Plant Growth, Development and Reproduction PDF

CHAPTER IV PLANT GROWTH, DEVELOPMENT AND REPRODUCTION  CONCEPTS OF GROWTH AND DEVELOPMENT ❖Growth - An irreversible change in the size of a cell, organ or whole organism. - It includes increase in number of cells, and weight and enlargement of the cells in terms of width, length,...

CHAPTER IV PLANT GROWTH, DEVELOPMENT AND REPRODUCTION  CONCEPTS OF GROWTH AND DEVELOPMENT ❖Growth - An irreversible change in the size of a cell, organ or whole organism. - It includes increase in number of cells, and weight and enlargement of the cells in terms of width, length, diameter or area. ❖Differentiation - Cells taking on specialized form and function. ❖Plant Development - The orderly and progressive change from seed germination through juvenility, maturity, flowering and fruiting. Phases of Plant Growth and Development  I. VEGETATIVE PHASE 1.Germination stage Germination refers to the presumption of active growth on the part of the embryo, leading to the rapture of the seed coat and the subsequent emergence of the young plant. Water imbibition Enzyme activation Initiation of embryo growth - Water is absorbed in - Water absorbed in seed - New materials begin to be the hilum tissues activates the synthesized, reflected by an - Cells become turgid various enzyme increase in the size of the - Seed grows in volume systems. root-shoot axis. - Seed coat becomes - Break down stored food more permeable. - Transfer of nutrients from Rapture of seed coat and cotyledons to the emergence of young plant growing points - Synthesis of new materials through respiration  Factors affecting seed germination 1. Seed maturity There is a higher percent of germination in matured seed than in immature seeds. 2. Environmental factors a. Water This is essential for enzyme activation, thus permitting breakdown, translocation and use of reserve storage material. c. Temperature The optimum temperature for germination of most seeds is between 15°C and 30°C. The maximum temperature for most species is from 35°C and 45°C. ……..requirements for seed germination c. Air Oxygen is required for germination of most species, carbon dioxide concentration higher than 0.03% retards germination and nitrogen gas has no influence. Respiration increases sharply during seed germination. If oxygen concentration is reduced, germination of most seeds is retarded. d. Light Both light intensity and light quality influence germination. The chemical reaction is controlled by the wavelength of light absorbed in plant cells by the same chemical pigment controlling floral induction – the Phytochrome (a photo reversible protein pigment). 2. Seedling stage The stage of plant growth and development just after germination and before the juvenile stage of the plant. Two kinds of Growth Epigeal growth Hypogeal growth Growth in dicot plants Growth in monocot plants Both the cotyledon and hypocotyl is The plumule and the hypocotyl raised above the ground. The emerges above the soil cotyledon will serve as the food for surface leaving the cotyledon the new plant during its early stage. below the soil. 3. Juvenile Stage A stage of vigorous growth during which the plant cannot be readily induced to a reproductive type of growth. Growth is very fast because meristematic cells divide and elongates very fast. Description a. Exponential type of growth b. Not capable of reproductive type of growth c. Has the ability to rejuvenate d. It develops morphological characteristics – e.g. thorniness e. Not receptive to external stimuli 4. Transition Stage The plant gradually losses the characteristics of a juvenile plant and slowly acquires the characteristics of a mature plant. Descriptions: a. Decrease in growth rate b. It becomes receptive to external conditions c. The plant decreases the ability to rejuvenate 5. Maturity Stage The stage when the plant becomes potentially capable of flowering.  II. REPRODUCTIVE PHASE This is the stage where the plant normally produces flowers, fruits and seeds.  Microsporogenesis It refers to the formation of microspores or pollen grains inside the microsporangium. Sporogenous tissue The Sporogenous tissue undergo (diploid-2n) meiotic divisions (I and II) to form microspore tetrads (4 microspores) Tube/Vegetative Meiosis Cell which are haploid (n). Generative Cell These microspores mature to form pollen grains (male gametes). Pollens or microspores are very tiny round structures. After the formation, they dry up and become powdery and Each microspore will develop into are liberated from the anther to the pollen grain containing the generative environment. nucleus and tube nucleus. Megasporogenesis & Megagametogenesis During megasporogenesis, the megasporocyte (diploid) divides meiotically to form four megaspores. Out of these four megaspores, three degenerates and only one remains functional. Then, the functional megaspores divide mitotically to form 2-celled embryo sac. Further subsequent mitotic division finally forms the embryo sac.  The Embryo Sac Six of the eight nuclei are surrounded by cell walls and organized into cells. The two cells (polar nuclei) are situated in the large central cell. Three cells at the micropylar end forms the egg apparatus. It is made up of two synergids and one egg cell. Three cells at the chalazal end is known as antipodals. The matured embryo sac is 7 celled and 8 nucleated.  Pollination - It is the transfer of the pollen from the stamen of a plant to the stigma of the flower. Upon reaching the stigma of the flower, pollen grain germinates. It then absorbs nutrients secreted by the stigma and the pollen tube begins to grow. The tube nucleus remains close to the growing tip of the pollen tube and eventually disintegrates. Meanwhile the generative nucleus divides into two male gametes or nuclei and will enter the embryo sac through the pollen tube.  Methods of Pollination a. Self-Pollination (augamy) - It is the transfer of pollen from the stamens to the pistils within the same flower or between flowers in the same plant. - Examples of self-pollinated crops: bunch grapes, peach, plum, strawberry, orange, lemon, tomato, pepper, eggplant, legumes and snapdragon. b. Cross Pollination (heterogamy) - It is the transfer of pollens from the stamens of a flower of one plant to the pistil of the flower of another plant. - Examples: apple, peach, cabbage, lettuce, onion, carrot, cucumber, aster, azalea, zinnia, etc.  Fertilization Fertilization is the fusion of the nuclei of male and female gametes to form a zygote. The generative One male sperm cell fuses The second male sperm nucleus divides with the egg cell and forms cell fuses with the two polar mitotically producing the fertilized egg or zygote. nuclei and forms the two haploid sperm The zygote develops into endosperm nucleus, which cells/nuclei. embryo. will eventually develops into endosperm of the seed.  Double Fertilization In flowering plants, two sets of fertilization takes place. ✓ One occurs between the first sperm nucleus and egg cell; ✓ The other takes place between the second sperm nucleus and the 2 polar nuclei. Thus, fertilization in flowering plants is referred to as double fertilization. ❖ If, for some reasons the egg cell fails to be fertilized, the synergids (also called help cells) assume the role of the egg cell. In case fertilization occurs between the egg cell and the male nuclei, the synergids disintegrate soon after fertilization. The antipodal cells also get disorganized, sometimes even before fertilization.  Summary of Double Fertilization in Plants After fertilization, the egg cell surrounds itself with a cell wall and is known as oospore  Fate of floral parts after fertilization Floral Part Fate after fertilization a. Sepals*, petals & stamens Will wither and drop off b. Ovary Becomes the fruit i. Ovary wall Fruit wall ii. Ovule Seed iii. Integuments Seed Coat (Testa) iv. Fertilized egg Embryo *The sepals may either fall off or may remain intact in a dried and shriveled form. Guavas show such dried sepals very clearly. In eggplant, it remains.  III. SENESCENCE Senescence refers to the erosive process that accompany ageing prior to death. Forms of Senescence a. Whole plant/complete senescence It is the ageing and death of the entire plant except for seeds. b. Organ /Partial senescence The deterioration and death of plant organs such as leaves, fruits and flowers.  SEED DORMANCY Seed dormancy is a condition wherein the seed is viable but fails to germinate even in the presence of favourable environmental conditions imposed upon it. The ability of the seeds to delay their germination until the time and place are right is an important survival mechanism in plants. Seed dormancy could be due to physical causes, physiological causes and due to double dormancy.  Types of Seed Dormancy 1. Physiological or Internal dormancy The failure of the seed to germinate due to inhibitors or substances that blocks the germination process. This can be caused by, immature embryo or presence of chemical inhibitors such as ABA (Abscissic Acid) and Coumarin. 2. Physical Dormancy This is caused by hard impervious seed coats that prevents the imbibition of water and the entrance of oxygen. 3. Double Dormancy Both physical and physiological blocks are present in a seed and can only be broken by exposing it to cold temperature.  How to break physical dormancy 1. Stratification It is the practice of exposing imbibed seeds to cool or warm temperature conditions for a few days prior to germination in order to break dormancy. Types: a. Warm Stratification Moist seeds are stored under 15-250C. This promotes germination as a result of microbial decomposition of the seed covering. b. Cold Stratification Moist seeds are stored under 5-100C. The stratification media are moist soil, sand and peat, vermiculite. …….how to break physical dormancy 2. Scarification This refers to the alteration of seed coat to render it permeable to gas and water exchange. Methods: a. Mechanical methods It involves the use of abrasive materials to wear out the seed coat such as sand papers, iron, etc. b. Hot water treatment Seeds are subjected to about 170- 212 deg. F to soften the seed coat and renders it permeable to gas and water. c. Chemical Scarification It involves the use of Sulphuric acid to scarify the impervious seed coat. …….how to break physical dormancy 3. Dry Storage - This is necessary for seeds that will not germinate immediately after harvest. 4. Embryo culture - The aseptic growth of the excised embryo in an artificial media.  Seed Viability Seed viability is the measure of how many seeds in a lot are alive and could develop into plants that will reproduce under appropriate field conditions. Determination of seed viability 1. Germination tests 2. Biochemical tests - quicker but are not accurate as the germination test  Maintaining Viability of Recalcitrant Seeds Recalcitrant seeds are those that tolerate only slight desiccation after harvest, if at all, and consequently are usually sensitive to chilling temperatures as well. a. Since recalcitrant seeds cannot withstand much drying, they should be kept well when stored mixed with sawdust moistened to about 10% at relative temperature (7-100C). a. Moistened charcoal could also be used and a combination of half-fine charcoal and sterilized sawdust is a better medium combined with a low temperature of 7-100C.  Maintaining Viability of Orthodox Seeds Orthodox seeds are seeds that have the ability to endure extreme freezing and drying conditions during ex situ conservation. a. Decreasing the moisture of the seed by proper drying. b. Storing the seeds at low temperature. c. Keeping low the oxygen in the immediate vicinity of the seeds. d. After drying, seeds are kept in air tight containers, possibly with desiccants – a chemical which absorbs moisture (silica gel, calcium chloride, charcoal). Harrington’s Thumb Rules: (1) For each 1% decrease in moisture content, the storage life of the seed is doubled and (2) For each 10°F (5.6 °C) decrease in storage temperature, the storage life of a seed is doubled.  Seed Storage ✓ If seeds are to be kept in a long period, coat them with fungicides to protect them from microorganisms. ✓ Containers for ordinary seeds range from simple polyethylene to a more sophisticated cellophane, aluminum and vacuum sealed cans. ✓ For home storage, cans or bottles can be used provided that they can be tightly sealed. Sealing the lids with paraffin will keep the bottle or cans ait-tight. OTHER CONCEPTS RELATED TO PLANT GROWTH LIEBIG’S LAW OF MINIMUM THE LAW OF LIMITING FACTORS THE LAW OF DIMINISHING RETURNS  LIEBIG’S LAW OF MINIMUM “If one of the essential plant nutrients is deficient, plant growth will be poor even when all other essential nutrients are abundant”. Formulated by the German scientist Justus von Liebig in the 19th century. Example: When fertilizer prices — especially of nitrogen (N) and phosphate (P2O5) products — are high, some growers reduce or even eliminate applications of micronutrient or secondary nutrient fertilizers that provide balanced potassium (K), magnesium (Mg) and sulfur (S). But von Liebig’s Law tells us that if a soil is deficient in, say, Magnesium, yields will be depressed regardless of how much N-P-K product you apply.  THE LAW OF LIMITING FACTORS “The rate of a physiological process will be limited by the factor which is in shortest supply. Any changes in the level of the limiting factor will affect the rate of reaction”. Example: Photosynthesis requires basic components like water, sunlight in proper intensity, chloroplast, temperature, carbon dioxide and chlorophyll present in certain required amount. Any of these factors if present in scarcity will affect the rate of photosynthesis.  THE LAW OF DIMINISHING RETURNS “If one input in the production of a commodity is increased while all other inputs are held fixed, a point will eventually be reached at which additions of the input yield progressively smaller, or diminishing, increases in output” It is also called principle of diminishing marginal productivity.  PLANT MOVEMENTS TROPISM It is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. NASTIC MOVEMENTS Nastic movements are plant movements that occur in response to environmental stimuli but unlike tropic movements, the direction of the response is not dependent on the direction of the stimulus.  Tropic Movements 1. Phototropism It is the movement of plant organs in response to light. Auxins are responsible for phototropism. It could be positively phototropic (stems generally show a curvature toward the source of light) or negatively phototropic (roots which grow away from the A stem manifesting source of light). positive phototropism as it bend towards the source of light (window). ………….. tropic movements 2. Geotropism or Gravitropism It is the unidirectional response of plants to gravitational pull. a. Positive geotropism The organ grows downward toward the direction of the pull of gravity (center of the earth). e.g. The primary root b. Negative geotropism The organ moves upward in opposite direction to the pull of gravity. e.g. The shoots c. Diagravitropic The organ, grows perpendicular to the pull of gravity. e.g. Stolons and rhizomes ………….. tropic movements 3. Chemotropism This is the plant movement in response to a chemical substance. Example: Plant roots elongate towards a supply of essential mineral nutrients. 4. Hydrotropism The growth of the plant parts (i.e. the roots) in response to moisture or water. The root exhibits positive hydrotropic response by moving towards the water source. Note: Hydrotropism in roots is stronger than root gravitropism ………….. tropic movements 5. Heliotropism This is also called “solar tracking”. It is a plant movement in which the organs of plants track the sun across the sky. The responding organ may be oriented perpendicular, parallel, or obliquely to the sun’s rays. Compass plant (Silphium laciniatum) Sunflower (Helianthus annuus) ………….. tropic movements 6. Thigmotropism It results to the curvature and the coiling of tendrils or entire stems on supports. It also occurs in other plant organs such as leaves, petioles, and roots. The curvature 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: The tendrils of Momordica charantia (Ampalaya), Phaseolus vulgaris (Beans), Sechium edule (Chayote)  Nastic Movements 1. Epinasty It is the bending of an organ, such as petioles, leaves, and peduncles, toward the ground not due to gravity. The bending response is due to higher rate of longitudinal growth at the upper than at the lower side of the organ. ……nastic movements 2. Nyctinasty The sleep movement (opening and closing) of plant organs, such as leaves and flowers, due to day and night periods of daily rhythm and change in temperature. Photonastic (caused by change in light intensity) and thermonasty (caused by change in temperature) Photonastic movement in Night Jasmine Thermonastic movement in plants as in closes its flower by night and Rhododendrons as it closes its leaves at opens it by day. low temp and opens at high temperature. ……nastic movements 3.Seismonasty This is the movement in plants in response to touch as well as other forms of physical contact or mechanical disturbance such as shaking, wounding, wind, raindrops, and intense heat or burning. In the case of the sensitive plant (Mimosa pudica), a leaflet, leaf, or group of leaves rapidly folds and bends in response to the external stimulus.  ……nastic movements 4. Thigmonasty or haptonasty These movements are exemplified by the closing of the insect-eating plant venus flytrap (Dionaea muscipula) and the bending of the glandular hairs of sundew (Drosera sp.) as a result of contact with an insect. The leaves of a sundew are covered with long, nectar-tipped tentacles which traps the insect that stops for a sip. The struggles of an insect causes the leaf to slowly curl around its trapped prey. They Sundew also cue the leaf to produce digestive You can't fool a sundew. Non-nutritious matter that enzymes that dissolve the captured insect, falls onto it may cause a leaf to begin to curl, but it and the plant absorbs the liquid, nutrient- soon rejects and releases the offending annoyance. rich soup.