PLANT RESPONSES TROPISM is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the species. The word tropism comes from the Greek trope (“to turn” or “to change”). Tropisms are usually named for the stimulus involved and may be either positive (towards the stimulus) or negative (away from the stimulus). Phototropism is the growth response of a plant in response to light direction.
Different parts of a plant exhibit different reactions to light. Stems exhibit positive phototropism while most roots exhibit negative phototropism. Geotropism is the growth response of a plant in response to gravity. Roots exhibit positive geotropism while stems and leaves exhibit negative geotropism. Hydrotropism is the growth response of a plant to water. Roots exhibit positive hydrotropism. Thigmotropism is the growth response of a plant to physical contact (touch). Plants that cling to physical structures such as walls exhibit positive thigmotropism.
Chemotropism is the growth response of a plant to a particular chemical. Roots grow toward useful minerals in the soil but away from acids. PLANT HORMONES Auxins are a class of plant hormones (or plant growth substances) with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant’s life cycle and are essential for plant body development. functions -Stimulates cell elongation -Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture -Stimulates differentiation of phloem and xylem Stimulates root initiation on stem cuttings and lateral root development in tissue culture -Mediates the tropistic response of bending in response to gravity and light -The auxin supply from the apical bud suppresses growth of lateral buds -Delays leaf senescence -Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission -Can induce fruit setting and growth in some plants -Involved in assimilate movement toward auxin possibly by an effect on phloem transport -Delays fruit ripening -Promotes flowering in Bromeliads Stimulates growth of flower parts -Promotes (via ethylene production) femaleness in dioecious flowers -Stimulates the production of ethylene at high concentrations GIBBERILLIN Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. All gibberellins are derived from the ent-gibberellane skeleton. The structure of this skeleton derivative along with the structure of a few of the active gibberellins are shown above.
The gibberellins are named GA1…. GAn in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterised , is GA3. Function – Stimulate stem elongation by stimulating cell division and elongation. -Stimulates bolting/flowering in response to long days. -Breaks seed dormancy in some plants which require stratification or light to induce germination. -Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves. -Induces maleness in dioecious flowers (sex expression). Can cause parthenocarpic (seedless) fruit development. -Can delay senescence in leaves and citrus fruits. Abscisic acid is a single compound unlike the auxins, gibberellins, and cytokinins. It was called “abscisin II” originally because it was thought to play a major role in abscission of fruits. At about the same time another group was calling it “dormin” because they thought it had a major role in bud dormancy. The name abscisic acid (ABA) was coined by a compromise between the two groups.
Though ABA generally is thought to play mostly inhibitory roles, it has many promoting functions as well(Arteca, 1996; Mauseth, 1991; Raven, 1992; Salisbury and Ross, 1992). Function -Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis). -Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. -Induces seeds to synthesize storage proteins. -Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase. -Has some effect on induction and maintanance of dormancy. Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense. Cytokinins are compounds with a structure resembling adenine which promote cell division and have other similar functions to kinetin. Kinetin was the first cytokinin discovered and so named because of the compounds ability to promote cytokinesis (cell division). Though it is a natural compound, It is not made in plants, and is therefore usually considered a “synthetic” cytokinin (meaning that the hormone is synthesized somewhere other than in a plant).
The most common form of naturally occurring cytokinin in plants today is called zeatin which was isolated from corn (Zea mays). Function -Stimulates cell division. -Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture. -Stimulates the growth of lateral buds-release of apical dominance. -Stimulates leaf expansion resulting from cell enlargement. -May enhance stomatal opening in some species. -Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis. DIFFERENCE BETWEEN SHORT DAY PLANTS
Short-day and long-day plants exhibit a response to photoperiodism, or the changes in light and dark in a twenty-four-hour cycle. Short-day plants form flowers when the days become shorter than a critical length, while long-day plants form flowers when the days become longer than a critical length. Short-day plants bloom in late summer or autumn in middle latitudes. Examples of short-day plants are chrysanthemums, goldenrods, poinsettias, soybeans, and ragweed. Long-day plants bloom in spring and early summer. Some examples of long-day plants are clover, irises, and hollyhocks.
Florists and commercial plant growers can adjust the amount of light a plant receives to force it to bloom out of season. A short day plant is a plant that flowers only when it is exposed to light for a short period of time, like in early spring or fall, approximately 12 hours. Chrysanthemums and strawberries are short day plants. A long day plant flowers only when it is exposed to light for a long period of time, like in the summer. Lettuces, spinach, and other different types of wheat are long day palnts. Short- need a lot of night long- need a lot of day.
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