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In Plant Sensing and Communication, Richard Karban provides the first comprehensive overview of what is known about how plants perceive their environments, communicate those perceptions, and learn. Facing many of the same challenges as animals, plants have developed many similar capabilities: they sense light, chemicals, mechanical stimulation, temperature, electricity, and sound. Moreover, prior experiences have lasting impacts on sensitivity and response to cues; plants, in essence, have memory. Nor are their senses limited to the processes of an individual plant: plants eavesdrop on the cues and behaviors of neighbors and—for example, through flowers and fruits—exchange information with other types of organisms. Far from inanimate organisms limited by their stationary existence, plants, this book makes unquestionably clear, are in constant and lively discourse.
Product Details
| ISBN-13: | 9780226264844 |
|---|---|
| Publisher: | University of Chicago Press |
| Publication date: | 06/18/2015 |
| Series: | Interspecific Interactions |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 240 |
| File size: | 4 MB |
About the Author
Read an Excerpt
Plant Sensing & Communication
By Richard Karban
The University of Chicago Press
Copyright © 2015 The University of ChicagoAll rights reserved.
ISBN: 978-0-226-26484-4
CHAPTER 1
Plant Behavior and Communication
1.1 Plants and animals are different but also similar
We all learn intuitively that plants are not like us in many ways. Ask a child about the differences between plants and animals and the answer won't be about photosynthesis. They'll say, "Plants can't move" or "Plants don't do anything." It is true that plants don't appear to do the things that many of us find most interesting about humans and other animals—moving, communicating with one another, and displaying a great diversity of sophisticated behaviors that depend upon the particular situations in which they find themselves. But this intuition about plants is incorrect; plants sense many aspects of their abiotic and biotic environments and respond with a variety of plastic morphologies and behaviors that are often adaptive. In addition, plants communicate, signaling to remote organs within an individual, eavesdropping on neighboring individuals, and exchanging information with other organisms ranging from other plants to microbes to animals.
Plants lack central nervous systems; the mechanisms coordinating plant sensing, behavior, and communication are quite different from the systems that accomplish similar tasks in animals. The challenges that face plants are similar to those facing animals—finding resources, avoiding predators, pathogens, and abiotic stresses, acquiring mates, placing offspring in situations where they are likely to be successful. The modes of selection are also basically similar for plants and animals. As a result, natural selection has led to the evolution of solutions to these challenges that are often analogous. Nonetheless, there are many important differences between plants and animals that have led to very different adaptations, including the behaviors that will be considered in this book.
Although the issues that plants and animals face have similarities, their habits, abilities, and circumstances tend to be different in many ways. Most plants are capable of producing their own food, allowing them to spend much of their lives as factories converting resources (light, water, CO2) into organic tissues. Since these resources are rapidly renewable, vegetative organs of many plants move relatively short distances and remain rooted in the soil. Higher plants are constructed of repeated modular units (leaves, branches, roots) that are far less specialized than the organs of higher animals (White 1984). Many important processes are carried out by plant organs that are less centralized than their counterparts in higher animals. For example, plants lack a central nervous system, and consequently, phenotypic expression is determined locally in many cases. Plants acquire resources from many different organs, above ground and below, thus avoiding restrictions that would be imposed by one or a small number of mouths. Diverse plant structures arise from undifferentiated meristems that have the potential to produce any cell type, including germ cells and somatic cells. In addition, these diverse plant organs can be produced repeatedly during the lifespan of an individual. Important plant organs are generally found in multiple, redundant copies, making any single organ more expendable than similar organs in most animals. This open-ended growth form allows plants enormous developmental flexibility, an important attribute that was recognized by the Greek botanist Theophrastus approximately 300 years BC (White 1984, Herrera 2009). Developmental flexibility allows plants to respond to environmental cues and change morphology, adding or shedding organs in response to current or anticipated conditions and allowing plants to "forage" for light, water, and soil nutrients and to allocate resources to reproduction, growth, or storage.
The philosopher Michael Marder (2012, 2013) has recently introduced the idea that plants sense their environments by focusing attention towards some cues more than others. The attention is dynamic, allowing plants to selectively respond to shifting, current stimuli. Many studies of plant responses to resource heterogeneity attest to the ability of plants to sense many stimuli (chapter 2) and selectively respond (e.g., chapters 5–8). In addition, different cues vie for the attention of sense organs or receptors that are distributed throughout the plant's tissues. Plant sensing is not contemplative, but active, and translates into various behaviors (see section 1.2.1). Unlike memory, which is also displayed by plants but is biased towards past events, plant attention as described by Marder is focused on current stimuli.
As humans, we have a tendency to compare plants to humans and other higher animals. Such comparisons can be useful at times, since our awareness and understanding of human behavior and communication is so much better developed. However, such comparisons can range from counterproductive to absurd, if done uncritically. For example, several authors assert that plants can appreciate and benefit from hearing particular kinds of music, most famously in the popular book, The Secret Lives of Plants (Tompkins and Bird 1973). The hypothesis that plants may respond to music is not itself absurd, since plants can sense and respond to electromagnetic radiation and acoustic energy (Telewski 2006, Gagliano et al. 2012b). However, asserting that plants benefit from music without carefully controlled experiments is not science and has hurt progress in this field and acceptance of these ideas. In summary, while plants don't appear at first glance to behave, in some cases they have evolved functions that are analogous to those in animals, but with different mechanisms and capabilities.
1.2 Working definitions
1.2.1 Plant behavior
Before attempting to determine whether plants exhibit behaviors including communication, it seems reasonable to agree upon a set of criteria that define these phenomena. This is more difficult than it sounds. Although animal behavior is a relatively mature field, practitioners of that field cannot reach a consensus about what constitutes behavior (Levitis et al. 2009). Early behaviorists defined the term quite restrictively; Tinbergen (1955), for example, wrote that behavior included "the total movements made by an intact animal." This definition excludes plants (and other taxa) and it also fails to include inactivity or decisions to not reproduce as behavior, as well as changes in traits not involving physical movement. Some behaviorists wish to differentiate between intentional, purposeful behaviors from actions that result as unintended consequences of other processes. This distinction has proven to be problematic since it is virtually impossible to determine an animal's intent. A recent survey of behavioral biologists found little concordance about what phenomena could be considered behaviors although approximately half of the respondents identified plant responses to light as a behavior (Levitis et al. 2009).
The possibility that plants exhibit behavior is not a new suggestion. Charles Darwin's grandfather, Erasmus Darwin (1794:107), speculated that "vegetable life seems to possess an organ of sense to distinguish varying degrees of moisture, another of light, another of touch, and finally another analogous to our sense of smell." As is often the case in biology, Charles Darwin, who seems to have foreshadowed much of modern biology, provided detailed descriptions of many plant species that moved in response to light, gravity, and contact (Darwin 1880). In a more recent attempt to explicitly define behavior to include plants, Jonathan Silvertown and Deborah Gordon (1989) described behavior as a response to an event or environmental change during the course of the lifetime of an individual. Responses ultimately are the result of physiological changes that have a biochemical basis. Behavior differs from other physiological and biochemical reactions by occurring rapidly relative to the lifespan of the individual and requires a response to a stimulus. Furthermore, behavioral responses need not be permanent, and can be reversed if the stimulus changes. For example, the decision to expand a shoot into a sunny patch is reversible in the sense that it can be stopped and additional resources allocated to other tissues should that shoot become shaded. However, the resources that have been allocated to that shoot cannot be fully recovered. This definition of behavior does not include changes that are the result of ontogeny (Silvertown and Gordon 1989, Silvertown 1998). For example, the changes that occur as a seed germinates and expands its cotyledons and then its true leaves are not considered behavioral responses since they are part of a developmental program that is not plastic, once initiated. This definition of behavior is similar to one used by plant biologists to describe phenotypic plasticity (Bradshaw 1965), and behavior may be considered a form of plasticity that occurs rapidly and reversibly in response to a stimulus.
1.2.2 Plant sensing, eavesdropping, communication, cues and signals
Communication can be considered a behavior that provides information from a sender to a receiver. Communication also provides information that can cause the receiver of that information to respond (behave). As was the case for behavior, there is no agreed-upon definition of what constitutes communication either for animals or for plants (Scott-Phillips 2008, Schenk and Seabloom 2010). Most definitions require that receivers respond to cues or stimuli (Karban 2008). This requirement is considered necessary but not sufficient by most workers who study animal behavior since it includes situations in which receivers respond to cues from their abiotic environment. In keeping with accepted definitions, I will regard responses to stimuli as examples of plants sensing cues but not communicating. How plants sense their environments is fascinating in its own right and will be discussed at length later in this book. I will restrict my use of the term "communication" to situations in which emission or display of a cue is plastic and the response of the receiver is conditional on receiving the cue. For example, a plant that always attains a short compact growth form because of its genes is not responding to cues in a proximate, short-term sense. A plant that adjusts its morphology depending upon the cues that it receives from its neighbors may or may not be considered to be communicating.
Definitions of communication tend to emphasize either the exchange of information from a sender to a receiver (Smith 1977, Hauser 1996) or the requirement that the transfer of information be favored by natural selection (Maynard Smith and Harper 1995, Scott-Phillips 2008). Cues provide the receiver with accurate estimates of the relative probabilities of alternative conditions. Both the amount of information and its value to the receiver can be quantified, although doing so in a meaningful way can be challenging (Wilson 1975, Bradbury and Vehrencamp 1998).
Some authors consider communication to have occurred if information has been transferred that elicits a response in the receiver without regard to benefits, while others require that the sender, the receiver, or both benefit from the exchange (Wilson 1975, Bradbury and Vehrencamp 1998, Maynard Smith and Harper 2003). They distinguish between cues that do not necessarily benefit the sender and signals that increase the sender's fitness. The term "signal" is reserved for those situations in which exchange of information is beneficial for both the sender and the receiver. Receivers that respond to cues (as opposed to signals) may eavesdrop on the sender or may be engaging in communication with the sender. For example, an herbivore that locates its host plant by the volatile cues that the plant emits is eavesdropping on cues that the plant emitted for some purpose other than attracting herbivores. According to the authors cited above, "true communication" occurs when providing information in the form of a signal is not accidental but benefits the sender. True communication can occur when signals are transferred between cells, organs within an individual, or different individuals. Some authors require that the signal must have evolved because of the effects that it causes and therefore that both the sender and the receiver must experience a benefit by communicating (Maynard Smith and Harper 2003, Scott-Phillips 2008). This definition has many advantages and can explain the evolution of refinements to effective signaling.
The various definitions of communication can make reading this literature confusing. I have summarized the requirements associated with various terms in table 1.1. There is a consensus that communication occurs only when the signal is sent, is received, and causes a response (fig. 1.1). Communication requires that all three steps be present, although this book will consider each of the steps independently since plants sense and respond to environmental cues even when the cue was not intentionally sent by a living organism. One problem with the definition is that a signal that is missed by a receiver may be identical to one that causes a response. Is it a signal in one case but not in the other? This problem can be fairly easily resolved by stipulating that a signal will cause a response on average (Scott-Phillips 2008). A more serious problem with an adaptationist definition that explicitly relies on establishing that signaling arose because it benefited the sender and the receiver is that determining why a trait evolved is extremely difficult (Gould and Lewontin 1979, Endler 1986). For example, pigments make flower petals visible to insect pollinators, suggesting that they may be considered signals (Fineblum and Rausher 1997, Gronquist et al. 2001). However, these same pigments also deter floral herbivores, suggesting that they may have evolved for this purpose and that they should be considered cues, not signals, in communication with pollinators. This makes identifying true communication with any certainty almost impossible in many instances.
1.3 Plant sensing and communication—organization of this book
In this book we will consider all three of the steps shown in fig. 1.1: what are the cues that plants emit, what are the cues that plants respond to, and how do plants or the organisms with which they are communicating change as a result? Although any single step is not sufficient for communication, each step can be fascinating regardless of what we call it.
fig. 1.1 is a graphical representation of the scope of this book, and it will be used repeatedly to show the relationships between the various sections. Plants have considerable sensory capabilities, as receivers of various animate and inanimate cues (chapter 2). Sensitivity to cues can be influenced by past experiences, and plant responses are considered as learning in chapter 3. The properties of different cues that are involved with communication are examined in chapter 4, as are the mechanisms by which plants receive cues and emit them. Plants sense and respond to heterogeneity in resources (chapter 5) and herbivores (chapter 6). Communication between plants and animal visitors affect plant reproduction—particularly pollination and seed dispersal (chapter 7). Diverse interactions between plants and microbes are discussed in chapter 8. Sensing and communication affect plant fitness and can drive macroevolutionary patterns (chapter 9). An understanding of plant sensing and communication can lead to many useful application, considered in chapter 10.
CHAPTER 2Plant Sensory Capabilities
2.1 Plants sense their environments
Plants can sense many qualities about their environments. In this chapter, we will consider some of the environmental qualities that plants sense: light, chemicals, touch and gravity, temperature, sound, and electromagnetic forces. They do this using a variety of receptors and feedback mechanisms, including phytochrome receptors to detect light, mechanical sensing to detect gravity, and chemical feedbacks to detect CO2. The stimuli that plants sense include both abiotic factors and those caused by other plants, microbes, and animals. Because we are more familiar with the ability of animals to detect their environments, those abilities will be compared to the sensory capabilities of plants.
Plants are also affected by their previous experiences; plant learning and memory will be discussed in chapter 3. Chapter 4 will explore what we know about the cues and signals used by plants to acquire sensory information and the cues that they produce that other organisms sense and respond to.
Plants live in a diversity of habitats: from deserts to rainforests, rooted in soil and free floating in water, under full sun or in full shade. These conditions often change over short spatial scales such that the seeds from a single mother may germinate in very different situations. Similarly, conditions may change rapidly so that an individual experiences a great range within its lifetime. As a result of this uncertainty and variability, plants have evolved the ability to sense their environments and to respond to their current and expected conditions.
(Continues...)
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Table of Contents
AcknowledgmentsChapter 1. Plant Behavior and Communication
1.1 Plants and animals are different but also similar
1.2 Working definitions
1.3 Plant sensing and communication—organization of this book
Chapter 2. Plant Sensory Capabilities
2.1 Plants sense their environments
2.2 Plants sense light
2.3 Chemical sensing
2.4 Mechanical sensing—touch
2.5 Plant sensing of temperature, electricity, and sound
Chapter 3. Plant Learning and Memory
3.1 Do plants learn?
3.2 Learning, memory, and light
3.3 Learning, memory, and perception of chemicals, resources, pathogens, and herbivores
3.4 Learning, memory, and touch
3.5 Learning, memory, and cold
3.5 Transgenerational memory
Chapter 4. Cues and Signals in Plant Communication
4.1 The nature of cues and signals
4.2 Plant competition—light and hormonal cues
4.3 Cues used in plant defense
4.4 Cues and signals emitted by plants that animals sense
Chapter 5. Plant Responses to Cues about Resources
5.1 General characteristics of plant responses
5.2 Plants forage for resources
5.3 Integrating resource needs
Chapter 6. Plant Responses to Herbivory
6.1 Induced responses as plant defenses
6.2 Volatile signals and communication between ramets and individuals
6.3 Indirect defenses against herbivores
6.4 Visual communication between plants and herbivores
Chapter 7. Plant Communication and Reproduction
7.1 Pollination and communication
7.2 Seed dispersal and communication
Chapter 8. Microbes and Plant Communication
8.1 Microbes are critical for plant success
8.2 Plants recognize pathogens
8.3 Infested plants attract the microbial enemies of their attackers
8.4 Plants communicate with mycorrhizal fungi
8.5 Plants communicate with N-fixing bacteria
Chapter 9. Plant Sensing and Communication as Adaptations
9.1 Plant senses and emission of cues—adaptive traits?
9.2 Case studies of adaptations
Chapter 10. Plant Sensing and Communication in Agriculture and Medicine
10.1 Manipulating the sensing and communication process
10.2 Manipulating resource acquisition and allocation
10.3 Manipulating tolerance to abiotic stress
10.4 Manipulating resistance to pathogens
10.5 Manipulating resistance to herbivores
10.6 Manipulating reproductive timing and effort
10.7 As a source of medicines
10.8 Plant sensing in the future of mankind
References
Index