Plant Growth and Development PDF

PLANT GROWTH REGULATORS DR. R. RAVINDHRAN  GROWTH & DEVELOPMENT Growth is a continuous process GROWTH Irreversible change in mass, i.e. increase in size, volume and weight of any part of plant’s body. It means quantitative increase in plant body. E.G. CELL DIVISION...

PLANT GROWTH REGULATORS DR. R. RAVINDHRAN  GROWTH & DEVELOPMENT Growth is a continuous process GROWTH Irreversible change in mass, i.e. increase in size, volume and weight of any part of plant’s body. It means quantitative increase in plant body. E.G. CELL DIVISION CELL ENLARGEMENT. Development is phase to phase process DEVELOPMENT Irreversible change in state. It means the qualitative change in plant body. E.G. SEED SEEDLING VEGETATIVE MATURATION FLOWERING.  Plant’s growth and development are under the control of two sets of internal factors.  Nutritional factors such as the supply of carbohydrates, proteins, fats and others constitute the raw materials required for growth.  Proper utilization of these raw materials is under the control of certain “CHEMICAL MESSENGERS” which can be classified into hormones and vitamins. HORMONE  The site of synthesis is different from the site of action. ( BEYLIS AND STARLING, 1902).  Plant hormones are physiologically active. VITAMIN  Vitamins are used in the same part without being transported.  Vitamins by themselves are not physiologically active. they act as co-factor of enzyme.  PLANT HORMONES THIMANN (1948) suggested using the term ‘phytohormone’ for hormones of plant.  Also termed as growth hormones growth regulators growth promoting substances growth factors etc.. growth substances all plant hormones are plant growth regulators but, all plant growth regulator are not plant hormones. PLANT GROWTH REGULATORS Plant growth regulators – include plant hormones (natural & synthetic), but also include non-nutrient chemicals not found naturally in plants that when applied to plants, influence their growth and development.  CLASSIFICATION OF PGR On the basis of origin NATURAL HORMONE: Produced by some tissues in the plant. also called endogenous hormones. E.g. IAA. SYNTHETIC HORMONE: Produced artificially and similar to natural hormone in physiological activity. also called exogenous hormones. E.g. 2,4-D, NAA etc. POSTULATED HORMONE: Also produced spontaneously in the plant body, but their structure and function is not discovered clearly. E.g. FLORIGEN, VERNALIN. On the basis of nature of function GROWTH PROMOTING HORMONES/GROWTH PROMOTER: Increase the growth of plant. E.g. AUXINS. GIBBERELLINS, CYTOKININS ETC. GROWTH INHIBITING HORMONES/GROWTH RETARDANT: Inhibit the growth of plant. E.g. ABA, ETHYLENE.  GROWTH REGULATORS 5 Recognized groups of natural plant hormones and growth regulators.  AUXINS  GIBBERELLINS  CYTOKININS  ETHYLENE  ABSCISIC ACID  AUXINS DERIVED FROM THE GREEK WORD "AUXEIN" MEANS- "TO GROW/INCREASE". AUXINS MAY BE DEFINED AS GROWTH PROMOTING SUBSTANCES WHICH PROMOTE GROWTH ALONG THE VERTICAL AXIS WHEN APPLIED IN LOW CONCENTRATION TO THE SHOOT OF THE PLANT. DISCOVERY OF AUXINS DISCOVERY OF AUXINS Arpad Paál (1919) - Asymmetrical placement of cut tips on coleoptiles resulted in a bending of the coleoptile away from the side onto which the tips were placed. F.W. Went (1926) successfully discovered and isolated this growth substance from Avena sativa (Oat) coleoptiles tips. Kogl and Haagen-Smit(1931) named it as “auxin”. OCCURRENCE AND DISTRIBUTION OF AUXINS occurs universally in all plants. where there is active growth there is auxin production. growing meristem and enlarging organs produces auxin. shoot apex produces much auxin than root apex. apical bud synthesizes more auxin than lateral buds. developing seeds contain more auxin than matured seeds. apical bud synthesizes six times more auxin than expanding leaves.  AUXINS NATURAL AUXIN: which are almost continuously produced by some tissues in the plants, also known as endogenous growth substances. E.g IAA (Indole acetic acid) SYNTHETIC AUXINS: produced artificially and similar to natural in their physiological activity.  IPA (Indole propionic acid)  IBA (Indole butyric acid)  NAA (Napthalene acetic acid)  2,4-D (2,4 – Dichlorophenoxy acetic acid)  2,4,5-T (2,4,5 – Trichlorophenoxy acetic acid) etc.  EFFECTS OF DIFFERENT AUXIN ON PLANT GROWTH AND DEVELOPMENT  CELL ELONGATION AND CELL DIVISION causes growth in coleoptiles and stem due to elongation of already existing cells. the main causes of cell elongation- by increasing the osmotic content, permeability of cell to water, wall synthesis. by reducing wall pressure. by inducing the synthesis of rna & protein which in turn lead to an increase in cell wall plasticity & extension. auxin also induces / promotes cell division within the cambial region.  APICAL DOMINANCE apical or terminal buds of many vascular plants are very active while the lateral buds remain inactive. removal of apical buds promotes lateral buds to grow. apical dominance is due to much higher auxin content in the apical buds than lateral buds.  EFFECTS OF DIFFERENT AUXIN ON PLANT GROWTH AND DEVELOPMENT  ROOT INITIATION application of IAA to cut end of a stem promotes root formation.  CONTROL OR PREVENTION OF ABSCISSION abscission does not occur when auxin content is high on distal end and low in the proximal end of abscission zone.  PARTHENOCARPY auxin induces parthenocarpy.  CALLUS FORMATION undifferentiated mass of parenchymatous tissue is known as callus. application of IAA causes cells to elongate & adventitious root.  GIBBERELLINS  Discovered by KUROSAWA, a japanese plant pathologist in 1928.  Rice plants infected by the fungus GIBBERELLA FUJIKUROI (SYNONYM: FUSARIUM MONILIFORME) showed excessive stem elongation.  Symptom is called ‘bakane’ diseases.  Chemical was extracted & purified and named as gibberellic acid (GA).  now 80 different gibberellins are available- GA1 to GA80 is available.  the most commonly occurring gibberellins is GA3. GIBBERELLINS (GA) have a regulatory function are produced in the shoot apex primarily in the leaf primordial (leaf bud) and root system stimulates stem growth dramatically  GIBBERELLINS stimulates cell division, cell elongation (or both) and controls enzyme secretions. E.g: dwarf cultivars can be treated with GA and grow to normal heights – indicates dwarf species lack normal levels of GA. involved in overcoming dormancy in seeds and buds. GA translocates easily in the plant (able to move freely) in both directions – because produced in not only shoot apex but also in the root structure. used commercially in: increasing fruit size of seedless grapes stimulating seed germination & seedling growth overcoming cold requirements – for some seed, application of GA foregoes the cold requirements (some seed require to be frozen or placed in the refrigerator for a period of time before they will germinate).  CYTOKININS  cytokinins promote another important process namely cell division.  developing embryo shows active cell division.  liquid endosperm of coconut called coconut water / milk contain cell division causing factors (kinetine).  similarly the developing endosperm of maize contain such factors (zeatin). found in all tissues with considerable cell division. E.g: embryos (seeds) and germinating seeds, young developing fruit. roots supply cytokinins upward to the shoots. interact with auxins to influence differentiation of tissues (may be used to stimulate bud formation).  CYTOKININS as roots begin to grow actively in the spring, they produce large amounts of cytokinins that are transported to the shoot, where they cause the dormant buds to become active and expand. tissue cultures use cytokinins to induce shoot development cytokinins may slow or prevent leaf senescence (leaf ageing or leaf fall).  ETHYLENE gaseous hormone produced in the actively growing meristems of the plant, in senescing ripening or ageing fruits, in senescing (ageing or dying) flowers, in germinating seeds and in certain plant tissues as a response to bending, wounding or bruising. ethylene as a gas, diffuses readily throughout the plant. may promote leaf senescing and abscission (leaf fall). increases female flowers in cucumbers (economically - will increase fruit production). degreening of oranges, lemons and grapefruit – ethylene gas breaks down chlorophyll and lets colors show through  INHIBITORS - ABSCISIC ACID (ABA) widespread in plant body – moves readily through plant ABA appears to be synthesized (made) by the leaves. interacts with other hormones in the plant, counteracting the growth – promoting the effects of auxins & gibberellins. involved with leaf and fruit abscission (fall), onset of dormancy in seeds and onset of dormancy (rest period) in perennial flowers and shrubs ABA is effective in inducing closure of stomata in leaves, indicating a role in the stress physiology in plants. (E.g: increases in ABA following water, heat and high salinity stress to the plant) PHOTOPERIODISM DISCOVERY The concept of photoperiodism was given by W.W. Garner & H.A. Allard of U.S Department of Agriculture,studied flowering in Maryland mammoth variety of Tobacco plant in 1920.  PHOTOPERIODISM THE BIOLOGICAL MEASUREMENT OF THE RELATIVE LENGTHS OF DAY AND NIGHT THE CONTROL OF FLOWERING- CHANGE IN DAY LENGTH PHOTOPERIOD MECHANISM IN THE LEAVES “FLORIGEN” HORMONE FLOWER BUDSThe hormone which induces flowering- florigen FLOWERING  CLASSIFICATION OF RESPONSES SDP:- Flowers when day length is shorter than critical day length. Eg. - SOYA BEAN, CHRYSANTHEMUM LDP:- Flowers when day length is longer than critical day length. E.g- OAT, RADISH, SPINACH LSDP:- Flower after a sequence of long days followed by short days. E.g- JASMINE, BRYOPHYLLUM SLDP:- Flower after a sequence of short days followed by long days. E.g- WINTER RYE DNP:- are insensitive to day length. Flowering is controlled by endogenously. E.g-BALSAM, MAIZE SDP- Short-day Plants, LDP- Long-day Plants, LSDP- Long-short Day Plants, SLDP- Short-long Day Plants, DNP- Day-neutral Plants It has significant role in bud dormancy Control of vegetative trait Tuberization in plants Bulb formation Simultaneous leaf fall in deciduous tree Dark carbon fixation in CAM plants  CRITICAL DAY LENGTH Critical day length is the photoperiod required to induce flowering. it varies from species to species. E.g- Xanthium(SDP) requires a critical day length of 15.5hrs(15.5 light/8.5 dark).if the plant gets less than 8.5hrs of dark it fails to flower. Critical photoperiod mustn't be exceeded in short day plants & should always be exceeded in long day plants. There is no relation with total day length. a single photoperiodic cycle which induces flowering-inductive cycle & its effect is called photoperiodic induction.  THE NIGHT BREAK PHENOMENON- For plants with a critical night length, a short flash of light in the middle of the night would make the plant behave as if it had been exposed to a long day THE QUALITY OF THE LIGHT- The wavelength of the light used is important COLOU WAVELEN SHORT- LONG- R GTH DAY DAY FAR > 700 nm Stimulate Reverses RED s LIGHT RED 670-680 Inhibits Stimulate LIGHT nm s  THE PHOTOPERIOD MECHANISM Phytochrome exists in two versions which are inter-convertible PR that absorbs red light PFR that absorbs far red light  The above observations indicate presence of some pigment in the leaf which must be photoreversible.  Several expts indicate that light is absorbed by a photoreversible pigment –phytochrome  This is a bluish biliprotein and exists in 2 interconvrtible form- Pr & Pfr  Pr form of phytochrome absorbs red light & converts into Pfr form.  The Pfr form of phytochrome absorbs far red light & converts to Pr form under continuous darkness. SIGNIFICANCE- the yield of tubers,corms,bulbs & rhizomes can be increased. Vegetative crops like raddish, carrot, sugarcane can be made to remain vegetative for longer periods. Annuals may be grown twice or thrice. Winter dormancy & autumnal fall can be prevented by increasing light hours.  VERNALIZATION PROMOTING FLOWERING WITH COLD  VERNALIZATION  Vernalization is the process whereby flowering is promoted by a cold treatment given to a fully hydrated seed or to a growing plant.  Dry seeds do not respond to the cold treatment.  Due to vernalization the vegetative period of the plant is cut short resulting in an early flowering.  also called as yarovization.  Without the cold treatment, plants that require vernalization show delayed flowering or remain vegetative.  In many cases these plants grow as rosettes with no elongation of the stem. HISTORY  KLIPPART,1857- first noticed the low temperature requirement for flowering while working with winter wheat and spring wheat.  LYSENKO,1938-used the term VERNALIZATION for a low temperature promotion of flowering in plants.  CHOURAD ,1960- defined VERNALIATION AS “ACQUISITION OR ACCELERATION OF THE ABILITY TO FLOWER BY A CHILLING TREATMENT”.  VERNALIZATION For vernalization the seeds are allowed to germinate for some time and then are given cold treatment 0 ̊c to 5 ̊c. the period of cold treatment varies from few days to many weeks. after the cold treatment the seedlings are allowed to dry for sometime and then sown. Vernalization prepares the plant for flowering. the cold stimulus usually perceived by the apical meristems, but in some species all dividing cells of roots and leaves may be the potential sites of vernalization E.g. LEENNARIO BIENNIS. vernaliztion induces the plant to produce a hormone called vernalin.it was discovered by melcher(1936). the vernalization stimulus can be transmitted from one plant to another through grafting. the age of the plant is an important factor in determining the responsiveness of the plant to the cold stimulus and it differs in different species.  VERNALIZATION the suitable temperature for vernalization ranges between 1 to 6 ̊c. at higher temperature from 7 ̊c onwards response of the plant is decreased. a temperature of about 12 to 14 ̊c is most ineffective in vernalizing the plant. the vernalization is an aerobic process and requires metabolic energy. in the absence of oxygen cold treatment becomes completely inefficient. sufficient amount of water is also essential. vernalization of dry seeds is not possible. MECHANISM OF VERNALIZATION TWO THEORIES.. 1. PHASIC DEVELOPMENT THEORY 2. HORMONAL THEORIES.  PHASIC DEVELOPMENT THEORY Proposed by LYSENKO in 1934. According to this theory there is a series of phases in the development of a plant. each phase is stimulated by an environmental factor such as temperature, light, etc. Commencement of one phase will take place only after the completion of the proceeding phase. there are two phases 1.Thermophase 2.Photophase Thermophase depends on temperature. Vernalization accelerates thermophase. Thermophase should be followed by photophase which requires light.  HORMONAL THEORIES MELCHER (1939)  He proposed that chilling treatment induces the formation of a new floral hormone called Vernalin.  This hormone is transmitted to other parts of the plant.  He graphed a vernalized plant with an unvernalized plant.  The universalized plant also initiates flowering.  The hormone vernalin diffuses from the vernalized plant to the unvernalized plant and induces flowering. DEVERNALIZATION The reversion of vernalization by high temperature treatment is called devernalization. Devernalization is effected by treating the vernalized seeds or buds with high temperature. Lang et al (1957) demonstrated that application of gibberellins can replace the cold treatment for vernalization in certain biennial plants.