BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

8. PHYSIOLOGY OF MOVEMENTS

Many Living organisms are capable of movement. This allows them to navigate their environment and enables the Organism or its parts to attain the most advantageous orientation possible. Most animals exhibit The ability to move, or engage in locomotion, meaning they can freely change their Location. This allows them to avoid unfavorable environmental conditions and actively seek out favorable ones. The group of plants capable of locomotion is relatively limited (certain Bacteria, Algae, and Fungi), but specific types of Cells (spores, Gametes) are motile even in higher plants up to gymnosperms, such as the gametes of the cycad (Cycas) and Ginkgo (Ginkgo biloba). Sessile plants can orient specific Organs in space in response to environmental factors acting upon them, or execute specialized movement sequences in reaction to inducing stimuli, in order

to achieve the fullest possible adaptive responses, which are discussed in greater detail in the subsequent sections.

8.1. Basic Concepts of the Physiology of Irritability

A stimulus is defined as a physical or chemical signal that triggers a sequence of reactions in a Cell, the energy for which is supplied by the organism itself rather than the stimulus. Chemical stimuli are nowadays frequently referred to as signaling substances. Endogenous signaling substances (those produced within the organism itself) include, for example, phytohormones (see 7.6). Even a single flash of light lasting a fraction of a second, perceived by a previously darkened plant, can induce hours of growth inhibition and act as a stimulus. In contrast, the light that drives Photosynthesis in a green plant serves as an energy source and therefore cannot be termed a stimulus.¹ A stimulus thus acts as a trigger (releaser) of a characteristic process rather than its driving mechanism.

¹ Simultaneously with photosynthesis, processes for which light serves as a stimulus always take place in a plant. — Ed. note.

If The process of locomotion is induced by a stimulus, it is referred to as a taxis (see 8.2.1). Movements of organs or cells in a sessile plant that are elicited by a stimulus and whose direction is determined by it are called Tropisms (see 8.3.1). Tropisms manifest primarily as Changes in the direction of cell growth and differences in the growth rates of opposite sides of an organ; such growth movements generally proceed relatively slowly (from a few minutes to many hours). Nasty (see 8.3.2) occurs when a process is triggered by a stimulus, but the course of the process is nevertheless determined by the structural plan of the organ. Nasties are usually, though not always, based on changes in the osmotic potential of cells; in such cases, we are dealing with turgor movements (which are reversible in most cases). These often proceed very rapidly (for example, the Touch-induced folding of the gynostemium, or Column, of Stylidium from the family Stylidiaceae takes only 10–30 ms).

Overall, the response process elicited by a stimulus, regardless of whether the outcome is movement or another type of response (see also 7.7), can be divided into the following phases: stimulus perception (reception), stimulus Transduction, signal transmission, and the response phase.

The cellular system that perceives an "adequate stimulus"—i.e., a stimulus that determines the degree of stimulation—is called a receptor. In the simplest case, for example during the perception of light stimuli or signaling molecules, individual Proteins or Oligomeric Proteins serve as receptors; for other stimuli, particularly mechanical forces or gravity, the involvement of complex cellular structures is hypothesized. The impact of the stimulus transitions the receptor into an activated state, which sets off a characteristic cascade of reactions (secondary response) based on the activation or inhibition of cellular systems operating downstream of the receptor. This conversion of a stimulus into a cellular signal is sometimes referred to as excitation, a term borrowed from the physiology of Higher Nervous Activity, specifically neurophysiology. However, this term is best avoided in the context of plants.

The subsequent secondary response triggered by the activated receptor may, depending on the circumstances, directly modulate The activity of the target cell system responsible for the stimulus response; more frequently, however, multi-step signaling pathways, under certain conditions involving signal Amplification, are implemented through enzyme activity and/or the engagement of electrical processes, which furthermore enable diverse regulation and modulation via other cellular signaling pathways (cross-talking of signal pathways). Signaling pathways can be localized within a cell, but they may also run between cells over considerable distances. In such cases, one also speaks of signal transduction. The investigation of these processes in plants at THE MOLECULAR LEVEL is only just beginning. As in the previous chapter on developmental physiology, the presentation of concepts concerning molecular processes must henceforth be restricted to a few (and insufficiently elaborated) Examples.

Ultimately, the stimulus-controlled target cell systems that terminate the signaling pathways may be represented by proteins or genes. For instance, reversible turgor movements are based on changes in the activity of plant Ion Channels (see primarily 8.3.2), whereas irreversible growth movements are driven by changes not only in protein activity but also in their qualitative composition; consequently, they are invariably rooted in differential Gene activity (see 7.2.2.3).

A stimulus is capable of eliciting a reaction if its magnitude exceeds a certain threshold value (stimulus threshold). However, multiple subthreshold stimuli can also be perceived. This is evident from the fact that individual subthreshold stimuli arriving at short intervals (intermittent stimuli) can summate, allowing the reaction-triggering threshold value to be exceeded (stimulus summation). The magnitude of the stimulus threshold can undergo changes, for example, under the Influence of Environmental factors (adaptation). For instance, an etiolated seedling reacts much more sensitively to unilateral illumination than to uniform illumination from all sides.

The minimum time span during which a stimulus of a given strength must act to elicit a noticeable reaction is called the "presentation time." Near the stimulus threshold, the law of stimulus quantity applies, according to which the degree of stimulation R is defined as the product of stimulus intensity I and stimulus duration t:

R = It. (8.1)

The time interval from the onset of stimulus action to the recorded beginning of the response is called the reaction time, and the time interval from the cessation of the stimulus to the recorded beginning of the reaction is termed the latency period.

If the intensity of the reaction does not depend on how much the stimulus threshold is exceeded, and thus upon crossing this threshold—regardless of the duration and strength of the stimulus—a full-fledged reaction always ensues (for example, the touch-induced snap-closure of Dionaea leaf halves; see Box 4.4), this is referred to as an "all-or-none" response. Other reactions (such as phototropic ones, see 8.3.1.1) follow the law of stimulus quantity within wide limits.