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Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of .
Plant physiologists study fundamental processes of plants, such as photosynthesis, respiration, plant nutrition, plant hormone functions, , nastic movements, photoperiodism, photomorphogenesis, circadian rhythms, environmental stress physiology, seed germination, dormancy and function and transpiration. Plant physiology interacts with the fields of plant morphology (structure of plants), plant ecology (interactions with the environment), phytochemistry (biochemistry of plants), cell biology, genetics, biophysics and molecular biology.
First, the study of phytochemistry (plant chemistry) is included within the domain of plant physiology. To function and survive, plants produce a wide array of chemical compounds not found in other organisms. Photosynthesis requires a large array of , , and other compounds to function. Because they cannot move, plants must also defend themselves chemically from , and competition from other plants. They do this by producing and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract or herbivores to spread ripe seeds.
Secondly, plant physiology includes the study of biological and chemical processes of individual plant cells. Plant cells have a number of features that distinguish them from cells of , and which lead to major differences in the way that plant life behaves and responds differently from animal life. For example, plant cells have a cell wall which maintains the shape of plant cells. Plant cells also contain chlorophyll, a chemical compound that interacts with light in a way that enables plants to manufacture their own nutrients rather than consuming other living things as animals do.
Thirdly, plant physiology deals with interactions between cells, tissues, and organs within a plant. Different cells and tissues are physically and chemically specialized to perform different functions. and function to anchor the plant and acquire minerals in the soil. Leaf catch light in order to manufacture nutrients. For both of these organs to remain living, minerals that the roots acquire must be transported to the leaves, and the nutrients manufactured in the leaves must be transported to the roots. Plants have developed a number of ways to achieve this transport, such as vascular tissue, and the functioning of the various modes of transport is studied by plant physiologists.
Fourthly, plant physiologists study the ways that plants control or regulate internal functions. Like animals, plants produce chemicals called plant hormone which are produced in one part of the plant to signal cells in another part of the plant to respond. Many bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism. The ripening of fruit and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant.
Finally, plant physiology includes the study of plant response to environmental conditions and their variation, a field known as environmental physiology. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.
Despite this underlying similarity, plants produce a vast array of chemical compounds with unique properties which they use to cope with their environment. are used by plants to absorb or detect light, and are extracted by humans for use in . Other plant products may be used for the manufacture of commercially important rubber or biofuel. Perhaps the most celebrated compounds from plants are those with pharmacological activity, such as salicylic acid from which aspirin is made, morphine, and digoxin. Drug company spend billions of dollars each year researching plant compounds for potential medicinal benefits.
The following tables list chemical element nutrients essential to plants. Uses within plants are generalized.
| + Macronutrients – necessary in large quantities |
| Notes |
| Nucleic acids, proteins, hormones, etc. |
| Cellulose, starch, other organic compounds |
| Cellulose, starch, other organic compounds |
| Cellulose, starch, other organic compounds |
| Cofactor in protein synthesis, water balance, etc. |
| Membrane synthesis and stabilization |
| Element essential for chlorophyll |
| Nucleic acids, phospholipids, ATP |
| Constituent of proteins |
| + Micronutrients – necessary in small quantities |
| Notes |
| Photosystem II and stomata function |
| Chlorophyll formation and nitrogen fixation |
| Crosslinking pectin |
| Activity of some enzymes and photosystem II |
| Involved in the synthesis of enzymes and chlorophyll |
| Enzymes for lignin synthesis |
| Nitrogen fixation, reduction of nitrates |
| Enzymatic cofactor in the metabolism of nitrogen compounds |
Chlorophyll is the primary pigment in plants; it is a porphyrin that absorbs red and blue wavelengths of light while reflecting green. It is the presence and relative abundance of chlorophyll that gives plants their green color. All land plants and possess two forms of this pigment: chlorophyll a and chlorophyll b. , , and other photosynthetic contain chlorophyll c instead of b, red algae possess chlorophyll a. All chlorophylls serve as the primary means plants use to intercept light to fuel photosynthesis.
are red, orange, or yellow . They function as accessory pigments in plants, helping to fuel photosynthesis by gathering wavelengths of light not readily absorbed by chlorophyll. The most familiar carotenoids are carotene (an orange pigment found in ), lutein (a yellow pigment found in fruits and vegetables), and lycopene (the red pigment responsible for the color of ). Carotenoids have been shown to act as and to promote healthy eyesight in humans.
(literally "flower blue") are solubility flavonoid pigments that appear red to blue, according to pH. They occur in all tissues of higher plants, providing color in leaf, plant stem, , , and , though not always in sufficient quantities to be noticeable. Anthocyanins are most visible in the of flowers, where they may make up as much as 30% of the dry weight of the tissue. They are also responsible for the purple color seen on the underside of tropical shade plants such as Tradescantia zebrina. In these plants, the anthocyanin catches light that has passed through the leaf and reflects it back towards regions bearing chlorophyll, in order to maximize the use of available light
are red or yellow pigments. Like anthocyanins they are water-soluble, but unlike anthocyanins they are indole-derived compounds synthesized from tyrosine. This class of pigments is found only in the Caryophyllales (including cactus and amaranth), and never co-occur in plants with anthocyanins. Betalains are responsible for the deep red color of , and are used commercially as food-coloring agents. Plant physiologists are uncertain of the function that betalains have in plants which possess them, but there is some preliminary evidence that they may have fungicidal properties.
Plant hormones are that in small amounts promote and influence the cell growth, morphogenesis and differentiation of cells and tissues. Hormones are vital to plant growth; affecting processes in plants from flowering to seed development, dormancy, and germination. They regulate which tissues grow upwards and which grow downwards, leaf formation and stem growth, fruit development and ripening, as well as leaf abscission and even plant death.
The most important plant hormones are abscissic acid (ABA), , ethylene, , and , though there are many other substances that serve to regulate plant physiology.
Plants use four kinds of photoreceptors: phytochrome, cryptochrome, a UV-B photoreceptor, and protochlorophyllide a. The first two of these, phytochrome and cryptochrome, are photoreceptor proteins, complex molecular structures formed by joining a protein with a light-sensitive pigment. Cryptochrome is also known as the UV-A photoreceptor, because it absorbs ultraviolet light in the long wave "A" region. The UV-B receptor is one or more compounds not yet identified with certainty, though some evidence suggests carotene or riboflavin as candidates. Protochlorophyllide a, as its name suggests, is a chemical precursor of chlorophyll.
The most studied of the photoreceptors in plants is phytochrome. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of based on the length of day and night (photoperiodism) and to set circadian rhythms. It also regulates other responses including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.
Although a short day plant cannot flower during the long days of summer, it is not actually the period of light exposure that limits flowering. Rather, a short day plant requires a minimal length of uninterrupted darkness in each 24-hour period (a short daylength) before floral development can begin. It has been determined experimentally that a short day plant (long night) does not flower if a flash of phytochrome activating light is used on the plant during the night.
Plants make use of the phytochrome system to sense day length or photoperiod. This fact is utilized by and greenhouse gardeners to control and even induce flowering out of season, such as the poinsettia ( Euphorbia pulcherrima).
Environmental physiologists examine plant response to physical factors such as radiation (including light and ultraviolet radiation), temperature, fire, and wind. Of particular importance are water relations (which can be measured with the Pressure bomb) and the stress of drought or flood, exchange of gases with the atmosphere, as well as the cycling of nutrients such as nitrogen and carbon.
Environmental physiologists also examine plant response to biological factors. This includes not only negative interactions, such as competition, herbivory, disease and parasitism, but also positive interactions, such as mutualism and pollination.
While plants, as living beings, can perceive and communicate physical stimuli and damage, they do not feel pain as members of the Animal do simply because of the lack of any pain receptors, nerves, or a brain, and, by extension, lack of consciousness. Many plants are known to perceive and respond to mechanical stimuli at a cellular level, and some plants such as the venus flytrap or Mimosa pudica, are known for their "obvious sensory abilities". Nevertheless, the plant kingdom as a whole do not feel pain notwithstanding their abilities to respond to sunlight, gravity, wind, and any external stimuli such as insect bites, since they lack any nervous system. The primary reason for this is that, unlike the members of the Animal whose evolutionary successes and failures are shaped by suffering, the evolution of plants are simply shaped by life and death.
in plants are the result of differential cell growth, in which the cells on one side of the plant elongates more than those on the other side, causing the part to bend toward the side with less growth. Among the common tropisms seen in plants is phototropism, the bending of the plant toward a source of light. Phototropism allows the plant to maximize light exposure in plants which require additional light for photosynthesis, or to minimize it in plants subjected to intense light and heat. Geotropism allows the roots of a plant to determine the direction of gravity and grow downwards. Tropisms generally result from an interaction between the environment and production of one or more plant hormones.
results from differential cell growth (e.g. epinasty and hiponasty), or from changes in turgor pressure within plant tissues (e.g., nyctinasty), which may occur rapidly. A familiar example is thigmonasty (response to touch) in the Venus fly trap, a carnivorous plant. The traps consist of modified leaf blades which bear sensitive trigger hairs. When the hairs are touched by an insect or other animal, the leaf folds shut. This mechanism allows the plant to trap and digest small insects for additional nutrients. Although the trap is rapidly shut by changes in internal cell pressures, the leaf must grow slowly to reset for a second opportunity to trap insects.Adrian Charles Slack (1980). 9780262191869, Cambridge, Massachusetts : MIT Press. ISBN 9780262191869
Because the biology of plants differs with animals, their symptoms and responses are quite different. In some cases, a plant can simply shed infected leaves or flowers to prevent the spread of disease, in a process called abscission. Most animals do not have this option as a means of controlling disease. Plant diseases organisms themselves also differ from those causing disease in animals because plants cannot usually spread infection through casual physical contact. Plant tend to spread via or are carried by animal vectors.
One of the most important advances in the control of plant disease was the discovery of Bordeaux mixture in the nineteenth century. The mixture is the first known fungicide and is a combination of copper sulfate and lime. Application of the mixture served to inhibit the growth of downy mildew that threatened to seriously damage the France wine industry.
Stephen Hales is considered the Father of Plant Physiology for the many experiments in the 1727 book, Vegetable Staticks;Hales, Stephen. 1727. Vegetable Staticks http://www.illustratedgarden.org/mobot/rarebooks/title.asp?relation=QK711H341727 though Julius von Sachs unified the pieces of plant physiology and put them together as a discipline. His Lehrbuch der Botanik was the plant physiology bible of its time.
Researchers discovered in the 1800s that plants absorb essential mineral nutrients as inorganic ions in water. In natural conditions, soil acts as a mineral nutrient reservoir but the soil itself is not essential to plant growth. When the mineral nutrients in the soil are dissolved in water, plant roots absorb nutrients readily, soil is no longer required for the plant to thrive. This observation is the basis for hydroponics, the growing of plants in a water solution rather than soil, which has become a standard technique in biological research, teaching lab exercises, crop production and as a hobby.
Crop physiology steps back and looks at a field of plants as a whole, rather than looking at each plant individually. Crop physiology looks at how plants respond to each other and how to maximize results like food production through determining things like optimal plant density.
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