Plant Responses and Adaptations

1. Plant Responses and Adaptations • In what may be its last moments, an ant peers down into a pitcher plant's specialized leaf • The leaf is lined with slippery hairs and is filled with digestive enzymes that will extract nutrients from any unsuspecting prey

2. Plant Responses and Adaptations

3. Hormones and Plant Growth • Unlike most animals, plants do not have a rigidly set organization to their bodies • Cows have four legs, ants have six, and spiders have eight; but tomato plants do not have a predetermined number of leaves or branches • However, plants show distinct patterns of growth • As a result, you can easily tell the difference between a tomato plant and a corn plant, between an oak tree and a pine tree

4. Patterns of Plant Growth • Although plant growth is not determined precisely, it still follows general patterns that differ among species • What controls these patterns of development? • Biologists have discovered that plant cells send signals to one another that indicate when to divide and when not to divide, and when to develop into a new kind of cell

5. Patterns of Plant Growth • There is another difference between growth in plants and animals • Once most animals reach adulthood, they stop growing • In contrast, even plants that are thousands of years old continue to grow new needles, add new wood, and produce cones or new flowers, almost as if parts of their bodies remained “forever young” • As you have learned, the secrets of plant growth are found in meristems, regions of tissue that can produce cells that later develop into specialized tissues • Meristems are found at places where plants grow rapidly—the tips of growing stems and roots, and along the outer edges of woody tissues that produce new growth every year

6. Patterns of Plant Growth • If meristems are the source of plant growth, how is that growth controlled and regulated? • Plants grow in response to environmental factors such as light, moisture, temperature, and gravity • But how do roots “know” to grow down, and how do stems “know” to grow up toward light? • How do the tissues of a plant determine the right time of year to produce flowers? • How do plants ensure that their growth is evenly balanced—that the trunk of a tree grows large enough to support the weight of its leaves and branches? • The answers to these questions involve the actions of chemicals that direct, control, and regulate plant growth

7. Plant Hormones • In plants, the division, growth, maturation, and development of cells are controlled by a group of chemicals called hormones • A hormoneis a substance that is produced in one part of an organism and affects another part of the same individual • Plant hormones are chemical substances that control a plant's patterns of growth and development, and the plant's responses to environmental conditions

8. PLANT HORMONES • Chemicals that control the internal factors of plant growth • Organic compounds that are effective in small concentrations • Synthesized in one part of the plant and transported to target tissue elsewhere in the plant triggering a physiological response • Many hormones work together

9. Plant Hormones • The general mechanism of hormone action in plants is shown in the diagram • As you can see, the hormone moves through the plant from the place where it is produced to the place where it triggers its response • The portion of an organism affected by a particular hormone is known as its target cell or target tissue • To respond to a hormone, the target cell must contain a hormone receptor—usually a protein—to which the hormone binds • If the appropriate receptor is present, the hormone can exert an influence on the target cell by changing its metabolism, affecting its growth rate, or activating the transcription of certain genes • Cells that do not contain receptors are generally unaffected by hormones

10. Hormone Action in Plants   • Plant hormones are chemical substances that control patterns of development as well as plant responses to the environment • Hormones are produced in apical meristems, in young leaves, in roots, and in growing flowers and fruits • From their place of origin, hormones move to other parts of the plant, where target cells respond in a way that is specific to the hormone

11. Hormone Action in Plants  

12. Hormone Action in Plants   • Different kinds of cells may have different receptors for the same hormone • As a result, a single hormone may affect two different tissues in different ways • For example: • a particular hormone may stimulate growth in stem tissues but inhibit growth in root tissues

13. Auxins • The experiment that led to the discovery of the first plant hormone was carried out by Charles Darwin • In 1880, Darwin and his son Francis published a book called The Power of Movement in Plants • In this book, they described an experiment in which oat seedlings demonstrated a response known as phototropism • Phototropismis the tendency of a plant to grow toward a source of light

14. Auxins • The activity Effect of Light on a Growing Plant shows an experiment similar to the one carried out by the Darwins • Notice that the tip of one of the oat seedlings was covered with an opaque cap • This plant did not bend toward the light, even though the rest of the plant was uncovered • However, if an opaque shield was placed a few centimeters below the tip, the plant would bend toward the light as if the shield were not there • Clearly, something was taking place at the tip of the seedling

15. AUXINS • Hormones that regulate the growth of plant cells • Stimulates/inhibits cell elongation depending on concentration • Tropisms: • Phototropism: response to light • Geotropism (gavitropism): response to gravity • Thigmotropism: response to touch • Synthetic: • Weed killers: 2,4-D • Fruit harvest: naphthaleneacetic acid (NAA) • Harvest fruit at sametime • Stimulates root development

16. Auxins and Phototropism  • The Darwins suspected that the tip of each seedling produced substances that regulated cell growth • Forty years later, these substances were identified and named auxins • Auxins are produced in the apical meristem and are transported downward into the rest of the plant • They stimulate cell elongation • When light hits one side of the stem, a higher concentration of auxins develops in the shaded part of the stem • This change in concentration stimulates cells on the dark side to elongate • As a result, the stem bends away from the shaded side and toward the light • Recent experiments have shown that auxins migrate toward the shaded side of the stem, possibly due to changes in membrane permeability in response to light

17. Auxins and Gravitropism  • Auxins are also responsible for gravitropism, which is the response of a plant to the force of gravity • By mechanisms that are still not understood, auxins build up on the lower sides of roots and stems • In stems, auxins stimulate cell elongation, helping turn the trunk upright, as shown in photo • In roots, however, the effects of auxins are exactly the opposite • There, auxins inhibit cell growth and elongation, causing the roots to grow downward

20. Gravitropism in a Stem  • Auxins are responsible for the plant response called gravitropism • Auxins caused the tip of this tree stem to grow upright

21. Gravitropism in a Stem 

23. Gravitropism in a Stem  • Auxins are also involved in the way roots grow around objects in the soil • If a growing root is forced sideways by an obstacle such as a rock, auxins accumulate on the lower side of the root • Once again, high concentrations of auxins inhibit the elongation of root cells • The uninhibited cells on the top elongate more than the auxin-inhibited cells on the bottom of the root • As a result, the root grows downward

24. Auxins and Branching  • Auxins also regulate cell division in meristems • As a stem grows in length, it produces lateral buds • A lateral bud is a meristematic area on the side of a stem that gives rise to side branches • Most lateral buds do not start growing right away • The reason for this delay is that growth at the lateral buds is inhibited by auxins • Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited • This phenomenon is called apical dominance

25. Apical Dominance  • Apical dominance, shown here, is controlled by the relative amounts of auxins and cytokinins • During normal growth (A),lateral buds are kept dormant because of the production of auxins in the apical meristem • If the apical meristem is removed (B), the concentration of auxins drops

26. Apical Dominance 

27. Apical Dominance  • Although not all gardeners have heard of auxins, most of them know how to overcome apical dominance • If you snip off the tip of a plant, the side branches begin to grow more quickly, resulting in a rounder, fuller plant • Why does this happen? • When the tip is removed, the apical meristem—the source of the growth-inhibiting auxins—goes with it • Without the influence of auxins, meristems in the side branches grow more rapidly, changing the overall shape of the plant

28. Auxinlike Weed Killers  • Chemists have produced many compounds that mimic the effects of auxins • Because high concentrations of auxins inhibit growth, many of these compounds are used as herbicides, which are compounds that are toxic to plants • Herbicides include a chemical known as 2,4-D (2,4-dichlorophenoxyacetic acid), which is used to kill weeds • A mixture containing 2,4-D was used as Agent Orange, a chemical defoliant sprayed during the Vietnam War

29. CYTOKININS • Promote cell division • Influence the development of root, stems, and differentiation of xylem and phloem

30. Cytokinins • Cytokinins are plant hormones that are produced in growing roots and in developing fruits and seeds • In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout • Cytokinins also delay the aging of leaves and play important roles in the early stages of plant growth

31. Cytokinins • Cytokinins often produce effects opposite to those of auxins • For example, auxins stimulate cell elongation, whereas cytokinins inhibit elongation and cause cells to grow thicker • Auxins inhibit the growth of lateral buds, whereas cytokinins stimulate lateral bud growth • Recent experiments show that the rate of cell growth in most plants is determined by the ratio of theconcentration of auxins to cytokinins • In growing plants, therefore, the relative concentrations of auxins, cytokinins, and other hormones determine how the plant grows

32. GIBBERELLINS • Promote cell enlargement • Increasing length between nodes in stem • Elongation of stem • Taller plant • 65 different types • Stimulates seed germination • Promotes formation of seedless fruits

33. Gibberellins • For years, farmers in Japan knew of a disease that weakened rice plants by causing them to grow unusually tall • They called the disease the “foolish seedling” disease • In 1926, Japanese biologist Eiichi Kurosawa discovered that this extraordinary growth was caused by a fungus: Gibberella fujikuroi • His experiments showed that the fungus produced a growth-promoting substance that was named gibberellin

34. Gibberellins • Before long, other researchers had learned that plants themselves produce more than 60 similar compounds, all of which are now known as gibberellins • Gibberellins produce dramatic increases in size, particularly in stems and fruit • Gibberellins are also produced by seed tissue and are responsible for the rapid early growth of many plants

35. Ethylene • When natural gas was used in city street lamps in the nineteenth century, people noticed that trees along the street suffered leaf loss and stunted growth • This effect was eventually traced to ethylene, one of the minor components of natural gas

36. Ethylene • Today, scientists know that plants produce their own ethylene, and that it affects plants in a number of ways • In response to auxins, fruit tissues release small amounts of the hormone ethylene • Ethylene then stimulates fruits to ripen

37. Ethylene • Commercial producers of fruit sometimes use this hormone to control the ripening process • Many crops, including lemons and tomatoes, are picked before they ripen so that they can be handled without damage to the fruit • Just before they are delivered to market, the fruits are treated with synthetic ethylene to produce a ripe color quickly • This trick does not always produce a ripe flavor, which is one reason why naturally ripened fruits often taste much better

38. Plant Responses • Like all living things, plants respond to changes in their environments • Some biologists call these responses “plant behavior,” which is a useful way of thinking about them • Plants generally do not respond as quickly as animals do, but that does not make their responses any less effective • Some plant responses are so fast that even animals cannot keep up with them!

39. Tropisms • Plants change their patterns and directions of growth in response to a multitude of cues • The responses of plants to external stimuli are called tropisms, from a Greek word that means “turning” • Plant tropisms include gravitropism, phototropism, and thigmotropism • Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch

40. Gravitropism and Phototropism  • You have already read about gravitropism, the response of a plant to gravity, and phototropism, the response of a plant to light • Both of these responses are controlled by the hormone auxin • Gravitropism causes the shoot of a germinating seed to grow out of the soil—against the force of gravity • It also causes the roots of a plant to grow with the force of gravity and into the soil

41. Gravitropism and Phototropism  • Phototropism causes a plant to grow toward a light source • This response can be so quick that young seedlings reorient themselves in a matter of hours

42. Thigmotropism in a Grapevine  • Plant tropisms include gravitropism, phototropism, and thigmotropism • One effect of thigmotropism—growth in response to touch—is that plants curl and twist around objects, as shown by the stems of this grapevine

43. Thigmotropism in a Grapevine

44. Rapid Responses • Some plant responses do not involve growth • In fact, they are so rapid that it would be a mistake to call them tropisms • If you touch a leaf of Mimosa pudica, appropriately called the “sensitive plant,” within only two or three seconds, its two leaflets fold together completely • The secret to this movement is changes in osmotic pressure • Recall that osmotic pressure is caused by the diffusion of water into cells • The leaves are held apart due to osmotic pressure where the two leaflets join • When the leaf is touched, cells near the center of the leaflet pump out ions and lose water due to osmosis • Pressure from cells on the undersideof the leaf, which do not lose water, force the leaflets together

45. Rapid Responses • The carnivorous Venus' flytrap also demonstrates rapid responses • When a fly triggers sensory cells on the inside of the flytrap's leaf, electrical signals are sent from cell to cell • A combination of changes in osmotic pressure and cell wall expansion causes the leaf to snap shut, trapping the insect inside

46. Photoperiodism • To every thing there is a season • Nowhere is this more evident than in the regular cycles of plant growth • Year after year, some plants flower in the spring, others in summer, and still others in the fall • Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants • Plants such as spinach and irises flower when days are long and are therefore known as long-day plants

47. Photoperiodism • How do all these plants manage to time their flowering so precisely? • In the early 1920s, scientists discovered that tobacco plants flower according to the number of hours of light and darkness they receive • Additional research showed that many other plants also respond to periods of light and darkness, a response called photoperiodism • This type of response is summarized in the figure • Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth.

48. PHOTOPERIODISM • Plant response to changes in day length • Long-day plants: flower when exposed to longer days (Spring/Summer) • Short-day plants: flower when exposed to shorter days (Fall) • Day neutral plants: flowering not affected by length of day (tomato, dandelion)

49. Effect of Photoperiod on Flowering • Photoperiodism controls the timing of flowering and seasonal growth • The response of flowering, shown here, is controlled by the amount of darkness plants receive • Short-day plants, such as chrysanthemums, flower only when exposed to an extended period of darkness every night—and thus a short period of light during the day • Long-day plants, such as irises, flower when exposed to a short period of darkness or to a long period of darkness interrupted by a brief period of light

50. Effect of Photoperiod on Flowering