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9.24: Plant Responses - Biology

9.24: Plant Responses - Biology


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So what happens to a vineyard in the middle of winter?

The vines cannot die each year. Instead, the plants go into a state of dormancy, almost as if they are taking a long nap.

Plant Responses

Like all organisms, plants detect and respond to stimuli in their environment. Unlike animals, plants can’t run, fly, or swim toward food or away from danger. They are usually rooted to the soil. Instead, a plant’s primary means of response is to change how it is growing. Plants also don’t have a nervous system to control their responses. Instead, their responses are generally controlled by hormones, which are chemical messenger molecules.

Plant Tropisms

Plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin. As shown in Figure below, auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bend toward the light.

Phototropism is controlled by the growth hormone auxin.

Daily and Seasonal Responses

Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Environmental stimuli that indicate changing seasons trigger other responses. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme cold and dryness of winter. Dormancy ensures that seeds will germinate and plants will grow only when conditions are favorable.

Responses to Disease

Plants don’t have immune systems, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees produce salicylic acid to kill bacteria. The same compound is used in many acne products for the same reason. Exciting new research suggests that plants may even produce chemicals that warn other plants of threats to their health, allowing the plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless.

Summary

  • Like all organisms, plants detect and respond to stimuli in their environment. Their main response is to change how they grow.
  • Plant responses are controlled by hormones. Some plant responses are tropisms.
  • Plants also respond to daily and seasonal cycles and to disease.

Review

  1. What is the primary way that plants respond to environmental stimuli? What controls their responses?
  2. Define tropism. Name one example in plants.
  3. State ways that plants respond to disease.
  4. Why is it adaptive for plants to detect and respond to daily and seasonal changes?

Plants have a number of sophisticated uses for light that go far beyond their ability to photosynthesize low-molecular-weight sugars using only carbon dioxide, light, and water. Photomorphogenesis is the growth and development of plants in response to light. It allows plants to optimize their use of light and space. Photoperiodism is the ability to use light to track time. Plants can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Phototropism is a directional response that allows plants to grow towards, or even away from, light. Positive phototropism is growth towards a light source (Figure 1), while negative phototropism (also called skototropism) is growth away from light.

Figure 1. Azure bluets (Houstonia caerulea) display a phototropic response by bending toward the light. (credit: Cory Zanker)

The sensing of light in the environment is important to plants it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. Together, the two are called a chromoprotein.

The red/far-red and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.

Horticulturalist

The word “horticulturist” comes from the Latin words for garden (hortus) and culture (cultura). This career has been revolutionized by progress made in the understanding of plant responses to environmental stimuli. Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower, or fruit production by understanding how environmental factors affect plant growth and development.

Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night, plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed by applying plant hormones. Recently, considerable progress has been made in the development of plant breeds that are suited to different climates and resistant to pests and transportation damage. Both crop yield and quality have increased as a result of practical applications of the knowledge of plant responses to external stimuli and hormones.

Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens, and in the production or research fields. They improve crops by applying their knowledge of genetics and plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture majors add studies in economics, business, computer science, and communications.


16.4 Plant Features and Responses

Plants live just about everywhere on Earth. To live in so many different habitats, God created different features that allow them to survive and reproduce under a diversity of conditions.

Plant Features

All plants are made to live on land. Or are they? There are plants that live on land and others that live in watery habitats.

Features Suited for Water

Aquatic plants are plants that live in water. Living in water has certain advantages for plants. One advantage is, well, the water. There’s plenty of it and it’s all around. Therefore, most aquatic plants do not need features for absorbing, transporting, and conserving water. They can save energy and matter by not growing extensive root systems, vascular tissues, or thick cuticles on leaves. Support is also less of a problem because of the buoyancy of water. As a result, adaptations such as strong woody stems and deep anchoring roots are not necessary for most aquatic plants.

Living in water does present challenges to plants, however. For one thing, pollination by wind or animals isn’t feasible under water, so aquatic plants may have features that help them keep their flowers above water. For instance, water lilies have bowl-shaped flowers and broad, flat leaves that float. This allows the lilies to collect the maximum amount of sunlight, which does not penetrate very deeply below the surface. Plants that live in moving water, such as streams and rivers, may have different features. For example, cattails have narrow, strap-like leaves that reduce their resistance to the moving water (see Figure below).

Adaptations to Extreme Dryness

Plants that live in extremely dry environments have the opposite problem: how to get and keep water. Plants that are created to live in very dry environments are called xerophytes. Their features may help them increase water intake, decrease water loss, or store water when it is available.

The saguaro cactus pictured in Figure below has features that help it in all three ways. When it was still a very small plant, just a few inches high, its shallow roots already reached out as much as 2 meters (7 feet) from the base of the stem. By now, its root system is much more widespread. It allows the cactus to gather as much moisture as possible from rare rainfalls. The saguaro doesn’t have any leaves to lose water by transpiration. It also has a large, barrel-shaped stem that can store a lot of water. Thorns protect the stem from thirsty animals that might try to get at the water inside.

Living up in the Air

Plants called epiphytes grow on other plants. They obtain moisture from the air and make food by photosynthesis. Most epiphytes are ferns or orchids that live in tropical or temperate rainforests (see Figure below). Host trees provide support, allowing epiphyte plants to obtain air and sunlight high above the forest floor. Being elevated above the ground lets epiphytes get out of the shadows on the forest floor so they can get enough sunlight for photosynthesis. Being elevated may also reduce the risk of being eaten by herbivores and increase the chance of pollination by wind.

Epiphytes don’t grow in soil, so they may not have roots. However, they still need water for photosynthesis. Rainforests are humid, so the plants may be able to absorb the water they need from the air. However, many epiphytes have leaves or other structures for collecting rainwater, fog, or dew.

The leaves of the bromeliad shown in Figure below are rolled into funnel shapes to collect rainwater. The base of the leaves forms a tank that can hold more than 8 liters (2 gallons) of water. Some insects and amphibians may spend their whole life cycle in the pool of water in the tank, adding minerals to the water with their wastes. The tissues at the base of the leaf are absorbent, so they can take in both water and minerals from the tank. God took care of the tiniest details and made everything work together perfectly!

BBC Earth – Drinking Monkeys and Bathing Birds

Plant Responses

Like all organisms, plants detect and respond to stimuli in their environment. Unlike animals, plants can’t run, fly, or swim toward food or away from danger. They are usually rooted to the soil. Instead, a plant’s primary means of response is to change how it is growing. Plants also don’t have a nervous system to control their responses. Instead, their responses are generally controlled by hormones, which are chemical messenger molecules.

Plant Tropisms

As you read earlier in this chapter, plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin. As shown in Figure below, auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bend toward the light.

Daily and Seasonal Responses

Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Environmental stimuli that indicate changing seasons trigger other responses. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme cold and dryness of winter. Dormancy ensures that seeds will germinate and plants will grow only when conditions are favorable.

TED Ed: How Plants Tell Time video:

Responses to Disease

Plants don’t have immune systems, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees produce salicylic acid to kill bacteria. The same compound is used in many acne products for the same reason. Exciting new research suggests that plants may even produce chemicals that warn other plants of threats to their health, allowing the plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless.

TED Ed Can plants talk to each other?

Creation Moment: Plants Really Do Talk to Each Other:

KQED: Plant Plague: Sudden Oak Death

Devastating over one million oak trees across Northern California in the past ten years, Sudden Oak Death is a killer with no cure. But biologists now are looking to the trees’ genetics for a solution. See http://www.kqed.org/quest/television/plant-plague-sudden-oak-death for more information.

Lesson Summary

  • Plants live just about everywhere on Earth, so God created features that allow them to survive and reproduce under a diversity of conditions. Various plants have different features to live in the water, in very dry environments, or in the air as epiphytes.
  • Like all organisms, plants detect and respond to stimuli in their environment. Their main response is to change how they grow. Their responses are controlled by hormones. Some plant responses are tropisms. Plants also respond to daily and seasonal cycles and to disease.

Lesson Review Questions

Recall

1. List special challenges that aquatic plants face.

2. What are xerophytes? Give an example.

3. Identify three general ways that plants can live in extreme dryness.

4. Describe how epiphytes can absorb moisture without growing roots in soil.

5. What is the primary way that plants respond to environmental stimuli? What controls their responses?

6. Define tropism. Name one example in plants.

7. State ways that plants respond to disease.

Apply Concepts

8. Apply the concept of symbiosis to epiphytes and their host plants. Do you think they have a symbiotic relationship? If so, which type of symbiotic relationship do you think they have? Explain your answer.

Think Critically

9. Why are epiphytes found mainly in rainforest ecosystems?

10. Why is it helpful for plants to detect and respond to daily and seasonal changes?

Points to Consider

In this chapter you read about the cells, tissues, and organs that make up plants. You also read about plant life cycles. Like plants, animals are complex organisms with tissues and organs. Animals also have life cycles.


Plant Responses to Drought Stress | Genetics

In this article we will discuss about the plant responses to drought stress.

With the exception of flooding, the major drought stresses result in water-deficit stress. The stressed plants exhibit striking responses by producing plant hormone abscisic acid (ABA), which plays an important role in tolerance against drought. Abscisic acid can induce the expres­sion of stress relieving genes. More than half of the drought-inducible genes are also induced by abscisic acid.

However, dehydration responsive genes are not induced by external application of ABA. These studies clearly show the existence of different signal transduction pathways in­volved in the expression of stress relieving genes. The drought responsive genes expression involves existence of ABA-independent as well as ABA-dependent signal transduction pathways.

The drought inducible gene products are, enzymes involved in particu­lar metabolic pathway, regulatory proteins or proteins with specific properties. These proteins are also involved in the regulation of other genes by signal transduction in the drought stress response. Kazuko (2002) classified this into two groups.

The gene products of the first group include:

(1) Functional proteins that directly protect macromolecules and membranes (LEA proteins, osmotin, antifreeze proteins, chaperones and mRNA binding proteins, etc.),

(2) The proteins maintain water movement through membranes (water-channel proteins and mem­brane transporters),

(3) The enzymes catalyzing the biosynthesis of various osmoregulators (proline, betaine and sugars, etc.),

(4) Detoxifying enzymes to maintain normal cellular func­tions (glutathione s-transferase, soluble epoxide hydrolase, catalase, superoxide dismutases and ascorbic peroxidases).

The gene product of the second group includes:

(1) Transcription factors (DREB, MYC, MYB, bZIP),

(2) Protein kinases (MAP kinases, CDP kinases, ribosomal-protein kinase and transcription-regulation protein kinase, etc.),


Learning Objectives

  • grow plants in a controlled manner under sufficient and deficient iron supply
  • use genetically distinct plants and monitor plant growth in response to iron and in the presence and absence of a functional FIT gene (wild type versus fit mutant)
  • reveal responses at physiological (Fe reductase activity) and molecular level (gene expression of FRO2, FIT, reference gene)
  • apply current state of the art methodology for gene expression (RT-qPCR)
  • summarize key steps in nutrient acquisition using the example of iron

V. Discovery of the Arabidopsis TCH genes

Control experiments serendipitously revealed the existence of touch-inducible genes in plants ( Braam & Davis, 1990 ). These genes, originally called the TCH genes and isolated by differential cDNA library screening, are strongly and rapidly up-regulated in expression in response to touch ( Braam & Davis, 1990 ). The TCH genes were identified by their dramatic mRNA accumulation in plants sprayed with gibberellins. However, further northern analyses revealed that TCH expression increased in plants sprayed with a variety of hormones, for example, auxin, cytokinin and abscisic acid, and indeed by a spray of simply water ( Braam & Davis, 1990 ). Finally, a simple mechanical stimulation, administered by touching the rosette leaves and bending them down or back and forth, was found to be sufficient to elicit a dramatic enhancement of TCH expression ( Braam & Davis, 1990 ). The differential cDNA screening technique tends to identify preferentially genes with high expression levels and indeed the handful of TCH genes proved to be only the tip of the iceberg. Over the past 14 years, many other genes have been discovered, often unintentionally, to have mechano-stimulus-inducible expression ( Ling et al., 1991 Perera & Zielinski, 1992 Gawienowski et al., 1993 Botella & Arteca, 1994 Botella et al., 1996 Mizoguchi et al., 1996 Oh et al., 1996 Royo et al., 1996 Shirsat et al., 1996 Eldick et al., 1997 Mauch et al., 1997 Gilmour et al., 1998 Arteca & Arteca, 1999 Gadea et al., 1999 Hirsinger et al., 1999 Tatsuki & Mori, 1999 Müssig et al., 2000 Oufattole et al., 2000 Lee et al., 2005 ).


Low phosphorus supply constrains plant responses to elevated CO2: A meta-analysis

Mingkai Jiang, Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia.

Max Planck Institute for Biogeochemistry, Jena, Germany

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Land Surface-Atmosphere Interactions, Technical University of Munich, Munich, Germany

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

ARC Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Department of Forest Resources, University of Minnesota, St Paul, MN, USA

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Max Planck Institute for Biogeochemistry, Jena, Germany

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Mingkai Jiang, Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia.

Max Planck Institute for Biogeochemistry, Jena, Germany

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Land Surface-Atmosphere Interactions, Technical University of Munich, Munich, Germany

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

ARC Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Department of Forest Resources, University of Minnesota, St Paul, MN, USA

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Max Planck Institute for Biogeochemistry, Jena, Germany

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

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Abstract

Phosphorus (P) is an essential macro-nutrient required for plant metabolism and growth. Low P availability could potentially limit plant responses to elevated carbon dioxide (eCO2), but consensus has yet to be reached on the extent of this limitation. Here, based on data from experiments that manipulated both CO2 and P for young individuals of woody and non-woody species, we present a meta-analysis of P limitation impacts on plant growth, physiological, and morphological response to eCO2. We show that low P availability attenuated plant photosynthetic response to eCO2 by approximately one-quarter, leading to a reduced, but still positive photosynthetic response to eCO2 compared to those under high P availability. Furthermore, low P limited plant aboveground, belowground, and total biomass responses to eCO2, by 14.7%, 14.3%, and 12.4%, respectively, equivalent to an approximate halving of the eCO2 responses observed under high P availability. In comparison, low P availability did not significantly alter the eCO2-induced changes in plant tissue nutrient concentration, suggesting tissue nutrient flexibility is an important mechanism allowing biomass response to eCO2 under low P availability. Low P significantly reduced the eCO2-induced increase in leaf area by 14.3%, mirroring the aboveground biomass response, but low P did not affect the eCO2-induced increase in root length. Woody plants exhibited stronger attenuation effect of low P on aboveground biomass response to eCO2 than non-woody plants, while plants with different mycorrhizal associations showed similar responses to low P and eCO2 interaction. This meta-analysis highlights crucial data gaps in capturing plant responses to eCO2 and low P availability. Field-based experiments with longer-term exposure of both CO2 and P manipulations are critically needed to provide ecosystem-scale understanding. Taken together, our results provide a quantitative baseline to constrain model-based hypotheses of plant responses to eCO2 under P limitation, thereby improving projections of future global change impacts.

Open Research


23.6 Plant Sensory Systems and Responses

In this section, you will explore the following questions:

  • How do red and blue light affect plant growth and metabolic activities?
  • What is gravitropism?
  • What are examples of plant hormones, and how do they affect plant growth and development?
  • What are the differences between thigmotropism, thigmonastism, and thigmogenesis?
  • How do plants defend themselves against predators and respond to wounds?

Connection for AP ® Courses

Why do some flowering plants bloom in the spring, whereas others bloom in the summer or fall? Why do the stems of plants grow upward and bend toward light, while roots grow downward? Why do the leaves of deciduous plants in northern climates turn colors in the fall and eventually fall off, while evergreens keep their needles all year around? Is it true that putting an unripe avocado in a paper bag will hasten the ripening process? Can the same tactic work on bananas?

Like most other organisms, plants have the ability to detect and respond to environmental change this ability is an adaptation favored by natural selection. Flowering plants react to light, gravity, infection by pathogens, drought, and, as with the Venus flytrap, touch. Animals have two systems on which they can rely to detect and respond to stimuli: the nervous and endocrine systems. Plants, however, only have chemical control mechanisms at their disposal. In addition to the phytochromes, plants evolved hormones that allow them to respond to environmental changes. Like animal hormones, plant hormones are chemical signaling molecules that trigger a cellular response through a signal transduction pathway.

Although you do not have to memorize a laundry list of plant hormones and their activities, you should understand the basic mechanism of how they trigger a response. Examples that we’ll learn about in this chapter include auxins, which cause plant stems to grow and bend toward light, and gibberellins, which stimulate cell growth and breaks seed dormancy. Plants also produce other chemicals to protect against biotic stimuli, including herbivory and parasitism. The fact that plants produce chemicals should not be surprising our pharmaceutical industry depends on these substances. For example, species of the foxglove plant produce digitalis, a powerful heart medicine, whereas the opium from the poppy is the source of many narcotics.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 3 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.C Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis.
Essential Knowledge 2.C.1 Plants use negative and positive feedback mechanisms to maintain their internal environments and respond to external environmental changes.
Science Practice 4.2 The student can design a plan for collecting data to answer a particular scientific question.
Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
Science Practice 6.1 The student can justify claims with evidence.
Learning Objective 2.17 The student is able to evaluate data that show the effect(s) of changes in concentrations of key molecules on negative feedback mechanisms.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.C Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis.
Essential Knowledge 2.C.1 Plants use negative and positive feedback mechanisms to maintain their internal environments and respond to external environmental changes.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.18 The student can make predictions about how organisms use negative feedback mechanisms to maintain their internal environments.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.D Growth and dynamic homeostasis of a biological system are influenced by changes in the system’s environment.
Essential Knowledge 2.D.1 Cellular activity in plants is affected by interactions with biotic and abiotic factors.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.22 The student is able to refine scientific models and questions about the effect of complex biotic and abiotic interactions on all biological systems, from cells and organisms to populations, communities, and ecosystems.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.D Growth and dynamic homeostasis of a biological system are influenced by changes in the system’s environment.
Essential Knowledge 2.D.4 Plants have nonspecific immune responses to defend against infections and other threats.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.30 The student can create representations or models to describe nonspecific immune defenses in plants and animals.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.E Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination.
Essential Knowledge 2.E.3 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes.
Science Practice 4.2 The student can design a plan for collecting data to answer a particular scientific question.
Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
Science Practice 6.1 The student can justify claims with evidence.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.39 The student is able to justify scientific claims, using evidence, to describe how timing and coordination of behavioral events in organisms are regulated by several mechanisms.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.E Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination.
Essential Knowledge 2.E.3 Plant responses to stimuli are adaptations favored by natural selection.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.40 The student is able to connect concepts in and across domain(s) to predict how environmental factors affect responses to information and change behavior.
Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.B Expression of genetic information involves cellular and molecular mechanisms.
Essential Knowledge 3.B.2 Signal transmission within and between plant cells mediates gene expression.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 3.22 The student is able to explain how signal pathways mediate gene expression, including how this process can affect protein production.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.29][APLO 2.30]

Animals can respond to environmental factors by moving to a new location. Plants, however, are rooted in place and must respond to the surrounding environmental factors. Plants have sophisticated systems to detect and respond to light, gravity, temperature, and physical touch. Receptors sense environmental factors and relay the information to effector systems—often through intermediate chemical messengers—to bring about plant responses.

Plant Responses to Light

Plants have a number of sophisticated uses for light that go far beyond their ability to photosynthesize low-molecular-weight sugars using only carbon dioxide, light, and water. Photomorphogenesis is the growth and development of plants in response to light. It allows plants to optimize their use of light and space. Photoperiodism is the ability to use light to track time. Plants can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Phototropism is a directional response that allows plants to grow towards, or even away from, light.

The sensing of light in the environment is important to plants it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. Together, the two are called a chromoprotein.

The red/far-red and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.

The Phytochrome System and the Red/Far-Red Response

The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (

667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (

730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein therefore, exposure to red light yields physiological activity. Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system (Figure 23.38).

The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark, or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.

Once the phytochrome system evolved, plants adapted it to serve a variety of needs. Unfiltered, full sunlight contains much more red light than far-red light. Because chlorophyll absorbs strongly in the red region of the visible spectrum, but not in the far-red region, any plant in the shade of another plant on the forest floor will be exposed to red-depleted, far-red-enriched light. The preponderance of far-red light converts phytochrome in the shaded leaves to the Pr (inactive) form, slowing growth. The nearest non-shaded (or even less-shaded) areas on the forest floor have more red light leaves exposed to these areas sense the red light, which activates the Pfr form and induces growth. In short, plant shoots use the phytochrome system to grow away from shade and towards light. Because competition for light is so fierce in a dense plant community, the evolutionary advantages of the phytochrome system are obvious.

In seeds, the phytochrome system is not used to determine direction and quality of light (shaded versus unshaded). Instead, is it used merely to determine if there is any light at all. This is especially important in species with very small seeds, such as lettuce. Because of their size, lettuce seeds have few food reserves. Their seedlings cannot grow for long before they run out of fuel. If they germinated even a centimeter under the soil surface, the seedling would never make it into the sunlight and would die. In the dark, phytochrome is in the Pr (inactive form) and the seed will not germinate it will only germinate if exposed to light at the surface of the soil. Upon exposure to light, Pr is converted to Pfr and germination proceeds.

Plants also use the phytochrome system to sense the change of season. Photoperiodism is a biological response to the timing and duration of day and night. It controls flowering, setting of winter buds, and vegetative growth. Detection of seasonal changes is crucial to plant survival. Although temperature and light intensity influence plant growth, they are not reliable indicators of season because they may vary from one year to the next. Day length is a better indicator of the time of year.

As stated above, unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise. By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle. In addition, leaves retain that information for several days, allowing a comparison between the length of the previous night and the preceding several nights. Shorter nights indicate springtime to the plant when the nights become longer, autumn is approaching. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly. Short-day (long-night) plants use this information to flower in the late summer and early fall, when nights exceed a critical length (often eight or fewer hours). Long-day (short-night) plants flower during the spring, when darkness is less than a critical length (often eight to 15 hours). Not all plants use the phytochrome system in this way. Flowering in day-neutral plants is not regulated by daylength.

Career Connection

Horticulturist

The word “horticulturist” comes from the Latin words for garden (hortus) and culture (cultura). This career has been revolutionized by progress made in the understanding of plant responses to environmental stimuli. Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower, or fruit production by understanding how environmental factors affect plant growth and development.

Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night, plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed by applying plant hormones. Recently, considerable progress has been made in the development of plant breeds that are suited to different climates and resistant to pests and transportation damage. Both crop yield and quality have increased as a result of practical applications of the knowledge of plant responses to external stimuli and hormones.

Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens, and in the production or research fields. They improve crops by applying their knowledge of genetics and plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture majors add studies in economics, business, computer science, and communications.

The Blue Light Responses

Phototropism—the directional bending of a plant toward or away from a light source—is a response to blue wavelengths of light. Positive phototropism is growth towards a light source (Figure 23.39), while negative phototropism (also called skototropism) is growth away from light.

The aptly-named phototropins are protein-based receptors responsible for mediating the phototropic response. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore. In phototropins, the chromophore is a covalently-bound molecule of flavin hence, phototropins belong to a class of proteins called flavoproteins.

Other responses under the control of phototropins are leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood.

In their 1880 treatise The Power of Movements in Plants, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the tip of the plant (the apical meristem), but that the response (bending) took place in a different part of the plant. They concluded that the signal had to travel from the apical meristem to the base of the plant.

In 1913, Peter Boysen-Jensen demonstrated that a chemical signal produced in the plant tip was responsible for the bending at the base. He cut off the tip of a seedling, covered the cut section with a layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend. A refinement of the experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant did bend towards the light. Therefore, the chemical signal was a growth stimulant because the phototropic response involved faster cell elongation on the shaded side than on the illuminated side. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem. Most activation occurs on the lit side, causing the plant hormone indole acetic acid (IAA) to accumulate on the shaded side. Stem cells elongate under influence of IAA.

Cryptochromes are another class of blue-light absorbing photoreceptors that also contain a flavin-based chromophore. Cryptochromes set the plants 24-hour activity cycle, also know as its circadian rhythem, using blue light cues. There is some evidence that cryptochromes work together with phototropins to mediate the phototropic response.

Link to Learning

Use the navigation menu in the left panel of this website to view images of plants in motion.

  1. Green light is absorbed by the plant. The seedling would not bend, but it would grow tall and spindly as if grown in the dark.
  2. Green light is not absorbed by the plant. The seedling would not bend, but it would grow tall and spindly as if grown in the dark.
  3. Green light is not absorbed by the plant. The seedling would not grow tall and spindly, but it would bend.
  4. Green light is absorbed by the plant. The seedling would not grow tall and spindly, but it would bend.

Plant Responses to Gravity

Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.

Amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.

The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER), causing the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone IAA to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion, causing the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses—involving the entire cell in the gravitropism effect—have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.

Growth Responses

A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially every cell in a plant can produce plant hormones. They can act in their cell of origin or be transported to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a distant site for action, and they act alone.

Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors.

Auxins

The term auxin is derived from the Greek word auxein, which means "to grow." Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance—the inhibition of lateral bud formation—is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.

Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Cytokinins

The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally occurring or synthetic cytokinins are known to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds, and cytokinins that promote bushier growth.

Gibberellins

Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.

GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection (Figure 23.40).

Abscisic Acid

The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls. However, more recent studies indicate that ABA plays only a minor role in the abscission process. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA inhibits stem elongation and induces dormancy in lateral buds.

ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds.

Ethylene

Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas (C2H4). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps.

Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocadoes, in a sealed paper bag to accelerate ripening the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes.

Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.

Nontraditional Hormones

Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.

Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest.

Oligosaccharins also play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues. Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids.

Plant Responses to Wind and Touch

The shoot of a pea plant winds around a trellis, while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.

The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch,” and tropism implying “direction.” Tendrils are one example of this. The meristematic region of tendrils is very touch sensitive light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand (Figure 23.14). Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.

A thigmonastic response is a touch response independent of the direction of stimulus Figure 23.24. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.

Everyday Connection for AP® Courses

The mimosa plant is also known as the sensitive plant, because its leaves are sensitive to touch and will fold inward and droop. Leaves in their normal state are shown on the left.

Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.

Link to Learning

Use the menu at the left to navigate to three short movies: a Venus fly trap capturing prey, the progressive closing of sensitive plant leaflets, and the twining of tendrils.

  1. anywhere, because the whole surface of the leaf responds to touch
  2. only on the margins of the leaves where insects usually land
  3. in the center of the leaf, where the touch-sensitive hairs are located
  4. on the petiole followed by the center of leaf which signal the presence of a wandering insect

Defense Responses against Herbivores and Pathogens

Plants face two types of enemies: herbivores and pathogens. Herbivores both large and small use plants as food, and actively chew them. Pathogens are agents of disease. These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers.

The first line of defense in plants is an intact and impenetrable barrier. Bark and the waxy cuticle can protect against predators. Other adaptations against herbivory include thorns, which are modified branches, and spines, which are modified leaves. They discourage animals by causing physical damage and inducing rashes and allergic reactions. A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes.

Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic, and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore.

Mechanical wounding and predator attacks activate defense and protection mechanisms both in the damaged tissue and at sites farther from the injury location. Some defense reactions occur within minutes: others over several hours. The infected and surrounding cells may die, thereby stopping the spread of infection.

Long-distance signaling elicits a systemic response aimed at deterring the predator. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host. The plant may activate abscission of injured tissue if it is damaged beyond repair.


Application of chitosan on plant responses with special reference to abiotic stress

Chitosan is a natural biopolymer modified from chitins which act as a potential biostimulant and elicitor in agriculture. It is non-toxic, biodegradable and biocompatible which favors potentially broad application. It enhances the physiological response and mitigates the adverse effect of abiotic stresses through stress transduction pathway via secondary messenger(s). Chitosan treatment stimulates photosynthetic rate, stomatal closure through ABA synthesis enhances antioxidant enzymes via nitric oxide and hydrogen peroxide signaling pathways, and induces production of organic acids, sugars, amino acids and other metabolites which are required for the osmotic adjustment, stress signaling, and energy metabolism under stresses. It is also known to form complexes with heavy metals and used as tool for phytoremediation and bioremediation of soil. Besides, this is used as antitranspirant compound through foliar application in many plants thus reducing water use and ensures protection from other negative effects. Based on such beneficial properties, chitosan is utilized in sustainable agricultural practices owing to changing climates. Our review gathers the recent information on chitosan centered upon the abiotic stress responses which could be useful in future crop improvement programs.

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A Level Biology: Plant Responses (Plant Hormones)

I have been teaching Biology for 17 years and I have struggled to find concise and engaging resources for teaching Biology, which is why I have set about creating my own. I hope that my resources are enjoyed by your pupils and of course support their learning of such an interesting and great subject. We are currently offering a 50% discount to all members who sign up now for our website for the first year, just copy and paste the code 3711E747FB when prompted during the signing up process.

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A visually appealing and well laid out A level worksheet pack focusing on plant responses and the plant hormones involved. There are three worksheets included in the pack, designed for pupils to work through independently. Once completed these worksheets will create a set of useful and detailed revision notes for students.

Worksheets cover the following:

  1. Overview of plant responses and the roles of the 5 main plant hormones. This worksheet comes with 5 information sheets, one for each hormone, that pupils can use to complete the table element of the worksheet.
  2. Plant responses to the environment
  3. Types of plant response (tropisms)

Answer sheets are provided for all three worksheets.

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I have been teaching Biology for 17 years and I have struggled to find concise and engaging resources for teaching Biology, which is why I have set about creating my own. I hope that my resources are enjoyed by your pupils and of course support their learning of such an interesting and great subject. We are currently offering a 50% discount to all members who sign up now for our website for the first year, just copy and paste the code 3711E747FB when prompted during the signing up process.


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