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PLANT RESPONSES TO STIMULI. Chapter 21. A. Plant Growth Regulated by the action of hormones. Hormone = a chemical messenger produced in one part of a plant & usually transported to another, where it elicits a response.
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PLANT RESPONSES TO STIMULI Chapter 21
A. Plant Growth Regulated by the action of hormones. Hormone = a chemical messenger produced in one part of a plant & usually transported to another, where it elicits a response. Plants have 5 major classes of hormones: auxins, gibberellins, cytokinins, ethylene & abscisic acid.
1. Auxins First group of plant hormones to be discovered (phototrophic experiments of Charles & Francis Darwin, 1870s).
Auxins are synthesized in shoot tips, young leaves & seed embryos. Major Actions • promote elongation of cells (shoots, leaves & embryos) • inhibit growth of lateral buds • inhibit leaf & fruit abscission • stimulate synthesis of ethylene
IAA (indolacetic acid) is the most active naturally occurring auxin. Commercial uses: • stimulate adventitious root growth in cuttings • stimulate some plants to produce fruit (seedless) without being fertilized • 2,4-D (synthetic auxin) kills broadleaf weeds, but not grasses
2. Gibberellins Discovered by Japanese botanists studying “foolish seedling disease” in rice (1926). Gibberellins are synthesized in young shoots & developing seeds. Major Actions • promote elongation of cells (shoots, leaves & seeds) • stimulate flowering & fruit development • stimulate seed germination
Commercial uses: • elongate flower stems of cyclamen plants • lengthen stems of grapes
3. Cytokinins Name derived from fact that cytokinins stimulate cytokinesis. Cytokinins are synthesized in roots, embryos & fruits. Major Actions • stimulate cell division (shoots, roots, leaves & seeds) • stimulate growth of lateral buds • delay leaf senescence
Effects of cytokinins are influenced by auxin concentration. Apical meristem intact - auxin suppresses lateral bud growth [apical dominance]. Apical meristem removed - cytokinin concentration increases, stimulating lateral bud growth.
Commercial uses: • extend shelf life of leafy vegetables • keep cut flowers fresh • promote branching in Christmas trees
4. Ethylene Only plant hormone that is a gas. Ethylene is synthesized in all parts of plants, especially ripening fruits, nodes of stems, & dying leaves. Major Actions • promotes fruit ripening • stimulates leaf & flower senescence
stimulates leaf & fruit abscission Stem Leaf stalk Abscission layer Commercial uses: • ripens fruits that are picked green
5. Abscisic acid (ABA) Referred to as the “stress hormone” because it helps plants cope with adverse conditions (severe drought, onset of winter). ABA is synthesized in mature leaves & plants under stress.
Major Actions • inhibits growth • closes stomata • induces & maintains seed dormancy Commercial uses: • inhibits the growth of plants that are to be shipped.
B. Plant Movement 1. Tropic Movements Plant growth directed toward or away from an environmental stimulus. • Phototropism Growth in response to unidirectional light. Plant shoots are positively phototropic. Bending results from auxin accumulation on shaded side of plant.
Gravitropism Growth in response to gravity. Plant roots are positively gravitropic. Plant shoots are negatively gravitropic. Believed that amyloplasts in root cells function as statoliths (gravity detectors). Shift in statoliths signals redistribution of auxin.
Thigmotropism Growth in response to touch. Passion vine tendrils exhibit positive thigmotropism. Coiling is controlled by auxin & ethylene.
2. Nastic Movements Plant movements that are not oriented with respect to a stimulus. • Thigmonasty A nastic response to touch. Ex. leaflet folding of “sensitive plant”
Photonasty (“sleep movement”) A nastic response to daily rhythms of light & dark. Ex. movement of prayer plant leaves
C. Response to Seasonal Changes Many plant processes (flowering, seed germination, senescence, dormancy) occur at specific times of the year. Plants track seasons by measuring photoperiod(relative lengths of daylight & darkness).
1. Flowering • Long-day plants bloom when light periods are longer than some critical length. Flower in spring or early summer • Short-day plants bloom when light periods are shorter than some critical length. Flower in late summer or fall
Day-neutral plants do not rely on photoperiod to stimulate flowering. Flower spring, summer & fall Long-day & short-day plants actually respond to length of night rather than length of day. Thus, short-day plants require a specific period of uninterrupted darkness to flower.
Plants measure photoperiod with help of phytochrome, a blue pigment molecule that exists in 2 forms: • Pr= red-absorbing (660nm) • Pfr= far-red-absorbing (730nm)
In daylight: Red light** Far-red light Pr PfrPr Pfr (rapid conversion) (rapid conversion) In darkness: Pr <--------- Pfr (slow spontaneous conversion)
2. Seed Germination Phytochrome affects seed germination. In many weeds: • red light stimulates germination Pr Pfr • far-red light inhibits germination Pfr Pr If seeds are buried too deeply in soil, Pfr is lacking & germination does not occur.
D. Circadian Rhythms Plant responses that occur daily. Circadian rhythms are regulated by biological clocks, which are controlled: • internally by genes • externally by environmental factors
Examples of circadian rhythms include: • flowering of the evening primrose • photonastic movements of prayer plant • opening of stomata • secretion of nectar • solar tracking (heliotropism) of sunflowers
