1. Chapter 30 PLANT RESPONSES TO STIMULI

2. A. Hormones and Plant Growth 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.

3. 1. Auxins First group of plant hormones to be discovered (phototrophic experiments of Charles & Francis Darwin, 1870s).

4. Auxins are synthesized in shoot tips, young leaves & seed embryos. Major Actions promotes elongation of cells (shoots, leaves & embryos) 교과서 그림 15.2 참조 inhibits growth of lateral buds inhibits leaf & fruit abscission stimulates synthesis of ethylene

5. IAA (indoleacetic acid) is the most active naturally occurring auxin. Commercial uses: stimulates adventitious root growth in cuttings stimulates some plants to produce fruit (seedless) without being fertilized 2,4-D (synthetic auxin) kills broadleaf weeds, but not grasses

6. 2. Gibberellins Discovered by Japanese botanists studying “foolish seedling disease” in rice (1926). Gibberellins are synthesized in young shoots & developing seeds. Major Actions promotes elongation of cells (shoots, leaves & seeds) stimulates flowering & fruit development stimulates seed germination

7. Commercial uses: elongates flower stems of cyclamen plants lengthen stems of grapes and increase fruit size

8. 3. Cytokinins Name derived from fact that cytokinins stimulate cytokinesis. Cytokinins are synthesized in roots, embryos & fruits. Major Actions stimulates cell division (shoots, roots, leaves & seeds) stimulates growth of lateral buds delays leaf senescence

9. 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.

10. Commercial uses: extends shelf life of leafy vegetables keeps cut flowers fresh promotes branching in Christmas trees

11. 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

12. stimulates leaf & fruit abscission Stem Leaf stalk Abscission layer Commercial uses: ripens fruits that are picked green

13. 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.

14. Major Actions inhibits growth closes stomata induces & maintains seed dormancy Commercial uses: inhibits the growth of plants that are to be shipped.

15. B. Plant Movement 1. Tropic Movements Plant growth directed toward or away from an environmental stimulus. Phototropism Plant shoots are positively phototropic. Bending results from auxin accumulation on shaded side of plant. Growth in response to unidirectional light.

16. 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.

17. Thigmotropism ( 굴촉성 ) Growth in response to touch. Passion vine tendrils exhibit positive thigmotropism. Coiling is controlled by auxin & ethylene.

18. 2. Nastic ( 경성운동 ) Movements Plant movements that are not oriented with respect to a stimulus. Thigmonasty ( 경촉 ) A nastic response to touch. Ex. leaflet folding of “sensitive plant”

19. Nyctinasty (“sleep movement”) A nastic response to daily rhythms of light & dark. Ex. movement of prayer plant leaves at night (Maranta)

20. 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).

21. 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

22. Day-neutral plants do not rely on photoperiod to stimulate flowering. 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. Flower spring, summer & fall

23. 1-minute flash of light

24. Plants measure photoperiod with help of phytochrome, a blue pigment molecule that exists in 2 forms: P r = red-absorbing (660nm) P fr = far-red-absorbing (730nm): 종자발아, 기공의 열림, 개화 조절

25. In daylight: Red light** Far-red light  ( 흡수 )  P r  P fr P r  P fr (rapid conversion) (rapid conversion) In darkness: P r <--------- P fr (slow spontaneous conversion) **Sunlight consists of much more red light than far-red light. Thus, most of the phytochrome is converted to the P fr form during daylight hours.

26. Effects of alternate flashes of red & far-red light ( 마지막 섬광이 중요 ) No short Night effect Short Night effect No short Night effect due to far-red (Red light affects on flowering.)

27. 2. Seed Germination Phytochrome affects seed germination. In many weeds: red light stimulates germination P r  P fr far-red light inhibits germination P fr  P r If seeds are buried too deeply in soil, P fr is lacking & germination does not occur.

28. 3. Senesence a seasonal plant response occurs at different rates in different species leaf senesence is characterized by destruction of chlorophyll and production of anthocyanins in autumn dormancy is a period of decreased metabolism

29. 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

30. Examples of circadian rhythms include: flowering of the evening primrose nyctinastic movements of prayer plant opening of stomata secretion of nectar solar tracking (heliotropism) of sunflowers evening primrose ( 앵초 )