Plant Growth and Development 2023 PDF

Chapter 7 Plant Growth and Development At the end of this chapter, students should be able to:  Recognize the plant hormone: auxin, gibberellin, cytokinin, abscisic acid and ethylene  Explain the plant response: tropism, taxism and nastism  INTRODUCTION All cells of a plant d...

Chapter 7 Plant Growth and Development At the end of this chapter, students should be able to:  Recognize the plant hormone: auxin, gibberellin, cytokinin, abscisic acid and ethylene  Explain the plant response: tropism, taxism and nastism  INTRODUCTION All cells of a plant develops from the zygote. Zygote produces a number of cells which organize into tissues and organs. Development is the sum of two processes: growth and differentiation. During the process of development, a complex body organisation is formed that produces roots, leaves, branches, flowers, fruits, and seeds, and eventually they die.  Growth and Development What is growth? Growth and development are necessary for plants to survive Growth is defined as an irreversible permanent increase in the number, size, volume or mass of a cell or organ or whole organism. It is accompanied by metabolic processes i.e. anabolic and catabolic process, that occur at the expense of energy. Example:- expansion of a leaf, elongation of stem etc. The main characteristics of growth are :- 1. Cellular growth 2. Cell division 3. Cell expansion 4. Cellular differentiation Growth are measurable by a variety of parameters some of which are: increase in fresh weight, dry weight, length, area, volume and cell number. Primary growth protoderm procambium Leaf primordia Apical meristem Ground meristem Forming axillary bud Secondary growth Vascular cambium xylem Ray parenchyma cork Phloem with bands of fibers  Growth and Development What is development? Development is that include all changes that an organism goes through during its life cycle from germination of the seed to senesces. Development = growth + differentiation Differentiation occurs when cells take on a special form and function. Pattern formation is determined by the plane of cell division It is a phase to phase process Eg: seed----seedling…vegetative maturation….flowering Growth and Development What controls growth and development? Plant growth and development are under the control of two sets of internal factors 1. Nutritional factors such as the supply of carbohydrates, proteins, fats and others constitute the raw materials required for growth 2. Proper utilization of these materials is under the control of certain “chemical messengers” which can be classified into hormones and vitamins Plant Hormones and Plant Growth Regulators HORMONES PLANT GROWTH REGULATORS The term Hormone is derived from a Plant growth regulators are Greek word “hormao” which means chemical substances (natural “to stimulate” or synthetic) that influence the They are also called phytohormone. growth and differentiation of Phytohormones are organic plant cells, tissues and organs. substances produced naturally by Plant growth regulators the plants which in low concentration will increase, function as chemical decrease and modify the growth messengers for intercellular and development of plant. communication. Hormones are chemicals produced by plants that regulate the growth processes.  All plant hormones are plant growth regulators, but all plant regulators are not plant hormones.  They work together coordinating the growth and developments of plant cells. Plant Hormones and Plant Growth Regulators Plant Hormones and Plant Growth Regulators Plant hormone / Phytohormone are active in very low concentrations, are produced in certain parts of the plants are usually transported to other parts where they elicit specific biochemical, physiological, or morphological responses. They are also active in tissues where they are produced. They work together coordinating the growth and development of cells. They could be indole compounds (indole-3-acetic acid, IAA); adenine derivatives (N6-furfurylamino purine, kinetin), derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, GA3) or gases (ethylene, C2H4). Plant hormones can be divided into two classes: Growth promoters: Auxins, Gibberellins, Cytokinin Growth inhibitors: Ethylene gas, Abscisic acid  Growth promoters  Can promote plant growth in two ways:  Stimulating cell division in meristems to produce new cells.  Stimulating elongation in cells.  Auxins, Gibberellins, Cytokinin Plant hormones (In general) Auxin (for cell elongation) Gibberellin (cell elongation + cell division = increase growth) (induce seed germination) Cytokinin (cell division + inhibit senescence) Abscisic acid (abscission of leaves & fruits + induce seed dormancy + overcome water stress) Ethylene (promote senescence, epinasty, and fruit ripening) 1. Auxin  Auxin Flavors:  Indoleacetic Acid (IAA)  Phenylacetic Acid (PAA)  4-chloroindoleacetic Acid (4-chloroIAA)  Indolebutyric Acid (IBA) Use of Auxin in agriculture Rooting of cuttings Seedless fruit production (parthenocarpy) Weedicide /herbicide – 2,4-D (highly toxic to broad leaved plants/dicots), MCPA (Methyl-Phenoxy Acetic Acid) Tissue culture – together with cytokinin shows successful callus formation, root differentiation. Fruit setting – 2,4,5-T is used for improved setting of berries Timing of flower, fruit and leaves – 2,4 D is used for defoliation of cotton plant before harvesting NAA is used for fruit thinning in apple Prevention of lodging in cereals Promotion of flowering – application NAA causes uniform flowering in pineapple leading to development of uniform sized fruits Preventing of premature dropping of fruits Germination – IAA, IBA is most widely used in soaking seeds for germination Cell elongation Tip of shoot detect light and auxin is produced. (phototropism) Auxin causes cell elongation in the stem. – If light comes from an angle, auxin accumulated on the shaded side caused cell elongation and shoot bends towards light. Apical dominance Apical dominance refers to the inhibition or suppression of the growth of lateral buds or axillary buds The growth pattern of pines tree indicates strong apical dominance. Bushy plants have weak apical dominance. If the terminal or apical tip of the plant is removed lateral buds will elongate and grow and the plants may become bushier. Root initiation Auxins promote root initiation. Auxin induces both growth of pre-existing roots and adventitious root formation, i.e., branching of the roots. As more auxin is transported down the stem to the roots, the overall development of the roots is stimulated. In horticulture, auxins, especially napthaleneacetic acid (NAA) and indole-3-butyric acid (IBA) are commonly applied to stimulate root initiation when rooting cuttings of plants. – However, high concentrations of auxin inhibit root elongation. 2. Gibberellins (GA) Discovered by Kurosawa (Japanese plant pathologist) in 1928 Rice plant infected by the fungus (Giberella fujikuroi (Synm: Fusarium moniliforme) showed excessive stem elongation = “Bakane” disease Chemical was extracted & purified and named as Gibberellic acid (GA) There are more than 80 different gibberellin compounds in plants but only gibberellic acid (GA3) and GA4+7 are often used in plant tissue culture They are synthesized in young leaves, roots and shoots and transport to other parts of the plant. Site of production Apical meristems (shoots and root tips), Young leaves, Embryo Germinating seed Fruits, tubers Synthesis: Acetyl CoA Production increases in the dark 2. Gibberellins (GA) Functions: Promote stem elongation Break seed dormancy – seed germination Bolting in rosset plant – dwarf plant to growth tall Increase the size of leaf and fruit Induce parthenocarpic fruit o/ development of seedless fruit Discovered in association with Foolish disease of rice (Gibberella fujikuroi) uninfected infected Found as the toxin produced by some fungi that caused rice to grow too tall GA in breaking seed dormancy GA also have a fundamental role in breaking seed dormancy and stimulate germination The endosperm of seeds contains protein and carbohydrate reserves upon which a developing embryo relies for energy ad nutrition These reserves must be mobilized and transported to the embryo A range of hydrolytic proteolytic enzymes break down endosperm starches and proteins into smaller, more easily transported molecules, such as sugars and amino acids. Agricultural application of GA 3. Cytokinins (CK) First cytokinins discovered is kinetin by Skoog and Miller (1973) from corn-kernels and coconut milk. The most active, naturally-occurring cytokinin is zeatin More cytokinin induces growth of shoot buds, while more auxin induces root formation Main functions of cytokinins are to stimulate cell division, and differentiation in plant roots and shoots 3. Cytokinin (CK) Growth inhibitors 4. Abscisic acid (ABA) Drought resistance Abscisic acid is the key internal signal that facilitates drought resistance in plants Under water stress conditions, ABA accumulates in leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss. ABA causes the opening of efflux K+ channels in guard cell plasma membranes, leading to a huge loss of this ion from the cytoplasm. The simultaneous osmotic loss of water leads to a decrease in guard cell turgor, with consequent closure of stomata. 5. Ethylene Promote fruit ripening Under natural conditions, fruits ripening undergo a series of changes, including changes in colour, declines in organic acid content and increases in sugar content In many fruits, these metabolic processes often coincide with a period of increased respiration, the respiratory climacteric Based on ripening behavior, fruits can be classified into two: Climacteric: displaying climacteric rise in respiration non-climacteric: manifest continuous down drift in respiration and ethylene evolution Fruit ripening PLANT RESPONSE  Plant response Plants respond to different stimuli usually by showing growth responses A stimuli is anything in the environment that causes a response in an organism May be come from outside (external) or inside (internal) the plant Plants respond to changes in the environment by growing their stems, roots, or leaves toward or away from the stimulus.  Tropis m  Tropism are movements of plant that respond to the direction of the external or environmental stimulus (e.g. temperature, humidity, light irradiance).  When the movement is towards the stimulus, it is called positive tropism.  When the movement is away from the stimulus, it is called negative tropism.  Several forms of tropism:  Phototropism (respond to light)  Geotropism or Gravitropism (respond to gravity)  Thigmotropism (respond to touch)  Chemotropism (respond to chemical)  Hydrotropism (respond to water) Phototropis m Phototropism is the growth movement of plant organs in response to light. Shoots bend toward light - positive phototropism (allows shoots to capture more light mediated by the plant hormone auxin) Roots bend away from light - negative phototropism The seedlings turn in the direction of the sun throughout the day is an example of phototropism In general, tropisms involve cell elongation or suppression of cell elongation on one side of a plant, causing the plant to grow in a particular direction.  Geotropism or Gravitropism  Geotropism is growth movement of plant in respond to gravity.  Positive gravitropism - the root grows downward toward the direction of the of gravity mediated by gravity sensing cells in root cap  Negative gravitropism - the shoot, moves upward in opposite direction to gravity, mediated by auxin - causes lower side of stem to elongate. The bending of shoot away from the pull of gravity. Bending of root toward the center of the earth Thigmotropism The growth movement of plants in response to touch of the solid object or physical contact. mediated by auxin and ethylene. It results to bend and the coiling of entire stems on supports or in other plant organs such as leaves, petioles, and roots. The bend is due to differential growth, that is, more cell division and elongation at the outer than at the inner side in contact with the support. Examples: bitter gourd (tendrils), long bean, cucumber. Chemotropism Chemotropism is the growth response or movement of a plant parts to a particular chemical stimulus, Eg: – Roots grow toward useful minerals in the soil but away from acids. – Growth of pollen tube towards ovules Hydrotropis m is the growth response of a plant to water stimulus. Roots exhibit Positive Hydrotropism (grow towards water). Shoot display a negative hydrotropism (growing away from water)  Nastism are non-directional movement of plant in response to external stimuli. The direction of response is predetermined by internal control mechanisms within the tissues and the structure of the plant organ itself, Unlike tropisms, there is no pronounced bending toward or against the direction of stimulus. The direction of movement is not determined by the direction of the stimulus. This response is reversible and repeatable. Several forms of nastism:  Thigmonasty (Mimosa, Venus flytrap)  Nyctinasty  Hydronasty Thigmona sty  Plant movement in response to touch or physical contact without regard to the direction of the stimulus.  closing of the insect-eating plant Venus’s flytrap (Dionaea muscipula)  movement of plant due to touch (Mimosa pudica). 1. Seismonastic movement The shock movement Movement in plants in response to touch as well as other forms of physical contact or mechanical disturbance such as shaking, wound, wind, raindrops and intense heat or burning. In the case of sensitive plant (Mimosa pudica), a leaflet, leaf or group of leaves rapidly folds and bends in response to the external stimulus 2. Haptonastic movement Haptonastic occurs due to stimulus of touch. Plant movement in response to touch of physical contact without regard to the direction of the stimulus. Thigmonastic movements are exemplified by the closing of the insect-eating plant Venus’s flytrap (Dionaea muscipula) and the bending of the glandular hairs of sundew (Drosera sp) as a result of contact with an insect  Nyctinasty (sleep movement) This is induced by diurnal variations in light and temperature, The sleep movement (opening and closing) of plant organs, such as leaves and flowers, due to day and night periods of daily rhythm. The leaves of many nyctinastic plants open during the day or part of the day and close at night. The diurnal movements. Photonasty Thermonasty Epinasty& Hyponasty Photonasty:  The principle stimulus of Photonasty is the photoperiod.  Example: Oxalis triangularis (Shamrock flower) DAY NIGHT Thermonasty: It takes place due to temperature changes. Example: Tulipa clusiana (Tulip flower) Rise Temperature Low Temperature downward-bending from hyponastic response is an growth at the top, for example, upward bending the bending down of a heavy of leaves or flower other plant parts, resulting from growth of the lower side. Hydronasty plant movement (the opening and closing of some flowers) to the changes of water or atmospheric humidity.