its growth and development produces the saprophytic organism.
Individually either the male sperm or the female egg cell is incapable of any division of its nucleus but the act of union between the two cells evidently supplies a mitogenic factor or factors which stimulate the united cell, i.e., zygote to become meristematic, may be after a short time lag, and initiate such fundamental changes in the nucleus of the cell which lead to division.
Why do the chromosomes continue to exist as resting nucleus in the sex cells for indefinite period and the sex cells even degenerate if they do not chance-unite in pairs? And what make the double chromosomes of the united zygotic nucleus to become shorter and separate out at metaphase? Again, we do not know the precise answers.
Physiological Aspects of Sex Determination in Plants:
IAA and other synthetic growth substances like 2, 4-D, NAA, etc., seem to be effective in the determination of the sex of individual plants and also in the development of sex organs.
In dioecious species of hemp (Cannabis sativa) genetically male plants could be induced to produce female flowers if during the period of differentiation of flower buds the third and fourth leaves of male plants were treated with napthaleneacetic acid. This probably suggests that sexuality here is determined by the concentration of native auxin during the period of primordium differentiation and that femaleness is associated with a relatively high auxin level.
In some Euphorbiaceae, e.g., Mercurialis, carbon monoxide gas in low concentrations sometimes reduced the number of male flowers in genetically monoecious types. This effect is presumably due to the effect of carbon monoxide on auxin. In some monoecious cucumber, painting of the lower surface of leaves with
NAA caused an increase in the proportion of female flowers, sometimes altogether suppressing the production of male flowers. In general it can be concluded from these observations that female flowers tend to differentiate under higher concentrations of auxins than do male flowers.
Gibberellins promote maleness; cycocel, the gibberellin antagonist, favours femaleness. Animal sex hormones do not elicit comparable responses in plant systems and there is no conclusive evidence that progesterone or steroidal substances which occur naturally in plants determine sex expression in plants.
If sex in dioecious plants is determined by specific substances (in all polypodiaceous ferns there seems to be a specific substance that stimulates the formation of male sex organs of prothalli), these have not yet been isolated nor can they be passed from one plant to another of opposite sex by grafting.
The length of the photoperiod may also affect the differentiation of sex. If seeds of Mercurialis annua are grown in short days instead of long days (in long days the plants produced were all staminate only) the percentage of pistillate plants significantly increased. Similar results were also obtained with unisexual heads in Ambrosia (Com-positae).
Cannabis sativa, growing under 16-hour photoperiod, produced plants which were about half staminate and half pistillate but if the day-length is shortened to 8-hour, the plants were about half bisexual, half females and none males. Again in general then, long days seem to favour production of male sex organs.
Senescence:
In annual herbs flowering is a signal for the death of the individual. In perennial plants, death is put off for many years and in Sequoia (Californian Red wood), or Pinus excels a life expectancy is prolonged to four or five thousand years. Nevertheless various organs of all plants senesce at various times during ontogeny. There is apparently no reason why a plant (an open system) growing in the field or in the green house where there is no lack of nutrients and environmental conditions are also congenial, should die. Various theories have been put forward for ageing in plants and animals.
These include (i) Wear and Tear theory, according to which like the parts of a machine, the various functional components of a cell gradually become less efficient and degenerate; (ii) the Toxin theory which assumes the accumulation with time of a toxic substance capable of interfering with some vital reactions of the cell (there is no tangible evidence for this theory); (iii) Gene repression theory according to which normal functioning of genes is gradually repressed with time resulting in their functioning at suboptimal rates; (iv) Error theory—which takes into consideration the probability of errors being committed in the gene duplication, transcription and translation processes over a period of time—resulting in the formation of abnormal proteins and (v) the Mutation theory according to which after a passage of a certain length of time mutation takes place in the genome, which proves lethal (ionising radiation accelerates such charges).
In the germinating seedling, the cells of the endosperm first degenerate; this is followed by the yellowing of the first and subsequent leaves. Once flowers are formed, the floral parts quickly senesce—the sepal being the last to senesce. In the annual plant after fertilization the entire plant yellows and dries up leaving only the fertilized ovule to continue its life activities. In each cell the endoplasmic reticulum gradually disintegrates accompanied by decreased functioning of chloroplasts, mitochondria and microsomes.
As senescence sets in, marked changes take place in metabolic processes like respiration, photosynthesis, and nucleic acid and protein synthesis. While DNA, RNA and protein syntheses decrease, hydrolases and basic proteins like histones are synthesized in larger quantities than previously and new isoenzymes are also produced. Degradative changes predominate due to enhanced activity of nucleases and proteinases. There is a marked loss of chlorophyll due to increase in the activity of chlorophyllase and decreased synthesis of chlorophylls.
In plant systems senescence can be delayed by the application of growth substances like cytokinins, auxins and gibberellins. The effect of cytokinins is most marked; auxin effects are prominent only in some trees and shrubs. Gibberellin effects are less marked. The decreases in DNA, RNA and protein synthesis are checked by all of them and the synthesis of basic proteins is inhibited. As chlorophyll breakdown is also controlled, leaves retain their greenness for a longer period of time.
How time affects metabolic processes and ultimately it determines the life span of the-, individual is unknown at present. It has been suggested that every organism possesses a gene, which is responsible for the termination of the individual.
Obviously the expression of such genes is delayed in perennial organisms by years and decades and sometimes centuries. However, in microorganisms which divide by cell fission, apparently there is no death, since the daughter cell divides after a few minutes or hours.
Although individuals die as the fertilized ovule gives rise to a new plant and the same processes of germination, growth, reproduction and senescence are repeated in the same sequence from generation to generation, the information and experience of each generation is passed on to the next until the species is totally exterminated. The first living cells, which originated some three billion years ago in an abstract sense, is still living in all the individuals which are living on this planet to-day.
Dormancy:
The seed, which is a fertilized ovule, usually does not germinate immediately after its development is completed. It usually needs a period of “rest”, in which metabolic activities are largely suspended. This period of “dormant” condition is known as “dormancy”. The period varies from a few days to several months or years. Seeds like groundnut have a dormancy period of about two weeks; rice also has a short dormancy period. Some seeds like those of jack-fruit sometimes may germinate within the fruit (viviparous germination).
The causes of dormancy may be varied. This may be due to (i) incomplete development of seed, resulting in immature embryos; (ii) hard seed coat, (iii) accumulation of inhibitors and (iv) sub- or supra-optimal levels of hormones essential for the early phase of seed germination. Some seeds also need exposure to light, temperature etc.
Seed coat or its associated components quite often are impermeable to gases and thus, the respiratory CO2 cannot diffuse out of the seed, thereby restricting aerobic respiration. Some seeds (e.g., legumes) are surprisingly impermeable to water. Some seed coats are so tough that the radicles and plumules cannot penetrate them.
Breaking the seed coat by scarification or even treatment with strong acids (some seeds germinate after treatment with concentrated HNO3 for 20 min.!) removes these hurdles and the seeds germinate. In nature seed coats are softened by the activity of microorganisms in soil. This is also helped by rain fall, which probably explains why weeds are so abundant in the rainy season.
Many seeds possess considerable quantities of inhibitors, which include abscisic acid, coumarins, phenolics, alkaloids, tannins, unsaturated lactones and cyanide releasing substances. Much of these are leached out during imbibition’s with water. Supra-optimal levels of growth substances are also responsible sometimes.
When the levels of growth substances are low, sometimes they can be germinated by the application of such substances; gibberellins and cytokinins are particularly useful in this respect. Other chemicals which have been used successfully in breaking dormancy include thiourea and KNO3. Ethylene chlorohydrin is used for breaking dormancy of potato buds.
Seeds like lettuce, need application of light for germination; red light is most useful since the phytochrome pigment (P730) is involved here. Dormancy of buds of trees is also influenced by photoperiods; short photoperiods in winter months may be responsible for bud dormancy. Some rosaceous seeds need low temperature treatment.
The chemical basis of dormancy is not understood. In dormant potato buds gene function is almost totally repressed. Depression takes place on treatment of the buds with ethylene chlorhydrin, which breaks dormancy.
Viability:
After a certain length of time, even when the period of dormancy is over, seeds do not germinate. They are then considered to be ‘non-viable’. Loss of viability apparently is related to ageing. In rice seeds there are an accumulation of auxins at relatively high concentrations, along with high levels of phenolics and ABA-like substances during storage. Age-induced formation of free radicals may also be a major cause, since chemicals which destroy free radicals increase the viability of seeds. The structural integrity and functional efficiency of organelles may also be irreversibly affected.
Germination:
The early reactions in seed germinations are physical. Cellular macromolecules absorb considerable quantities of water. The hydrogen bonding and dielectric properties of water presumably play important roles but how water triggers the initial reaction by which the quiscent seed is activated to resume its normal life activities is not understood.
One of the first major events is the release of gibberellins from some bound form which then interacts with the aleuone layer surrounding the endosperm, resulting in the formation and release of α-amylase; other hydrolases like proteinase, nuclease, cellulase, lipase, phosphatase etc. are all similarly produced. This leads to the production of monomers which are utilized for producing the energy required (via respiration) for the synthetic reactions which follow. The monomers like amino acids, nucleotides and monosaccharides are used for synthesis of proteins (structural and enzymic), nucleic acids and cell wall polysaccharides.
Release of auxin from some bound form and production of cytokinins (from some t-RNA initially?) help in the elongation and division of cells in the embryonic axis. There is a controversy regarding whether the m-RNA required for the early metabolic reactions is stable and does not have to be synthesised, but there is active synthesis of t-RNA and r-RNA undoubtedly. In photoblastic seeds phytochromes may trigger the early reactions—although how this is achieved, is not understood.
Light is also responsible for the phototropic curvature and chlorophyll synthesis. Gravitational stimuli lead to the differential distribution of auxins resulting in the geotropic curvature. The newly emerged seedling is subjected to practically the same environmental conditions as its parent was and the same events take place in the same sequence. So life continues.