Chapter 11: The control of growth and development.
Plant growth and development, and reproduction depend on the interaction of environmental factors with genetic makeup, or genotype. These two factors determine what an organism looks like, which is called the phenotype.Objectives
After completing this chapter, you should be able to:
* Explain the difference between phenotype and genotype
* Define growth and development as applied to plants
* Explain developmental biology
* Outline how growth and development are often limited to the environment
* Discuss how a plant's biological clock works
* Describe how reproductive cycles are attuned to biological clocks
* Recognize the primary receptor phytochrome (a protein-pigment)
* List and explain the mode of action of each of the hormones
* Describe plant response to different stimuli (tropisms), such as gravity, light, etc.
* Discuss why some plant movements may be temporary
Key Terms
phenotype
genotype
differentiation
diurnally
cold damage
water stress
water quality
evapotranspiration
duration
intensity
phytochrome
diurnal
seasonal cycles
biological clocks
photoperiodism
Pfr
red light
far-red light
hormones
gene activators
stimulates
inhibitors
endogenous
exogenous
auxins
abscisic acid
ethylene
plant growth regulators
coleoptiles
agar
indoleacetic acid (IAA)
tryptophan
auxin threshold
abscission layer
parthenocarpic fruits
apical dominance
alpha-amylase
aleurone layer
adenine
kinetin
postharvest physiology
controlled atmosphere
epinasty
growth retardants
callus
florigen
geotropism
polarity
thigmotropism
nastic movements
thermonasty
nyctinasty
seismonasty
Principles of Growth and Development
Even if a farmer plants the best quality seed available, the crop will be a failure if sunshine, water, and nutrients are not available in the right amounts at the right time. A light frost or a heat wave could spell disaster. In nature, the phenotype responds to the biotic and abiotic environments. Soil factors, precipitation, and sunlight must be adequate for the plant to complete its growth cycle during the right season. Consider the consequences for a seed that happens to germinate at the beginning of winter. If that particular species were a warm-season annual, the seed would be killed by the frost. Another plant might survive the frost, lie under snow during winter, and complete its life cycle the following spring and summer. Why does one survive and the other does not?
The answer lies in the genotype. The winter annual must have genes that code for protection from the cold. The warm-season annual does not have such genes. Scientists have learned a great deal about the expression of specific genes that are modified by the environment. Even though experiments can be designed to look at specific gene-environment interactions, the phenotype represents the totality of hundreds of thousands of genes, which have been influenced by the environment. Phenotype must be viewed as a summation of gene-gene interaction and gene-environment interaction.
You may have wondered why two seedlings derived from the same parent plant could grow at different rates, flower at different times, and assume shapes that are quite different. There are two basic reasons: variability and environment. Sexual reproduction leads to variability; for example, you do not look exactly like your brother or sister. Even identical twins possess slight differences. The environment also affects genetic makeup. It is certainly possible to identify a single gene as one that gives rise to the synthesis of a particular protein, which may represent an enzyme responsible for catalyzing a particular biochemical reaction. However, the timing of that gene expression, the amount of enzyme produced, and many other factors depend on the environment. Some genes are expressed during periods of light and others in the dark; some genes are turned only when it rains; others are activated with the onset of autumn. All these environmental factors, acting in concert, bring about the gene expression, which is manifested as the phenotype.
The term's growth and development are often used imprecisely. Sometimes the two are even used interchangeably, which is certainly incorrect. Growth refers to an increase in size of volume of a cell, tissue, or organism. It happens because cell division is accompanied by an increase in cell size. One can even refer to growth in populations, that is, an increase in numbers. There are some problems with the definition. A germinating seed imbibes water and therefore increases in volume: Should this uptake of water alone be considered growth of the seed? Probably not, although the uptake of water is also accompanied by many metabolic changes but is not considered growth. Some involve digestion of storage macromolecules; others involve the synthesis of totally new molecules. The clarification, the definition for growth, is usually modified to state an irreversible increase in size or volume. Thus, permanent increases in dry matter are interpreted as growth.
Development is a summation of all the activities leading to change in a cell, tissue, and organism. The life cycle of an organism, for example, is a progressive change from a single fertilized egg, the zygote, through embryonic changes, and finally the changes that accompany full maturity of the organism. Growth, then, is a part of development. Development is an orderly sequence of events dictated by a precise set of genes turning on and off at every stage. Compare the life cycle of an organism with a piano keyboard (the chromosomes). Music is produced only when the correct keys (the genes) are struck in unison and the harmonic effect is pleasing to the ear. The correct sequence of genes has been turned on at the right time. Even a "good" chord must occur at the right time or it fails to integrate the musical composition.
Another important concept is differentiation--the chemical and physical changes associated with the development process. A meristematic cell can become a vessel element. In a short time, that cell changes its metabolism, shape, and wall thickness, and finally the protoplast dies. Thus, while developing, the cell certainly underwent growth and differentiation. The two parts of the system constitute the developmental process.
Developmental biology is the study of how organisms, their cells, and their tissues achieve a final predictable form and function. It embodies genetics physiology, biochemistry, biophysics, and many other disciplines, but gene regulation is the key in development.
Limitations of Growth and Development (Stresses)
Cells, tissues, organs, and organisms do achieve their predictable form and function as the result of the environment exerting its influence on the genome, the entire complement of genes for the species. Many factors combine to give the overall environmental influence, such as temperature, light, and moisture. Even if a particular gene has been turned on, it may not be expressed, or the expression may be highly modified if the environment fails to meet certain limits of tolerance. Those environmental factors outside the normal range of tolerance for a given environmental insult cause stresses from many sources that tend to limit growth and reproduction capacity. With all the new technology, many plant genomes have been mapped. Not only does this tell us about the environmental effects on a particular organism but helps to explain many of the unknowns associated with a genome.
The dry dormant seed lying in the soil has little resemblance to the organism that will spring from the embryo if the proper conditions are met. Seeds carry with them a "backpack" of stored molecules to nurture them in the early stages of germination. The early hours are precarious ones, and even the slightest desiccation at the wrong time may cause death.
Although a few kinds of seed will germinate on the soil surface (some actually require light and must lie on the surface), most seeds require the insulating and protective qualities of the warm, moist soil for germination. If stresses are not imposed and if the seed has imbibed water and oxygen, the machinery of germination is set in motion. Macromolecules of starch, proteins, and oils are broken down, enzymes already present begin to function and new ones are rapidly synthesized, and metabolism gears up quickly. Respiration rates, for example, increase dramatically within a matter of hours, and the embryo begins to undergo mitosis and cell enlargement rapidly. Some cells of the embryo grow faster than others; the radicale is almost always the first organ to emerge. Why the radical grows faster than the plumules is unclear. It is part of the entire development sequence that causes an organism to turn genes on and off at different times during the life cycle. Wheather the genes actually get turned on and off is, again, determined by stress. The mystery is being unraveled but it will still take time because the interactions are complex and the answers elusive. We know that growth and development occur only within certain physiological limits imposed by the environment.
Temperature
Fluctuations in temperature occur diurnally and seasonably, and in some parts of the world the fluctuations are more pronounced than in others. As a general rule, greater seasonal variations occur as one moves toward the poles. Temperature extremes may be modified by factors such as altitude, clouds, relative humidity, ocean currents, and high and low pressure areas. Temperature affects each biochemical reaction in a particular way.
Generally, as the temperature increases within physiological limits--the tolerance range for that species--the rate of the reaction doubles for each 10[degrees]C rise in temperature. These are obvious limits; most biochemical reactions cease at approximately 43[degrees]C because the proteins are denatured and lose function at that point. This means that although the primary structure (amino acid sequences) may remain intact, the secondary, tertiary, and quaternary structures may collapse under several heat conditions. The enzyme then ceases all activity. Some enzymes can resist changes under intense heat. Consider for example the algae that manage to grow and reproduce in hot springs at or near the boiling point of water. Although there are exceptions, lower temperatures do not usually denature enzymes. Cold damage to plants may occur because the rate of the biochemical reactions is so slow that metabolic function is impossible. When temperatures fall below freezing, ice crystals may form and actually pierce membrane and cell walls. Although scientists do not fully understand how perennial plants are able to tolerate extremely low temperatures and survive, some believe the increasing viscosity of the cytoplasm of the cells to be a major factor in preventing ice crystal formation. Plant tissues also tend to dehydrate during the winter, and they rehydrate in the spring just as growth begins. Great gains in physiology and the biochemical makeup of the plant cells are presenting understanding of genes of various organisms and how they allow survival under extreme heat and cold while the genes of another organism fail to be regulated and the organism dies.
Water
Although it certainly is true that many modern cities import water through pipelines or canals for many miles, human migration and settlement has been historically tied to locals where fresh water is found. Although water has been discussed in various contexts in this book, it still deserves special attention as a factor in controlling plant growth (see Figure 11-1). As a medium for the support of all chemical reactions in living tissues, water dominates and controls the rates and combinations of reactions within living cells. Water stress in plant growth presents a major challenge for modern agriculture as growing human populations demand greater productivity. The stress problem is one of degree; each additional increment of water may increase productivity of plants a few percentage points, but the energy cost of water supply determines the amount of supplemental irrigation. Does one get enough in return to justify the additional cost?
[FIGURE 11-1 OMITTED]
In natural ecosystems, the water stress problem is quite different. Here, survival is the key. Productivity may be exceedingly low in some desert ecosystems, but if the species can survive and manage to reproduce, even occasionally, then there is a good chance for relative success under those environmental conditions, provided that some outside force does not intervene. Plant geneticists, physiologists, and biotechnologists are particularly interested in understanding the mechanisms underlying drought tolerance. Drought tolerant plants are now being developed through genetic engineering by using genes from drought tolerant plants and putting them into less tolerant plants to create a drought tolerant plant.
Soils
One of the most serious stresses is imposed by salts, both naturally occurring and artificially applied; plant owners sometimes fertilize too heavily. Even though a little bit is good, a lot is not necessarily better. Fertilizer burn actually comes about because water is pulled out of the plant and back into the soil: The plant cannot take up water fast enough to compensate for losses from the leaf surface.
Salt damage is far more subtle than are the effects of overfertilizing. Damaging salts, often not a part of the essential nutrients, are found in most irrigation water. Such salts contribute to the overall problem of water quality. So-called fresh water used for irrigation contains some salts; even bottled drinking water contains some salts to keep the water from tasting "flat." Deionized and distilled water, on the other hand, have most of the salts removed. In nature, both underground water and surface water acquire dissolved salts, and these are transported along with the irrigation water. When water is applied to plants, it leaves the soil and leaf surface through the process of evapotranspiration, which takes up much of it. As the water molecules escape into the atmosphere, they leave the salts in the soil or in the leaves, where they accumulate unless removed by runoff or leaching below the root zone. Salt accumulation is generally not a problem in zones where adequate rainfall occurs; but in arid and semiarid regions, comprising approximately one-third of the entire land surface of the earth; salt accumulation is a major problem. Rainfall is not great enough to leach salts out of the root zone. These are the same regions to which water is transported for irrigation. The Imperial Alley of California is a good example of prime farmland plagued by salts. The ecological problem is enormous and worsening progressively.
Light
For any novice plant grower, the importance of light becomes apparent very quickly. One of the first questions likely to be asked of a nursery sales person is whether a new houseplant needs a sunny window or shade. Too many new "botanists" decide that the plant is not getting enough light and immediately transfer it from the dark corner to the hottest south window. The change in light might be too drastic for the plant, and bleached or dead leaves might be the consequence.
The process of photosynthesis involves the conversion of light energy into a stored chemical energy as sugars or other organic molecules. So light is absolutely essential for sustained plant growth, but the amount and kind can vary greatly. Plant physiologists are concerned with three factors of light duration, intensity, and quality.
The duration is important in photosynthesis because the conversion process is an accumulative one. For all practical purposes, a plant can accumulate twice as much sugar in 10 hours of sunlight as it can in 5 hours. But there are limits, and the accumulation process may slow during very long periods. The season and distance from the equator dictate the amount of natural sunlight to fall on any given spot, and it is quite predictable for any given date.
In a desirable climate at the equator, every plant has 12 hours of sunlight each day in which to accumulate photosynthesis products; season has little if any effect on the amount of light received. But as one proceeds toward the north and south poles, the effect of this photoperiod becomes greater and greater. At the North Pole during summer, daylight is almost constant, but during the winter there the days are quite short. Few plants are photosynthesizing at that time of the year. The seasons are reversed at the South Pole, and days are longest there when they are shortest at the North Pole.
As expected, this finely synchronized system has important implications for reproduction. Like animals, plants reproduce during advantageous seasons. To be in the process of flowering at the time of the first blizzard or onset of winter could result in death of a particular plant and even extinction for the species. Through the process of natural selection, species have ensured that the onset of reproduction will be triggered at an appropriate time of the year. The photoperiod system is a foolproof way to accomplish this. Its predictability is absolute. If the plant species continues to grow in the same place, the length of day is a certain, and thus ideal. Most plants have developed a mechanism to signal and change the developmental process from strictly vegetative growth to reproductive growth, and still allow time for the reproduction processes to be complete before the environmental conditions change. This process does not depend on the accumulation of a given amount of photosynthesis, storage capacity is certainly important in providing energy for the reproductive process. Instead, a protein pigment called phytochrome represents the trigger itself. More will be said about this remarkable chemical in the latter part of this chapter.
Intensity
Light intensity is important in the growth and development of all plants. Some plants are adapted to a shady, woodland environment, whereas others survive only under sunlight. Light intensity is mediated not only by the shading of other leaves, but also by local weather conditions, including clouds. Sugarcane in Hawaii, for example, is not as productive as agriculturists would like because too many clouds surround the island. As a C4 plant (see Chapter 10), sugarcane is most productive under conditions of full sunlight. Many houseplants fail to survive because of low light intensity, whereas others die from excessively bright locations within the room.
Quality
Light quality is important in plant growth and development primarily because of the absorption spectrum of the chlorophyll and carotenoid molecules. Visible light without red or blue components will undermine the plant growth and possibly inhibit flowering. Most sunshine contains a complete wavelength distribution, but artificial lights tend to be deficient in certain proteins of the visible light. Incandescent lamps produce light primarily in the red and farred portions of the spectrum, whereas fluorescent lamps produce light primarily in the blue region. High-quality environment-control chambers where plant experiments are conducted are equipped with a mixture of both incandescent and fluorescent lamps to stimulate natural sunlight. With the correct mix of lamps, the quality can be duplicated rather well, but it is exceedingly difficult to get the intensity of natural sunlight. A special type of fluorescent lamp supplies more red light and therefore a more balanced spectrum than other fluorescent lamps.
Human Intervention
Each species has its own set of optimum conditions for growth and reproduction. By domesticating certain plants and animals, humans have learned about the environment best suited for those species. On the other hand, we really do not know the subtle requirements for optimum growth and reproduction for many species, even the domesticated ones. Each genome responds to the environment in a specific way. Change one or a few genes and the tolerance will change. One plant may be able to tolerate conditions near the south window very well (many of the succulent desert plants), but others do not have the right combination of genes to tolerate that level of heat and light intensity. Put the cactus in a dark corner and it will not thrive at all, yet a philodendron may grow very well there.
For some plants, one of the great stress factors is the human one. We may starve, overwater, underwater, overfertilize, overheat, freeze, desiccate, pollute, or crowd the plant to death. It certainly is not necessary for every person who ever looks at a plant to have a degree in agronomy, horticulture, or botany, but common sense concerning some of the principles of growth and development of plants can go a long way in making life greener, more pleasing, and rewarding.
ANIMAL HORMONES FROM PLANTS Phytoestrogens are chemicals produced by plants that act like estrogens in animal cells and bodies. They are often trace substances in food. These chemicals mimic and supplement the action of the hormones, estrogen, from the animal or human body. Phytoestrogens are a comparatively recent discovery, and researchers are still exploring the nutritional role of these substances' metabolic functions. Phytoestrogens were first discovered as causing "red clover" disease. This condition causes infertility in sheep that graze on red clover, a plant high in phytoestrogens. Why do plants produce phytoestrogens? They might be a form of plant defense, limiting predation of plant species by causing longterm infertility, increased risk of postnatal mortality, and other adverse reproductive effects on grazing herbivores. In the 1970s, phytoestrogens were first investigated as a treatment for postmenopausal symptoms. Phytoestrogens mainly fall into the class of flavonoids: the coumestans, prenylated flavonoids, and isoflavones are three of the most potent in this class. The best-researched are isoflavones, which are commonly found in soybeans and red clover. Lignan has also been identified as a phytoestrogen, although it is not a flavonoid. The estrogenic properties of these biochemicals have been shown to be due to their structural similarities to the hormone estradiol. Mycoestrogens produced by fungi also have similar structures and effects. According to research, flaxseed contains the highest total phytoestrogen (lignan) content. Isoflavones are found in high concentration in soybeans and soybean products like tofu, whereas lignans are mainly found in flaxseed. Today many over-the-counter products containing phytoestrogens are available to relieve menopausal symptoms.
Cycles
It is certainly not difficult to identify diurnal and seasonal cycles in our own lives. Work schedules, eating habits, body functions, and even psyches (many of us are cantankerous early in the morning!) are affected. A modern ailment is jet lag; rather uncomfortable physical emotional state brought about by passing through several time zones in a short period of time. Seasonal cycles, too, cause us to have changes in moods, and we eat and dress differently.
Plants too are carefully attuned to these biological clocks; the most important of which is the 24-hour clock. Perhaps you have never really thought about why our days are not 16 hours long, or maybe 32, or even 50. Would it make any difference to you if we had 8 hours of darkness year-round? Because of the physical design of our solar system, the earth rotates on its axis once every 24 hours to give us alternating exposure to the sun; at the same time each year, our planet follows an elliptical orbit around the sun. Because of the shape of the orbit, together with the tilt of the earth on its own axis, seasons are dictated for each precise location on the earth. Some other planet in some other solar system might have an entirely different biological clock. Ours is unique to the planet, and living organisms have systems for responding to the days and seasons.
The system certainly is not perfectly predicable. We cannot foretell what the exact temperature and relatively humidity will be on any particular day of the year from now, but we have learned to expect some limits. The plants and animals that thrive at any particular locality do so because their ancestors have survived a rigorous screening over many generations. Those that do not have the correct genes to allow for survival under those conditions are eliminated.
The day/night cycle allows a period of energy accumulation during the daylight hours; at onset, photosynthesis ceases. Daylight allows a plant to "recharge its batteries" and perhaps produce a little extra for storage. That's what agriculture is all about. Too much night, or even too much cloudy weather, could greatly reduce productivity. But is night actually necessary? Could plants photosynthesize in constant light? Experiments in controlled environmental chambers with constant light have shown that some species seem to perform perfectly in constant light, but others do not.
So how do plants use their biological clocks to maximum advantage? The seasonal cue is rather predictable in some localities, but not so predictable in others. Frost kill date and long-term meteorological averages can vary over several weeks. Thus, seasonal cues such as changes in precipitation and temperature are imprecise at best, but the photoperiod cue is absolute. Not all external control mechanisms are cued by day length increases. Nights become warmer, and the cue to the DNA calls for transcribing genes and synthesizing proteins responsible for renewed growth. Soon the buds break, cell division begins in the shoot apex, new leaves are produced, and a stem internode elongation occurs.
How a plant perceives changes in day length apparently has nothing to do with photosynthesis, and therefore chlorophyll is not involved. On the other hand, it is a light cue, so pigment--a molecule that becomes activated or excited--must be responsible.
The first clue about its identity came from two U.S. Department of Agriculture scientists in Beltsville, Maryland, in the 1920s. W. W. Garner and H. A. Allard were studying tobacco, which in Maryland normally flowers in the late summer. One plant in their tobacco field failed to flower and continued growing until frost time. Intrigued by the plant, the scientist took cuttings of it, which they grew in a greenhouse. Various experiments with fertilizers, irrigation, temperature, and other factors failed to induce flowering. Finally, these plants had somehow delayed flowering until winter. Seeds taken from the greenhouse were planted the following season and again failed to flower until winter. It was clear that the factor responsible for the initiation of flowering was the length of the day, all other factors being equal. The plants would simply not flower unless the length of day was shorter than a critical number of hours. Garner and Allard called this phenomenon photoperiodism and went on to work with other plants. Some species would flower only when the length of day was longer than some critical value and response to a change in the proportion of light and dark in a 24-hour day.
These scientists were able to categorize plants as long-day, short-day, and day-neutral type. Long-day plants flower in the summer, short-day plants flower in early spring or fall, and day-neutral plants will flower under a variety of light conditions. The absolute length of the light period is not the most important factor, but whether it is a longer or shorter period than some particular interval. Consider the common cocklebur (Xanthium strumarium), which will flower when the length of the light period is less than 16 hours. Many varieties of poinsettias, as shown in Figure 11-2a, as well as, chrysanthemums, as shown in Figure 11-2b, are also induced to flower when the day length is about 14 hours, or longer. Thus, wheat, as shown in Figure 11-3, is by definition a long day plant, as are many cereal crops.
[FIGURE 11-2a OMITTED]
The photoperiodism is at least partially controlled by a protein pigment called phytochrome. Unlike the other plant pigments, phytochrome is a very large protein molecule capable of existing in two different forms, referred to as Pr (phytochrome red) and Pfr (phytochrome far-red.) The Pr form can be made to change into the Pfr form if a red light of 660 nm wavelength is shown on the pigment in solution. This conversion occurs rapidly, in a matter of seconds or at most minutes. Once converted to the Pfr form, it can be reversed to the Pf form of far-red light if a 730 nm wavelength is shone on the solution. This reversal can go on indefinitely, and the pigment form at the end depends on the last light used to excite the molecule. The system can be dependent graphically.
[FIGURE 11-2b OMITTED]
[FIGURE 11-3 OMITTED]
Pr [right arrow] red light [right arrow] Pfr [right arrow] Far Dark-Red
Note that once the pigment is in the Pfr form, it can gradually revert to the Pr form in darkness, even if no far-red light is present. This change is very slow, and the length of the night is a factor in determining the ratio of Pr to Pfr. In nature, the pigment is primarily in the Pfr form at the end of the day because there is more red light in sunshine than far-red light.
This knowledge of photoperiodism has allowed plant scientists to take advantage of seasonal flowering and modify the flowering process to suit the needs of the grower. For example, when it is known how long it takes to bring chrysanthemum flowers into production after initiation, the length of the day and the date for peak flowering can be precisely determined. This timing is particularly critical for specialized sale, not only in chrysanthemums, but also Easter lilies and Christmas poinsettias.
Further experimentation has shown that the time-measuring drive is not controlled only by the phytochrome. It has not been possible to draw a conclusion about the ratios of Pr to Pfr and explain the phenomenon of long and short day plants.
The latest research on phytochrome is that phytochrome is a 3-D structure of the light detecting protein. Phytochrome is twisted into a molecular knot, an uncommon shape for any protein. Scientists theorize that the knot helps give phytochrome an overall stability as it snaps back and forth between two different forms in response to changes in light color.
Knowing the 3-D structure of phytochromes will allow researchers to determine the specific switching mechanism plants use to respond to light and how nanotechnology may also find a light-activated switch useful as they develop novel microscope devices.
Plant Hormones
Scientist are learning more about chemical messengers called hormones, which are indirectly responsible for much of the control of growth and development. Although it is difficult to demonstrate that a specific hormone can turn a particular gene on or off, the evidence shows that hormones are intimately involved in the regulation process. In animal systems, some hormones are in the gene activators; others clearly are not. It is temping to suggest that some plant hormones are also gene activators, but there is no direct experimental evidence to prove the point. Hormones may be both stimulates and inhibitors, accelerating growth in one tissue and inhibiting it in another. Sometimes the same hormone can perform both functions depending on its concentration.
The regulation of growth and development by all the internal (endogenous) and the external (exogenous) factors are partially due to these chemical messengers (see Figure 11-4). By definition, a hormone is an organic molecule synthesized in very small quantities in one part of an organism, when the molecule exerts some profound physiological effect. You might think the definition sounds suspiciously like that for an enzyme, but enzymes are synthesized within the same cell in which they operate. Hormones may be transported over great distances before they reach a target cell or cells. The definition for plant and animal hormones is the same. Animal hormones tend to be very specific: In the human body, for example, literally hundreds of them have been identified. Plant hormones, on the other hand, tend to be very general, and each kind of hormone may perform many different functions.
[FIGURE 11-4 OMITTED]
Currently, there are five general classes of known plant hormones auxins, gibberellins, cytokinins, abscisic acid, and ethylene. These molecules tend to be rather small, much smaller than the giant proteins that act as enzymes. Most of the plant hormones have molecular weights in the range of 200 to 300, which allows them to move through tissues with relative ease. Some animal proteins do act as hormones, but none are known in plants. The simplest kind of hormone is ethylene: C[H.sub.2] = C[H.sub.2]. Even though it is a very simple gas, it exerts a tremendous physiological influence on plants.
Many chemicals not found naturally in plants exhibit growth-regulating properties similar to those of the hormones. Since they are not synthesized in the plant, they do not meet the true criteria for a plant hormone and therefore are called plant growth regulators. By definition all hormones are plant growth regulators, but not all plant growth regulators are hormones.
Auxin
Although most hormones research has been carried out since 1940, the history of interest in hormones is much older. The earliest recorded observation leading to the discovery of plant hormones is that of Charles Darwin. In 1881, he and his son, Francis, reported on experiments performed with grass and oat coleoptiles in The Power of Movement in Plants. They first described the phenomenon of photoperiodism, the bending of plants toward a unidirectional light source. The Darwins placed molded lead caps on the tiny shoot apex and noted that the shoot tip did not bend; when the lead cap was removed, the plant responded to the light. Their notes recorded that "when seedlings are freely exposed to the lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend" (see Figure 11-5).
[FIGURE 11-5 OMITTED]
Essentially nothing was done about the Darwins' observations during the latter part of the nineteenth century and early part of the twentieth century. Then in 1926, the Dutch plant physiologist Frits W. Went, working in his father's oratory at the University of Utrecht, performed a remarkable set of experiments. Went found that the tip of the coleoptiles could be cut off and placed on a tiny block of agar (the gelatin-like material made from certain marine algae that maintains liquid in a semisolid state at room temperature). After about an hour, the coleoptile tip was removed and the agar block placed on one side of another decapitated oat coleoptile. In a short time, the upper part of the coleoptiles began bending away from the side of the agar block. These substances later diffused out of the block and into the second decapitated oat coleoptile and moved only down the side of the tissue cylinder directly below it, causing the cells to elongate more rapidly than those on the opposite sides of the cylinder.
This differential in the rate of growth on opposite sides of the cylinder of tissue caused the entire cylinder to bend. This experiment was particularly significant because it proved for the first time that the stimulus described by Darwin was a chemical one, rather than physical or electrical. The chemical substance was named auxin, after the Greek auxin, meaning "to increase." Although we now refer to any substance that will cause a similar bending response an auxin, the only naturally occurring one is indoleacetic acid (IAA).
Many chemicals have auxin-like properties, and some, such as 2,4-dichlorophenoxyacetic acid. (2,4-D), are used as herbicides or weed killers.
A low concentration of 2,4-D is far more active than the same concentration of IAA in many plants, and small amounts can cause excessive respiration, excessive cell expansion, and finally tissue death; however 2,4-D is used extensively in plant tissue culture as a growth regulator. A mixture of 2,4-D is closely related to 2,4,5-trichlorophenoxacetic acid (2,4,5-T), which is Agent Orange--used as a defoliant in the Vietnam War.
IAA is synthesized from the common amino acid tryptophan; it is found as a by-product of both fungal and animal metabolism, and a particularly rich source is pregnant horse urine. However, these organisms neither use nor respond to it. Although no hormones are found in truly large concentrations, auxin seems to be concentrated at the site of synthesis in embryos, apical meristems, and in young leaves and fruits--all actively growing tissues. Auxin has directional or polar movement, from the growing tips of the plant toward the base of the plant: Hardly any moves in the other direction. Movement generally occurs through parenchymal cells rather than through vascular tissue.
As with other hormones that exert tremendous influence at low concentrations, it is difficult to predict a response for auxin. It behaves differently as the condition changes; many dicots are more sensitive than monocots, and the root is more sensitive than the shoot. From time to time and place to place, tissue sensitivity changes, apparent that very high concentration of auxin is toxic; hence the use of 2,4-D and 2,4,5-T as weed killers. If extremely low concentrations of 2,4-D are used, it can be a very good synthetic auxin source for stimulation of growth.
Auxin Effects
Unquestionably one of the most important actions of auxin is Darwin's observed effects of cell elongation. Occurring as it does in meristematic tissues, auxin is apparently responsible for the rapid growth and elongation of tissues, and in the shoot apex it diffuses downward and causes the stem to elongate. Young, tender stems are most readily affected; and their cells elongate rapidly. If the shoot apex is removed from the plant, apical auxin supply is eliminated and the shoot stops elongating. If auxin is added to the cut surface, growth resumes. It appears the intact plants already have an optimum amount of auxin being produced in the shoot tips. And extra application generally fails to produce a response. The auxin threshold is apparently saturated under most growth conditions; the same concentration of auxin that causes shoot elongation will cause an inhibition of root growth. This differential is probably due to a difference in tissue sensitivity.
Although leaf growth does not appear to be directly controlled by auxin production, leaves do contain auxin. If for some reason, including seasonal triggers, the level of auxin in the leaf falls, there is a good chance that an abscission layer will form at the base of the petiole. Enzymes form that break down cell walls and cause the petiole to become very weak. The leaf eventually falls from the plant. Of course, this is normal for deciduous trees in the fall, but severe stress during the growing season can also cause premature leaf fall. Such stresses may be induced by drought, flooding, high and low temperatures, light stress, and a number of other factors. Abscission zone also forms at the base of the peduncle and may lead to premature fruit shedding.
One of the important commercial uses of auxin is to stimulate the production of adventitious roots on stem or leaf cuttings. Again, depending on the type of tissue, age, and a number of other factors, various concentrations of auxin may be needed to stimulate lateral roots. In commercial horticulture practices, various synthetic auxins are used not only to speed the rate of production, but also to increase the total number of roots produced.
Auxin is also important in the growth of fruit. Developing seeds are a rich source of auxin, and that source is responsible for the growth of the fruit surrounding the seed. If ovules fail to be fertilized or if for some reason the embryo aborts, the fruit will normally fail to develop. One of the best examples is the strawberry. The achenes are produced on the outer surface to the receptacle, and each one is responsible for a region of growth necessary to produce a normal accessory fruit. If the achenes on one side of the receptacle are removed when the fruit is small, that side of the fruit fails to develop, and an abnormal strawberry is produced. This is apparently why strawberry shape is so varied. It is possible to substitute auxins for achenes and cause a normal fruit to develop. Such fruits that develop without fertilization are called parthenocarpic fruits. Greenhouse tomatoes and cucumbers are often produced in this manner.
Auxin is also responsible for the phenomenon called apical dominance. If the shoot apex is removed, the level of auxin drops in the main stem, and the adjacent lateral buds (at the next lower node) are released from inhibition and begin to grow (see Figure 11-6). Buds at successively lower nodes may also begin to grow, but the influence is greatest at the bud nearest the shoot apex. If auxin is applied to these buds, they fail to grow, proving that they are inhibited by auxin moving basipetally (downward) from the shoot apex. Plants in nature that have one central leader, such as trees with a typical conical shape, are said to have very strong apical dominances. An oak tree, on the other hand, with much branching and no central leader, is said to have weak apical dominance. The ultimate in apical dominance is exhibited by columnar palm trees, as shown in Figure 11-7, which have a single meristem located at the shoot apex. Such trees die if the shoot apex is ever removed.
[FIGURE 11-6 OMITTED]
[FIGURE 11-7 OMITTED]
Mode of Action
Obviously a plant hormone with so many different functions (only a few have been described here) must influence regulation of several different systems. Certainly one of the most important is cell elongation. Detailed studies by many physiologists and biochemists have shown that auxin increases the plasticity or "stretch ability" of the cell wall. Since living cells normally have turgor pressure that allows the plasma membrane to push against the cell wall, if the wall becomes loosened, then the cell can stretch until the wall again becomes a significant barrier. As cells become older, they fail to respond to auxin as do young cells, and the elongation finally becomes impossible. Some rigidity is characteristic of older, mature tissues and is the foundation upon which structural wood is built.
Gibberellins
At approximately the same time that Went was performing his classic experiments with auxin, Japanese scientists were investigating a fungal disease that caused excessive stem elongation, stem weakening, and finally stem collapse and death of the rice plants. The disease was known locally as the bakanae, or crazy-seedling, disease of rice. The organism was finally identified as Gibberella fujikuroi, and the band of active growth compound causing bakanae was named gibberellic acid. Because scientific communication was so poor between the Orient and the Western world at the time, the information was not "discovered" by Westerners until British workers reported in 1955 that minute quantities of gibberellin acid would cause genetically dwarfed pea plants to grow to a normal size. Within a few years, dozens of laboratories were experimenting with gibberellins. In both dwarf pea and dwarf corn, the amount of gibberellin applied was directly proportional to the amount of increased growth up to that for a normal plant. The obvious but incorrect conclusion was that the normal pea and corn plants contain plenty of endogenous (synthesized within the plant body) gibberellin whereas dwarf mutants somehow failed to synthesize the hormone. But dwarf plants have even more gibberellin than do normal plants. Experiments suggest that light represses growth by causing a sensitivity of the tissue to endogenous gibberellin, perhaps through a gibberellin inhibitor in the dwarf plants. Dwarf pea and corn are the standard plants for gibberellin; bioassay is a method of quantitatively determining activity of a substance using a living organism. (Both straight growth and curvature of oat coleoptiles are used as bioassays for auxin).
As one might expect from the original rice disease, gibberellic acid proved to be a growth promoter. There are now more than 50 slightly different compounds, all closely related chemically, classified as gibberellins. Less than half occur naturally, and most have no known biological activity. Gibberellin has been extracted from seeds, young shoot and root tips, and young leaves. Its concentration diminishes greater in older tissues. In the 1950s, botanists found that some plants that flowered only under long days could be induced to flower under short days if given gibberellic acid. This exciting news led to the incorrect conclusion that gibberellin was unequivocally identified as the flowering hormone. It does cause flowering in some species, but not in others.
A good example of how a hormone acts as a chemical messenger is the effect of gibberellin on the germination of barley seed. In the normal germination process, following imbibition of water, various enzymes begin to break down macromolecules. Barley seeds have a large starchy endosperm that must be broken down into sugars to provide energy for the germinating seed. This process is triggered by the synthesis of the enzyme alpha-amylase, which causes starch to be converted in a process that was speeded up dramatically with the addition of gibberellic acid, and later studies showed that the gibberellin caused the production of the enzyme in the aleurone layer, a group of cells surrounding the endosperm. Even if exogenous gibberellin is not applied, the process eventually occurs: The embryo itself produces the gibberellin, which causes the synthesis of the enzyme, eventually breaking down the starch. If the seed is split in half so that the embryo is removed, no alpha-amylase is produced. It is still difficult to prove that the gibberellin is the direct trigger that turns the gene on and off, but the circumstantial evidence is most convincing.
The brewing industry uses gibberellin to speed up the conversion of starch to sugar in barley seeds. In beer brewing, malting barley is used as the sugar source for making alcohol. The malting process is rather slow if seeds are allowed to germinate normally, but if gibberellin is added to the germinating seeds, the conversion to sugar is greatly expedited and the entire process becomes far more efficient. Increased efficiency translates into income for the brewer.
Gibberellin has also been used commercially to increase the fruit size and cluster uniformity in Thompson seedless grapes. The embryo in each of these grapes fails to provide the optimum amount of gibberellin, and if the clusters are sprayed with a dilute concentration of gibberellin very early in fruit development, the ultimate size may be doubled or tripled, and the quality of the product is not usually changed.
Cytokinins
In the 1940s, workers at various laboratories began studying plant tissue culture as a means of isolating specific tissues or organs, removing them from external influence, and then studying nutritional and growth factors one at a time. Although tissues taken from various plants, including tobacco and carrots, grew for a while in culture, the cells eventually stopped dividing. Many culture media were tried, but liquid coconut milk proved to have some factor that immediately caused the cells to divide. After years of attempts at isolation, the factor was finally identified as a derivative of the purine adenine, the same substance found in nucleic acids. The compound was named kinetin, and now the entire class of compound isolated from plants that causes cell division is called cytokinins. Since cytokinins are so effective in promoting cell division, it was thought this compound or a similar one might be responsible for the uncontrolled growth of cancer cells. During the 1960s, a great deal of research effort was devoted to trying to associate cytokinins with tumor development in both plant and animal cells, but no direct connection was ever made. Instead, cytokinins appear to be naturally occurring compounds responsible for the control of the cell division process in meristematic regions. Cytokinins also seem to slow the turnover of proteins in plant tissues, which retard the aging, or senescence, process. For example, if cytokinins are sprayed on lettuce leaves, the storage life is greatly enhanced, and the product can be shelved for long periods. This allows supermarkets to distribute fresh vegetables more efficiently and ultimately buffers price fluctuations in a higher volatile market.
Abscisic Acid
Just as auxins, gibberellins, and cytokinins are generally considered to be promoters of growth, others are generally considered to be inhibitors of growth. They reduce the rate of growth or induce dormancy at critical times, such as the onset of fall and winter. The primary molecule that controls these inhibitory processes is abscisic acid. Originally identified in dormant buds of ash trees and potatoes, it was later found to be the same molecule that caused the abscission of leaves, flowers, and fruits. When days become shorter in the fall and temperatures begin to decrease, levels of abscisic acid gradually rise in the abscission zones of petioles. Leaves fall, and the level of abscisic acid in the dormant buds remains very high throughout the winter. With the onset of spring, levels of abscisic acid decrease at the same time that levels of auxin, gibberellin, and cytokinin begin to rise. Buds are activated and stems begin to grow and develop for the cycle to repeat itself.
Whenever a water stress occurs in leaves, abscisic acid levels rise in the guard cells, which triggers the movement of [K.sup.+] out into adjacent cells, making the solute less concentrated and causing water to move out of the cells. When the pressure decreases in the guard cells, the stomatas close, and transpiration is greatly reduced. Thus, it appears that abscisic acid is intricately involved in plant water stress.
Ethylene
For many years greenhouses were heated with open space heaters. Occasionally an attendant would arrive at work after a cold night to find that all the leaves had fallen off the plants. Various toxic gases were suspected, but many years went by before ethylene was recognized as a major component of exhaust gases and the association made between leaf abscission and ethylene. Even more powerful than abscisic acid, this gas can trigger unusual growth and development responses even in trace amounts. The ripening of fruit involves a number of chemical and anatomical changes, and in the process of ripening storage molecules change to soluble sugars, as in bananas. This is part of the respiratory process and is accompanied by the release of large quantities of ethylene. As bananas ripen naturally, the dark flecks that appear on the peel are pockets of concentrated ethylene. One can increase that concentration and speed the ripening process by enclosing the fruit in a plastic bag so that the ethylene given off by the ripening fruit is concentrated and not allowed to diffuse. Green stalks of bananas are ripened in this manner at the supermarket, or they are placed in a small gassing room and treated with bottled ethylene gas.
The study of postharvest physiology is a major segment of the field of plant science and horticulture. Scientists study the environmental and biochemical factors that influence the storage and ripening process of fruit and vegetables. Some fruits, such as banana and avocado, produce large amounts of ethylene and ripen very suddenly, and this ripening is accompanied by a rapid rise in the rate of respiration of that fruit tissue. This sudden increase in respiration is called the climacteric rise and at one time was the subject of many laboratory studies.
The storage of apples has received particular attention because apples are a major crop and subject to rapid deterioration under improper storage conditions. Postharvest physiologists have learned that if apples are stored in a controlled atmosphere of temperatures just above freezing, high relative humidity, low oxygen, and high carbon dioxide levels, storage life can be increased dramatically. It is now possible to store apples year-round with essentially no reduction in quality. This finding has obvious implications for availability of fresh fruits for the consumer and the stabilization of prices.
Although the effects of ethylene are well documented, some of the effects are so interrelated with auxin that it is difficult to determine which is causing the effects. In addition to its effects on fruit ripening, ethylene causes leaves to abscise, chlorophyll to leach, flower pigments to fade prematurely, and leaf petioles to grow more rapidly on the upper side and therefore curve down. The petioles response to ethylene is called epinasty and should not be confused with the phototropism attributed to auxin.
Growth Retardants
Many commercial growth regulators are advertised as growth retardants and are sold for the purpose of reducing pruning or mowing costs, decreasing stem length in flowering plants such as azaleas and poinsettias, and decreasing biomass production to reduce fire hazards. It appears that several of these compounds act by blocking the synthesis of gibberellin. Others may act as antiauxins, and in fact several possibilities exist for counteracting the effects of hormones.
Hormone Interactions
It is probably apparent by now that hormone action is rather complicated. Change the concentration and the promoter may become an inhibitor. Both auxin and gibberellin act on cell elongation; both ethylene and abscisic acid cause abscission. It is important to remember that hormones never act alone in any tissue, and the bottom line of growth and development depends on the relative concentrations of all the hormones acting in concert to produce the final product. Tissue culture experiments have revealed a great deal about the interaction of auxin, gibberellin, and cytokinin (see Figure 11-8). If the ratio is changed slightly, one may get callus (undifferentiated tissue); change the ratio again, and only roots may be produced; and if the ratio is just right, both roots and shoots may be produced, eventually leading to a complete new plant from clonal tissue. Many exciting results have been obtained in understanding how the chemical messengers do their work.
[FIGURE 11-8 OMITTED]
The Flowering Hormone
Thousands of hours of research have gone into the search for the flowering hormone. Various laboratories have isolated an extract that will cause a vegetative shoot apex to become reproductive and produce flowers, but so far no one has ever chemically identified the compound. The floral stimulus is obviously transported from leaves to the shoot apex. For example, it is possible to take a short-day plant such as cocklebur (Xanthium) and place it under long-day conditions so that the plant remains entirely vegetative. If a single leaf is enclosed by a dark box so that it receives only short-day light, the plant will flower. In addition, if a cocklebur plant is growing under long-day conditions and if a leaf from a flowering short-day plant is grafted onto the vegetative plant, the chemical stimulus is transferred to the apex and the noninduced plant will flower. This elusive chemical stimulus has been given the name of florigen, although nothing is known about its true chemical nature or structure. It seems particularly ironic that, in the age of modern miracles and molecular biology, such a compound could not be isolated, but scientists are coming closer to the conclusion that florigen may be a combination of substances perhaps including gibberellin and other hormones.
Plant Movements
You have already learned that some plant tissues have organs that are capable of modifying their rate of growth so that there is a change in direction. Darwin's experiments with the oat coleoptile showed that cells on the dark side of the stem elongate more rapidly than those on the light side. Went later demonstrated that the auxin levels are higher in the cells that elongate more rapidly, and even if the rate of cell division is unchanged, the cylinder of tissue will curve toward the light. An explanation of this phenomenon is that light causes a migration of auxin molecules from the light side to the dark side of the stem. It has been shown that seedlings kept in constant darkness elongate much more rapidly than seedlings grown in light (from the time of emergence from the soil). Such plants are said to be etiolated, and they have tiny underdeveloped leaves and weak, pale internodes. If stem tissues are analyzed, the etiolated (dark-grown and spindly) seedlings contain much more auxin. Etiolation in the dark helps the seedling to force its way through the soil before expending all its energy. Plants in shaded conditions tend to elongate faster than bright light-grown plants, which allows them to compete more effectively for the light.
Plant leaves and flowers also move a great deal, often exactly tracking the sun. For instance, sunflowers, as shown in Figure 11-9, track the sun from early morning to sunset, and leaves of many other plants do the same. This allows the plant to readjust constantly so that only perpendicular rays from the sun hit the leaf surface, and therefore the photosynthetic efficiency is increased.
[FIGURE 11-9 OMITTED]
Geotropism
The earth's gravitational pull may cause growth responses. Roots grow down into the soil (positive geotropism), and shoots grow upward from the ground (negative geotropism). All these tissues start out from the same embryo, originally consisting of only one cell. As soon as the first division of the fertilized egg occurs, a polarity is established, and it is possible to determine which direction is heads and which is tails, that is, shoot and root. Tissues in the same organism react in its tails, this is shoot and root. Tissues in the same organism react positively at one end and negatively at the other and one theory suggests differential tissue sensitivity to certain hormones, powerhouse auxin. Gravity may cause a higher concentration of auxin on the lower side of the tissue than on the upper side. Thus, it is possible for a given auxin concentration to accumulate on the lower side of the plant embryo and stimulate those cells to elongate faster. That tissue would curve upward and become the shoot. At the other end of the organism, the same concentration of auxin might be inhibitory to the cells and slow the rate of elongation. Thus, cells on the upper surface would grow more rapidly, and the tissue would curve downward and become the root.
Thigmotropism
Thigmotropism is a word derived from the Greek thigma ("touch") and refers to directional growth caused by plants touching a solid object. This phenomenon is often exhibited by climbing plants such as English ivy, which send out aerial roots when a shoot comes in contact with a wall, as shown in Figure 11-10. The roots attach the plant to the surface so that it may continue to climb upward. Hormones are undoubtedly involved in the contact response and subsequent differentiation of tissues.
[FIGURE 11-10 OMITTED]
Nastic Movements
Nastic movements are those made in response to some stimulus but are not oriented relative to the direction of the stimulus. Some of these movements are reversible and do not involve true growth. Others are true growth responses, as in the epinastic responses described for ethylene, and the changes are relatively permanent. Other nastic responses include thermonasty, as in the opening and closing response of tulips due to fluctuations in temperature; nyctinasty, the so-called sleep movements of leaves due to changes of turgor of certain cells located at the base of the petiole; and seismonasty, a response to shaking or some other mechanical disturbance.
The sensitive plant (Mimosa pudica) and the Venus flytrap (Dionaea muscipula), shown in Figure 11-11, both display leaf movements not related to differential growth (the response is much too rapid). They are due instead to turgor movements. Large, turgid cells act as a hinge for these organs. When the trigger is released, an electrical impulse is transmitted instantaneously to these storage cells, membrane properties are changed, and the water pressure is released. After a period of time, the water pressure is regained and the trap is reset.
[FIGURE 11-11 OMITTED]
Summary
1. The genetic composition of a plant-the genotype-is modified by the environment to produce a phenotype, the physical and chemical features that characterize the plant. The genotype specifies the potential characteristics of the organism, but whether they are in fact expressed depends on the type of environment.
2. Growth is denied as an irreversible increase in size or volume. Development is a summation of all the activities leading to changes in a cell, tissue, organ, or organism. Development is an orderly sequence of events dictated by a precise set of genes turning on and off at every stage. Differentiation comprises the chemical and physical changes associated with the development process.
3. Developmental biology is the study of how organisms, their cells, and their tissues achieve a final predictable form and function.
4. Growth and development are often limited by environmental extremes that suppress genetic function. Temperature, water, salt, light, and other factors combine to limit gene expression by influencing the turning on and turning off of specific genes.
5. Plants are carefully tuned to biological clocks, the most important being the 24-hour and the day/night cycles. Seasonal changes provide environmental signals to make certain changes in growth and development. Reproductive cycles are usually attuned to these environmental signals to make certain changes in growth and development. Reproductive cycles are usually attuned to these environmental cues and the flowering process in many plants due to photoperiodism. The primary receptor in this system is a protein-pigment that senses a change in the day length.
6. The primary plant hormones are auxin, gibberllic acid, cytokinin, abscisic acid, and ethylene. They act as stimulators and inhibitors of growth by directly or indirectly influencing gene regulation.
7. Plants respond to a unidirectional light source by compensatory growth, causing the cells on the dark side to grow more rapidly than those on the light side. The curving response occurs because of change of growth rate of localized cells, but the change in growth rate is brought about by a migration of auxin from the light side to the dark side. This bending response to light is called phototropism, and other tropisms include a response to gravity (geotropism) and a response to touch (thigmotropism).
8. Other plant movements may be temporary and occur in response to a change in membrane properties so that the turgor in certain target cells changes rapidly.
Something to Think About
1. Compare phenotype and genotype.
2. Define growth.
3. What is the difference in growth and development?
4. What is development biology the study of?
5. List the limiting factors in growth and development.
6. Explain plant biological clocks and what they control.
7. Name the primary plant hormones and tell which are promoters and inhibitors.
8. Discuss tropisms.
9. List the nastic movements of plants.
10. How do these movements work?
Suggested Readings
Fosket, D. E. 1994. Plant growth and development. San Diego: Academic Press.
Larcher, W., E. Huber-Sannwald, and J. Wiese. 1997. Physiological plant ecology (3rd ed.). New York: Springer.
Raghavan, V. 2007. Development biology of flowering plants. New York: Springer.
Taiz, L., and E. Zeiger. 2006. Plant physiology (4th ed.). Sunderland, MA: Sinauer Associates, Inc.
Internet
Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, and MSN Live Search, find more information by searching for these words or phrases: plant growth and development, photoperiod, phytochrome, plant biological clocks, short-day plants, long-day plants, day neutral plants, plant hormones, auxin, IAA, parthenocarpic fruit, apical, gibberellins, bioassay, cytokinins, abscisic acid, ethylene, postharvest physiology, flowering hormone, tropisms, and nastic movements.
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| Title Annotation: | PART 3: Function and Control |
|---|---|
| Publication: | Fundamentals of Plant Science |
| Date: | Jan 1, 2009 |
| Words: | 10365 |
| Previous Article: | Chapter 10: Energy conversions. |
| Next Article: | Chapter 12: Sexual reproduction and inheritance. |
| Topics: | |