1. Chapter 19: Plants Respond to Their Environments
Understanding how plants grow, develop, defend themselves, and survive extreme environments is critical to biology, as plants form the base of many food webs and chains. Also, in reading about plant hormones, you will be able to compare them and contrast them with animal (human) hormones, which will be tackled later on in this text. Welcome to Chapter Nineteen.
Regulating and defending while rooted in the ground
Lecture by Danielle DuCharme, Waubonsee Community College

2. Chapter 19-1 Opener
Plant hormones regulate the growth of this enormous pumpkin.

3. 19.1. Hormones help plants respond to their environments
Environmental variables influence plant growth.
Chemicals called hormones respond to environmental variables and influence the growth and development of the plants.

4. Environment affects the growth pattern of the arrowleaf plant.
In response to environmental conditions—in this case, depth of water—the arrowhead plant will grow different types of leaves. In deep water, long, ribbon-like leaves are grown. In shallow water, large round leaves are grown. And, on land, arrow-shape leaves develop. All of this is triggered by environmental conditions, but mediated by hormones.
Figure 19-1 Environment affects the growth patterns of the arrowleaf plant. The differences in growth forms are mediated by hormones.

5. Plant Hormones Are Chemical Signals
Hormones are produced in various places within the plant.
They may have their effect in the place where they are produced, or they may be transported to another part of the plant before they take effect.

6. Hormones
Hormones convey information about the physiological state of the plant’s tissues, or the environment in which the plant finds itself.
Hormones then regulate metabolism.

7. Hormone Action
Hormones may stimulate a certain response in the target cell.
Hormones can suppress an action in the target cell.

8. There Are Five Major Types of Plant Hormones
The five categories are:
Gibberellins
Auxins
Ethylene
Abscisic acid
Cytokinins
Each of these five types of hormones serves a different function and targets a specific location in the plant.
Gibberellins increase the speed of seed germination, and therefore are found in seeds, as well as apical meristem.
Auxins stimulate stem elongation, therefore they also target apical meristems.
Ethylene increases the speed at which fruit ripens, but is found all over the plant, including in fruits.
Abscisic acid inhibits growth and reproduction and is found in the leaves, fruits, roots, and seeds.
Cytokinins are the opposite and cause rapid cell division. They are found primarily in roots and fruits.
Figure 19-2 The location and function of five types of plant hormones.

9. Plant Hormones Are Different from Animal Hormones
Animal hormones are produced in glands and tissues and are transported for use.
Plant hormones are produced in many types of cells throughout the plant, and can be used on site.

10. Take-home message 19.1
Plant hormones are chemical signals produced by plant cells that enable the plant’s responses to environmental variables (such as amount of moisture, amount and direction of sunlight, and temperature) and influence its growth and development.

11. 19.2. Seed germination and stem elongation are stimulated by gibberellins
Gibberellins are a group of about 125 hormones that regulate a plant’s growth processes.
They stimulate cell division and cell elongation.

12. Gibberellins Have Four Main Types of Effects
Speeding seed germination
Stem elongation
Inducting early blooming of flowers
Enlargement of fruits

13. The Effects of Gibberellins
Giberellins produce enzymes that allow for quicker and more efficient use of the seed’s energy reserves. They also increase distance between nodes, making branch points further apart from one another. They can cause flower production without an environmental trigger, and the growth of larger fruits.
Figure 19-3 Physiological effects of gibberellins.

14. Gibberellins Have Powerful Growth-Stimulating Effects
Gibberellins can create radishes the size of beach balls, and cabbages 10 feet tall!
Figure 19-4 A giant, gibberellin-treated cabbage.

15. Take-home message 19.2
Gibberellins are a group of about 125 hormones, produced primarily in meristems and seeds, that regulate a plants growth processes, mainly by stimulating cell division and cell elongation.

16. 19.3 Seedlings grow and properly orient themselves under the direction of auxins.
Auxins are a small group of synthetic and naturally occurring hormones that stimulate and regulate a plant’s growth and development.

17. Auxins Auxins are found in apical meristems and immature plant tissue.
Auxins stimulate the expression of genes that promote cell division, stem elongation, and formation of roots.

18. The chief effects of auxins are of four types:
Stimulating shoot elongation
Controlling seedling orientation
Stimulating root branching
Promoting fruit development
Figure 19-5 Physiological effects of auxins. These Bear Grass stems are reaching toward the sunlight.

19. Auxins’ Influence on Plant Orientation
Auxins influence plant orientation.
Auxins cause plants to grow toward light.
Figure 19-6 Auxins cause a plant to grow toward light.

20. Synthetic Auxins
Synthetic auxins can use hormone properties to kill unwanted weeds.
These chemicals work by causing extreme growth of the plant, leading to plant death.
Figure 19-7 Too much of a good thing. Synthetic auxins are used to control weeds by causing such a high rate of growth, which results in plant death.

21. Take-home message 19.3
Auxins are a small group of naturally occuring horomones (and a larger group of synthetic variants) found primarily in meristems and immature plant tissue.

22. Take-home message 19.3
These hormones play several important roles in stimulating and regulating a plant’s growth and development, often by increasing the usually rigid cell wall’s flexibility and permeability.

23. 19.4. Other plant hormones also regulate flowering, fruit ripening, and responses to stress.
Ethylene, abscisic acid, and cytokinins also play critical roles in directing germination, sprouting, and fruiting.
They also initiate appropriate molecular and cellular activities when environmental conditions are favorable.

24. Ethylene Ethylene is a gas produced in every part of the plant.
This gas speeds the rate at which fruit ripens.
Figure 19-8 Ethylene speeds fruit ripening.

25. The Effect of Ethylene on Flowers
Ethylene also hastens the aging and dropping of leaves from trees and the death of flowers at specific times.
Figure 19-9 Florists fight the withering effects of ethylene on flowers.

26. Abscisic Acid
This hormone is synthesized primarily in leaves, fruits, and root tips.
This hormone has the general effect of inhibiting growth and reproductive activities under adverse environmental conditions.
Figure Not a good time to invest in growth. Abscisic acid inhibits plant growth in times of stress, such as during a spring ice storm.

27. Cytokinins
These hormones are stimulators of cell division throughout the plant body.
They are produced primarily in the roots and the fruits.
Figure Cytokinins stimulate cell division. The cells of the seed embryo are ready to divide and will form the shoot system of the plant.

28. Cytokinins Work with Auxins to Produce Four Main Types of Effects
Causing rapid cell division
Inducing seed germination
Initiating new branches from lateral buds
Retarding leaf death

29. Take-home message 19.4
In addition to auxins and gibberlellins, three types of hormones also play critical roles in plant regulation.

30. Take-home message 19.4
Among these multiple functions, ethylene induces and speeds fruit ripening, abscisic acid inhibits growth and reproduction under stressful conditions, and cytokinins stimulate cell division throughout the plant body.

31. 19.5. Tropisms influence plants’ direction of growth.
Plants use a variety of growth patterns called tropisms to grow toward or away from various environmental stimuli.
Plants can bend, curve and twist in response to light, gravity, and physical impediments.

32. There Are Three Main Types of Tropisms
Phototropism
Growth in response to a light source.
Gravitropism
Growth in response to gravity.
Thigomotropism
Growth in response to touch or physical contact with an object.

33. Phototropism
Cells in a plant’s stem grow unevenly, so the stem bends toward the light.
This allows the plant to photosynthesize more efficiently.
Figure Phototropism is plant growth that is influenced by the presence of light.

34. Heliotropism Heliotropism is a special type of phototropism.
This is growth in response to the position of the sun.

35. Gravitropism
This is the reason that stems grow upward, and roots grow downward.
This allows the leaves to access light, and the roots to access water in the soil.
Figure Plant stems grow away from the force of gravity.

36. Gravitropism
Starch-containing bodies, pulled by gravity, settle toward the bottom of the cells.
Auxin molecules migrate toward the starch causing uneven growth and causing stems and roots to grow in the right direction.

37. Thigmotropism
Climbing plants will wrap around structures, using tendrils—specialized thread-like leaves or stems.
Because of auxins, one side of the shoot will grow, while the other will not, inducing growth that allows tendrils to wrap around an object.
Figure Plant growth can be affected by touch or contact with physical objects.

38. Take-home message 19.5
Plants have a variety of growth patterns, known as tropisms, by which they grow toward or away from various environmental stimuli.

39. Take-home message 19.5
Phototropism is growth in response to light, gravitropism is growth in response to gravity, and thigmotropism is growth in response to touch or contact with physical objects.

40. 19.6. Plants have internal biological clocks.
Plants have a “biological clock,” an internal method of keeping time.
The internal clock of plants is continuously adjusted by environmental cues.
Plants have a “biological clock,” an internal method of keeping time, which enables them to initiate various biochemical and physiological actions at the appropriate time

41. The Purpose of a “Biological Clock”
The clock tells the plant when to turn on or turn off the expression of certain genes, which can orient leaves for efficient photosynthesis or make flowers accessible when pollinators are available.

42. Environmental Cues
The most important environmental cues that set and reset a plant’s biological clock are light-dark cycles and temperature cycles.
Plants schedule their daily activities so they happen at an optimal time. This will happen instinctually, whether the plant is indoors or outdoors. This is not a twenty-four hour clock, where things happen predictably at a specific time, or a specific day. This internal clock is driven by external, environmental cues.

43. The Biological Clock in Plants
Figure Great timing. Plants adjust their biological clocks in response to environmental cues such as light and dark cycles and temperature cycles.

44. Take-home message 19.6
Plants have internal methods of keeping time that enable them to initiate various biochemical and physiological actions at the appropriate time.

45. Take-home message 19.6
Plants constantly adjust their biological clocks to environmental cues, such as light-dark cycles and temperature cycles.

46. 19.7. With photoperiodism and dormancy, plants detect and prepare for winter.
Figure Plants flower and become dormant in accordance with the seasons.

47. Photoperiodism
Plants use the length of the dark period in a day, in a process known as photoperiodism, to regulate their flowering time and numerous other responses to seasonal changes.

48. The amount of daylight determines when they produce flowers.
Long-day plants
Short-day plants
Day-neutral plants
The amount of daylight determines when they produce flowers.
Plants fall into one of three categories when it comes to regulating flower production. Although called short-day and long-day plants, they are actually triggered by the amount of darkness or night vs. the amount of light in a given day. In long-day plants, the trigger is shorter amounts of darkness, which would cause these plants to flower in the spring. Short-day plants are triggered by longer periods of darkness, which would be found in the late summer or fall. Finally, day-neutral plants are triggered not by periods of dark or light, but by plant maturity.
Figure Day length and flowering. In long- and short-day plants, flowering is regulated by hours of darkness. In day-neutral plants, light and darkness do not affect flowering.

49. Timing
The critical amounts of daylight occur at different times of the year, when conditions are best suited for reproduction for a particular species.

50. How do plants regulate photoperiodism?
This appears to be related to the ratio of different light-sensitive pigments in the leaves of a plant at different points during the day and night.

51. Take home message 19.7
Plants exhibit photoperiodism, responding to seasonal changes over the course of a year, timing their production of flowers or initiation of winter dormancy, for example, according to environmental factors such as the length of the period of darkness each night.

52. Section 19-2 Opener
A plant defense: the stem of dog rose with thorns.

53. 19.8. Plants actively resist being eaten by herbivores.
Plants have evolved numerous defenses—these fall into four major categories:
Mechanical defenses
Chemical defenses
Mimicry and camouflage
Enlisting other organisms for “security”

54. Figure Plants have physical defenses to ward off predation by insects and other animals. Shown (left to right): Scotch thistle, holly, and the sensitive plants (mimosa pudica).

55. Chemical Defenses
Chemical compounds deter herbivory by making the plant toxic or by reducing the digestibility of the plant.
Figure Co-opting a toxin. Monarch butterfly caterpillars consume a poisonous plant’s chemicals and make them their own.

56. Beneficial Effects of Secondary Compounds
Spices
Medicines
Both tasty food and malaria treatments can be traced back to plants. In fact, for many thousands of years, native peoples have used plants as a food source, to spice otherwise bland foods, and for medicinal purposes. These compounds, which may be toxic to other animals or even to us, in different dosages have been used to our benefit.
Figure “Bio-prospectors” search for plant chemicals that have medicinal purposes. The dried roots of harpaogphytum (Devil’s Claw) are used by some people for its anti-inflammatory properties.

57. Mimicry and Camouflage
To avoid being eaten by insects, plants can mimic being already infested by insect eggs.
Figure No room for additional eggs. These passion flower leaves are covered with what appears to be butterfly eggs. The leaf spots are actually produced by the plant and the mimicry discourages a butterfly from selecting the site for egg laying.

58. Enlisting Other Organisms for “Security”
Plants seem to outsource some of their defenses to other species.
An example is ants on acacia plants in Central America.

59. Take-home message 19.8
Rooted in the ground, plants are targets for predators and pathogens. They actively resist infection and herbivory by producing toxic chemicals that make them distasteful, poisonous, or difficult to consume.

60. Section 19-3 Opener
Quiver trees are large succulents (Aloe dichotoma) that live in arid landscapes. These were photographed at Richtersveld National Park, South Africa.

61. 19.9 Special adaptations help some plants thrive in extreme habitats.
There are adaptations plants utilize in a variety of ecosystems, including:
super-dry habitats
salty environments
cold and windy habitats

62. Super-Dry Habitats In these habitats, plants have adapted features:
succulent leaves and stems
deep taproots
long-dormant seeds
In super-dry habitats, water storage is a need. The fleshy, water-storage tissues of succulents are best designed for these habitats, as these are the “camels” of the plant kingdom. Also, these plants are designed so that the least amount of evaporation occurs. This, in conjunction with deep taproots to find hidden, precious water, and seeds that can remain dormant until moisture is present, forms a set of characteristics only found in plants in extremely dry environments.
Figure Survival strategies for dry climate. Shown (left to right): finger and thumb succulent, mesquite tree, and wildflowers in the desert.

63. Salty Environments To survive in these habitats, plants can:
transport much of the absorbed salts to vacuoles within cells, or
transport much of the salt through the plant and excrete it from the leaves.
Figure Survival in salt water.

64. Cold and Windy Habitats
Plants here grow close to the ground, and have smaller-than-average leaves.
Also, plants tend to have shallow root systems.
Figure Survival in cold, windy habitats.

65. Take-home message 19.9
Plant evolution has produced adaptations to some of the most extreme habitats in the world, with plants managing to thrive where few animals can survive.