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Chapter 9 plants and their environment.

ABSTRACT

The abiotic atmospheric environment, including temperature, moisture, light, and wind, affects plant growth and development. The biotic atmospheric environment also affects plants. The abiotic edaphic environment includes water movement into the soil, soil water availability, soil aeration, soil pH, and saline soils. The biotic edaphic environment is filled with a variety of organisms; this chapter covers the four main categories found in this region, including microorganisms, arthropods, nonarthropods, and vertebrate animals, and their influence on plant growth and development.

Objectives

After reading this chapter, you should be able to

* recognize how abiotic and biotic atmospheric factors affect plant growth and development.

* understand how abiotic and biotic edaphic environmental factors influence plant growth and development.

Key Terms

abiotic

arthropods

atmospheric environment

available water

biotic

climate

cuticle

day-neutral plants

ECe number

edaphic environment

epinasty

etiolation

evapotranspiration

facultative fungi

field moisture capacity

fungi

gravitational water

hardened-off

hardpan

hardy plant

humidity

light intensity

light quality

long-day plants

mutualism

nematodes

nonarthropods

obligatory fungi

parasitic fungi

pathogen

pathogenic

photoperiod

plant environment

plant selection

relative humidity

saprophytic fungi

saturated soil

short-day plants

soil aeration

soil pH

soil salinity

tender plant

thatch

transpiration

turgid

water stress

wear

weather

wilting point

INTRODUCTION

Plants grow well only when they get what they need from the environment. Horticulture involves growing plants at times and in places that are not natural for the plant; for example, growing poinsettias for Christmas or lilies for Easter in Pennsylvania. Orchids grow outdoors in Hawaii but not in Pennsylvania, unless they are grown in greenhouses. Therefore, understanding the plant responses to the environment enables horticulturists to better create an artificial environment to grow them.

[FIGURE 9-1 OMITTED]

The plant environment is the above- and below-ground surroundings of the plant, which has a profound effect on the plant (Figure 9-1). The two main areas of the plant environment are the atmospheric environment, which is the above-ground portion of a terrestrial plant's environment, and the edaphic environment, which is the soil and area where plant roots are located. Both the atmospheric and edaphic environment can be broken down into the abiotic (nonliving factors) and biotic (living factors) that affect the environment.

When discussing the atmospheric environment, you must understand the difference between weather and climate. Weather is the combined effect of complex interactions among temperature, rainfall, wind, light, and relative humidity in a specific location. Weather factors that form patterns on a daily, weekly, monthly, and yearly basis and create the same pattern year after year are the climate.

Atmospheric conditions affecting plant growth are both abiotic and biotic. Abiotic factors that affect atmospheric conditions are light, temperature, air, moisture, and wind. The main source of light is from the sun. The three important components of light are light intensity, which is the actual quantity of light; light quality, which is the actual color or wavelength of light; and photoperiod, which is the length of the dark period that influences plant growth. Temperature is also an important abiotic factor because it determines when and where a given type of plant can be grown.

When discussing important abiotic factors that affect plant growth, air does not immediately come to mind; however, without air there would be no plant growth. The main components of air are nitrogen, oxygen, and carbon dioxide. Oxygen is required for almost all normal physiological processes in plants. Carbon dioxide is required for photosynthesis. Good-quality air must be available to maximize growth because pollutants such as ozone, sulfur dioxide, and others cause major reductions in plant growth and development.

Another important abiotic factor to consider is moisture, which is found in the air (in the form of humidity) and in the soil or growth medium (in the form of water). Water is used to keep plants turgid, which is when cells are full of water; anytime the plant is subjected to water stress, the overall potential yield of the plant is reduced. Water is also used by the plant to cool itself through the process of transpiration, which is the loss of water from the plant through the leaves in the form of vapor. Plants should be well watered with good-quality water because polluted water can cause a variety of problems leading to a reduction in crop yields.

Wind is another abiotic factor affecting plant growth by cooling the plant, reducing moisture, and reducing disease problems. In addition, wind causes plants to be smaller and more compact with a tougher cuticle, which is an impermeable, waxy material on the outside layer of leaves and stems that prevents water loss, thereby making the plant more resistant to numerous stresses. Wind can also be detrimental to plant growth by reducing plant size and subsequent crop yields or by reducing moisture to a point that causes water stress.

Biotic factors also have a profound effect on plant growth and development. Biotic factors affecting the atmospheric environment include insects and related pests; nematodes; diseases, such as viruses, fungi, and bacteria; weeds; and rodents; and other animals.

The edaphic environment, which is the soil and area where plant roots are located, can also be affected by abiotic and biotic conditions. Abiotic factors discussed in this chapter are water movement into the soil, soil water availability, soil aeration, soil temperature, soil pH, and soil salinity. Following irrigation or rainfall, water must move into the soil or waterlogging or drought can occur. Water movement into the soil is affected by the soil texture, soil structure, thatch, hardpans, and the presence of different layers in the soil together with a variety of other factors. Soil water availability is another important factor influencing plant growth and development. The soil must retain an adequate supply of available water to prevent water stress. For normal physiological processes to occur in plant roots, soils must be aerated to maintain the proper balance of [O.sub.2] and C[O.sub.2] in the soil. Soil aeration is also critical to a variety of microorganisms necessary for good soils. Proper soil temperature must also be maintained because it regulates all chemical reactions in the plant and the soil. Soil temperature is affected by the type of soil (sandy or clay) as well as atmospheric conditions such as air temperature, wind, and solar radiation. Because nutrient availability to the plant from the soil is affected by the pH of the soil, the pH range should be between 6.0 and 8.0, which is the optimal range for plant growth. Soil salinity, which is the amount of salt in the soil, is problematic in agriculture because it reduces crop yields in a number of important crops.

Biotic factors also have a profound effect on the edaphic environment. Soil organisms can be broken down into four groups: microorganisms, such as fungi and bacteria; arthropod animals, such as centipedes, ants, and grubs; nonarthropod animals, such as nematodes and earthworms; and vertebrate animals, such as mice and gophers. More details on biotic factors that affect the edaphic environment will be discussed in Chapter 12.

PLANT ENVIRONMENT

Plant growth and development is first affected by the plant's genetics; however, the plant environment, which is the above- and below-ground surroundings of the plant, also profoundly affects the plant. In fact, genetically identical plants can exhibit a dramatically different appearance when grown under different environmental conditions. Therefore, a grower must be knowledgeable in all aspects of the plant's environment to maximize plant growth and development for high yields and quality crops. The two main areas of the plant environment are the atmospheric environment, which is the above-ground portion of a terrestrial plant's environment, and the edaphic environment, which is the soil and area where plant roots are located. Both the atmospheric and edaphic environment can be broken down into the abiotic (nonliving factors) and biotic (living factors).

Atmospheric Environment

Climate and weather, although linked, are not the same. Weather is the combined effect of complex interactions among temperature, rainfall, wind, light, and relative humidity in a specific location. Weather factors have changing patterns on a daily, weekly, monthly, and yearly basis. The same pattern weather year after year is the climate. The sun influences atmospheric conditions, including temperature, moisture, light, and wind; these conditions, in turn, affect plant growth (Figure 9-2). Atmospheric conditions that have a profound effect on plant growth are discussed next.

Abiotic Factors

Abiotic factors are nonliving factors that affect the atmospheric environment, such as light, temperature, air, moisture, and wind.

Light. The main source of light for all plants is the sun. When discussing light, three components must be taken into consideration: light intensity, light quality, and photoperiod. Light intensity is the actual quantity of light. The highest light intensity occurs at noon. The maximum amount of light at this time is 2,000 [micro]E [m.sup.-2] [s.sup.-1] or 10,000 foot candles of light. The actual amount of light reaching the plant is affected by a variety of factors, such as clouds, trees, or buildings that cause shading. After the light reaches the leaf surface, it can be absorbed or reflected, so only a small amount of light is used in photosynthesis. Light intensity has a profound effect on plant growth. At high light intensities, plants are typically shorter, appear dark green, and have brittle leaves. Plants grown under low light, however, are tall as a result of etiolation, which is exaggerated growth of the stem caused by low light levels (Figure 9-3). The naturally occurring plant hormone indole-3-acetic acid (IAA) plays a key role in etiolation-type growth in plants.

[FIGURE 9-2 OMITTED]

[FIGURE 9-3 OMITTED]

Light quality is the actual color or wavelength of light. Plants absorb light to be used for photosynthesis in the visible portion of the spectrum, which is between 390 and 750 nanometers (Figure 9-4). Most of the light reaching the surface of Earth is in the visible light range. In this range, the three major types of plant pigments--chlorophylls, carotenoids, and phycobilins--absorb specific wavelengths and are involved in photosynthesis. The light range in which a green leaf absorbs light in the visible spectrum is in the blue (approx. 430 nm) and red (approx. 540 nm) regions. The green leaf absorbs light the poorest in the green region (approx. 540 nm) of the spectrum, thereby reflecting mostly green light and making the leaf look green to the naked eye. Light quality helps determine the height of a plant; for example, red/yellow light promotes elongation, whereas green/blue light inhibits elongation and promotes shorter plants. When supplying supplemental light, you must know the plant's requirement for light quality and quantity due to the wide variety of light sources available.

[FIGURE 9-4 OMITTED]

Photoperiod is the length of the dark period that influences plant growth. The plant photoperiod changes on a seasonal basis; in the fall, photoperiods are shortened, and in the spring, they become longer. Plants can be divided into three groups based on their photoperiod: short-day, long-day, and day-neutral. Short-day plants are plants that flower only when the dark period is greater than a certain critical length, because plants measure the length of the dark period in order to flower. Examples of short-day plants are shown in Table 9-1. Long-day plants only flower when the dark period is shorter than a certain critical length. Examples of long-day plants are shown in Table 9-2. Day-neutral plants determine flower initiation solely by the genotype and have no specific light requirement. Examples of day-neutral plants are shown in Table 9-3.

Temperature. Temperature is important for all physiological processes in plants. It determines when and where a given type of plant can be grown. Prior to planting, you should know the temperature extremes in your particular location. The U.S. Department of Agriculture (USDA) Plant Hardiness Map shows regions of the United States where plants can and cannot be grown. This map gives the range of average annual minimum temperatures for 11 zones throughout the United States. Although an excellent starting point for deciding what plants can be planted in your area, this should not be your only source of information. Check with your local weather authorities to determine the temperature extremes in your specific location.

Plants vary in their temperature requirements. Hardy plants are less sensitive to temperature extremes, whereas tender plants cannot tolerate cool weather. Examples of hardy and tender plants are shown in Table 9-4 and Table 9-5, respectively. Damage to the plant by adverse temperatures depends on the physiological state of the plant. For example, actively growing succulent plants with flower buds are more susceptible to frost damage than those in a dormant state. To improve the chances of survival of both hardy and tender plants, the plant should be hardened-off, or gradually subjected to cooler temperatures with less frequent watering.

Air. The main components of air are nitrogen, oxygen, and carbon dioxide. For normal physiological processes to occur in the plant, oxygen is required. Carbon dioxide is required for photosynthesis. When either oxygen or carbon dioxide is limited, plant growth is adversely affected. Another important function of the air is to hold water. The water content of the air is called humidity. A psychrometer is an instrument used to measure the relative humidity. Relative humidity is the ratio of the weight of water vapor in a given quantity of air to the total weight of water vapor that a quantity of air can hold at a given temperature (expressed as a percentage).

Good air quality is important to maximize plant growth. Environmental pollutants are becoming a major problem especially in heavily populated or high industry areas. Pollutants that cause significant reductions in plant growth and development are ozone, sulfur dioxide, peroxyacetyl nitrate (PAN) fluoride, and ethylene, plus a variety of pesticides, heavy metals, and others.

Moisture. Water is the most important requirement for plant growth. Water in the atmosphere comes from precipitation and evaporation. Plants use water for all biochemical and physiological processes. Water is required for photosynthesis and normal growth of the plant; when water is limited, photosynthesis is reduced, which leads to a reduction in plant growth and normal development. The plant cools itself during hot weather via transpiration, which is the loss of water from the plant through the leaves in the form of vapor. Nutrients and organic compounds are transported throughout the plant in water; at water deficits, this transport is dramatically reduced. Water keeps cells turgid, which means plant cells are full of water. Water stress occurs when a plant is unable to absorb an adequate amount of water to replace that lost by transpiration. When plants lose turgidity, they wilt; each time the plant wilts, its overall potential yield is decreased. Excessive water also kills the plant because oxygen in the roots is reduced.

Environmental pollutants cause many problems in agriculture. Pesticides and nutrients are pollutants that cause significant reductions in plant growth and development. Acid rain is caused when sulfur oxide and nitrogen oxide react with water in the atmosphere to form sulfuric acid and nitric acid, which reduces the pH of rain from a normal of pH 6 to as low as 4. The reduction in pH adversely affects the environment in a variety of ways; for example, acidified lakes show dramatically reduced fish populations. Acidification of the soil caused by acid rain alters nutrient availability. This leads to a variety of problems that reduce crop yields.

[FIGURE 9-5 OMITTED]

Wind. Wind is air in motion, which can be beneficial or harmful. Wind cools the plant when temperatures are high and reduces moisture on the plant and soil surfaces, which reduces disease problems. Wind also makes a plant shorter with a tougher cuticle, which is an impermeable, waxy material on the outside layer of leaves and stems that prevents water loss. By shortening the plant and making a tougher cuticle layer, the plant is more resistant to mechanical stress, pathogen (a disease-causing organism), or insect attack. Although wind can be beneficial, it can also cause critical problems. Some of the benefits of wind can also be detriments; for example, shorter plants with smaller leaves yield significantly less than normal size plants. Excessive wind can also cause desiccation, which accelerates the senescence process, leading to premature death. In addition, excessive wind can result in physical damage to the plant due to shear force. Wind can also spread fungal spores, weed seeds (Figure 9-5), pollutants, salt, and other pests.

Biotic Factors

Biotic factors are living organisms--ranging from animals and other plants (weeds) to microorganisms--that affect the atmospheric environment. More specifically, biotic factors can be insects and related pests; nematodes; diseases, such as viruses, fungi, and bacteria; weeds; rodents; and other animals. Climate affects the biotic factors; in turn, the biotic factors affect the biotic atmospheric environment. For any plant pest to become a problem, it must have a favorable environment. Both environmental and mechanical stress can adversely affect the biotic atmospheric environment by causing wear, which is the physical deterioration of a plant community resulting from excessive stress. Wear can come from a variety of sources in nature, including people, wind, and precipitation. These stresses disrupt the normal atmospheric environment surrounding the plant, resulting in a reduction in normal growth and development. Plant selection refers to selecting species that are tolerant to specific environmental stresses, thereby enabling the plant to withstand an adverse biotic atmospheric environment. Plants are being genetically engineered to make them more resistant to wear.

Edaphic Environment

The edaphic environment is the soil and area where plant roots are located. Abiotic and biotic factors affect this environment. Soil provides nutrients, water, gas exchange, and physical support to plant roots. The proper combination of sand, silt, and clay, plus organic matter, is very important for adequate nutrient and water retention, aeration and drainage (gas exchange), and physical support for the plant. Edaphic conditions that have a profound effect on plant growth are described next.

Abiotic Factors

Abiotic factors affecting the edaphic environment include water movement in the soil, water availability, soil aeration, soil temperature, soil pH, and soil salinity.

Water movement into the soil. Water movement into the soil is very important because if water does not penetrate the soil surface, waterlogging or drought can occur. The capability of water from precipitation or irrigation to enter the soil depends on soil texture, soil structure, and the presence of different layers in the soil, together with a variety of other factors. If the soil is subjected to water very rapidly, water will pool on the surface and eventually run off. The pooling of water also decreases the capability of water to penetrate the surface by blocking the surface pores.Water moves into the soil in two scenarios: when the soil is saturated, which is when all the pore spaces are filled with water, and when the soil is not saturated. When the soil is saturated, water movement is termed gravitational water, because water moves from the large pore spaces due to the pull of gravity. Water in a soil that is not saturated moves in response to a matrix potential gradient; in other words, the water moves from moist areas to drier areas of the soil. Water movement into the soil can be impeded in lawns by thatch, which is the layer of organic residue above the soil surface and just below the green leaves of the host plant (Figure 9-6). When thatch becomes too thick, it causes pooling and water runoff. The downward movement of water can also be impeded due to a hardpan, which occurs when soil is compressed into a very dense mass. This can be caused by the weight of heavy machinery or people. Heavy tractors and harvesting equipment can compress the soil, which causes a hardpan that restricts root growth and water movement, thereby reducing crop yields. Some weeds--such as annual bluegrass and white clover--can tolerate hardpans, which further decreases the plant's quality. In addition to hardpans, the rate at which water moves through the soil also depends on the normal composition of the different layers of soil.

[FIGURE 9-6 OMITTED]

[FIGURE 9-7 OMITTED]

Soil water availability. The capability of the soil to retain water is important for normal plant growth and development. When soil receives a large amount of water due to irrigation or a heavy rain, soil becomes saturated, and water freely drains in response to gravity. After all gravitational water has drained out of the large pore spaces leaving only the small pore spaces containing water, the soil is at field moisture capacity. This water may be lost from the soil surface by evaporation or from the leaf surface by transpiration; together these are called evapotranspiration. When water can no longer be absorbed by the plant, moisture stress occurs, causing the plant to wilt. At this stage, the plant is said to be at its wilting point. The difference between soil moisture at field capacity and the wilting point is called the available water (Figure 9-7).

[FIGURE 9-8 OMITTED]

Soil aeration. Soil aeration is the movement of air into the soil. In general, a mineral soil contains about 25 percent air. Keeping soils well aerated is important for maintaining the proper balance of [O.sub.2] and C[O.sup.2]. High levels of C[O.sub.2] in the soil are toxic, and low levels of O2 inhibit root growth. Clay soils are highly susceptible to aeration problems, whereas sandy soils are typically well aerated. Both tomato and pea plants are susceptible to oxygen deficiencies; they will show signs of epinasty, which is the downward bending of the petioles (Figure 9-8), wilt, and eventually die if the problem is not corrected. In general, at soil oxygen levels that are less than 10 or 12 percent, plants undergo stress that limits plant growth. Maintaining proper soil aeration and water aeration is important for hydroponically grown plants to maximize plant growth and subsequent crop yields. Oxygen levels in the soil are also critical to a variety of microorganisms that are necessary for good soils.

Soil temperature. The proper soil temperature is important for overall plant growth because temperature regulates all chemical reactions in the plant and soil. In general, plant roots stop growth when soil temperatures are 5[degrees]C (41[degrees]C) or colder. Soil temperature is affected by a variety of factors, one of which is the type of soil. Sandy soils warm up more quickly than clay soils due to the differences in their water content. Sandy soils drain faster, have less water, and have more air spaces than clay soils, which enables them to warm up faster than water-retaining clay soils. In addition, soil temperature is also affected by atmospheric conditions such as air temperature, wind, and solar radiation. Soil temperature can be modified to enhance crop production. Although it is difficult to control soil temperature in the field, there are many ways in which soil temperature can be modified. Applying plastic or mulch to the soil surface is one way to control soil temperature effectively in the field. Covering the soil with different colored plastics and mulches has been shown to be very effective (Figure 9-9). Using light-colored plastic or mulch reflects sunlight and lowers soil temperature; using dark-colored plastic or mulch absorbs light and warms the soil. Some growers use black plastic or mulches to raise soil temperature in order to start crops early.

Another way to modify soil temperature in the field is to promote adequate drainage either by modifying the soil texture by adding sand or by using raised beds to promote soil drainage and soil warming. Controlling soil temperature in a greenhouse is not as hard as in the field. Recent research has shown that supplemental root zone heating can have beneficial effects on plant growth because root zone temperatures in greenhouses fluctuate more than in the field.

[FIGURE 9-9 OMITTED]

Soil pH. Soil pH is the degree of acidity or alkalinity of the soil. The pH of the soil affects nutrient availability and should be maintained in the proper range to maximize crop yields. For example, nitrogen is most available between pH 6.0 and 8.0; as the soil becomes more acidic or more alkaline, the availability of nitrogen is reduced. In addition, nitrogen-fixing bacteria called Rhizobia work best in the pH range between 6.0 and 8.0. When the soil experiences low rainfall or it is poorly drained, salts generally accumulate and lead to an increase in soil pH. Soils exposed to heavy rainfall with good drainage typically have a lower soil pH. Examples of pH requirements for selected horticultural crops are shown in Table 9-6. Factors that affect soil pH and plant nutrition are discussed in more detail in Chapter 8, Media, Nutrients, and Fertilizers.

Saline soils. Soil salinity is the amount of salt (NaCl) in the soil. Salt stress is a major problem in agriculture because it reduces crop yields. Problems with salt buildup come from irrigation water and fertilizers. Millions of dollars are lost annually due to salt stress in plants. Selected examples of crop loss with increased salt as shown by ECe number (the units of electrical conductivity designated by milliSiemens/cm at 25[degrees]C) are shown in Table 9-7. When the salt levels in the soil get too high, plants exhibit similar symptoms to water stress, such as wilting, reduced plant growth, and, in some cases, the appearance of a green to bluish-green color. Plants have varying degrees of salt tolerance, as shown in Table 9-8.

Biotic Factors

Biotic factors are living organisms that affect the edaphic environment; they include a variety of living organisms, from animals and other plants (weeds) to microorganisms. Climate affects the biotic factors, which in turn affect the biotic environment. For any plant pest to become a problem, it must have a favorable environment. The edaphic environment is filled with a variety of simple and complex living organisms. These organisms can enhance this environment, thereby increasing crop yields, or they can be pests that damage plants. Soil organisms can be broken down into four groups: microorganisms, arthropod animals, nonarthropod animals, and vertebrate animals.

Microorganisms. The two classes of microorganisms found in the soil are bacteria and fungi. They can either be good or bad for the soil and plant. Bacteria are single-cell organisms that can occur in three different shapes--spherical, rod-like, or spiral--and are widespread in the edaphic environment. Bacteria can have a beneficial effect by causing the breakdown of dead organic material, thereby increasing the soil's organic matter content and the overall physical properties of the soil. Bacteria-plant interactions or symbioses have been shown to have beneficial effects in legumes. The bacteria Rhizobia spp. interacts with legume plant roots, enabling the roots to fix nitrogen, thereby reducing the need for nitrogen fertilization. Pathogenic (an organism that causes diseases) bacteria have rod-like shapes and thrive in soil with a pH of 6.5 to 7.5, which is the range that many plants also prefer. They enter the host through wounds or natural pores and cause a variety of problems, as shown in Table 9-9. Although bacterial diseases are difficult to control, bacterial disease-resistant cultivars are now available for many species.

Fungi, like bacteria, can also have both good and bad effects. Approximately 75 percent of all seed plants have some form of association with the fungi mychorrizae and its roots; this association is known as mutualism, which is when both the host and fungus benefit from the association. This type of association is similar to that observed with the bacteria Rhizobia spp. and its symbiotic relationship with legume roots. Much mychorrizal research has been done to understand this relationship better because the relationship enhances phosphorous uptake, which increases plant growth and development. The reproductive structures of fungi are called spores, which come in a variety of sizes, shapes, and colors. Spores are transported by wind, water, insects, birds, and others. Some fungi have coverings that protect them from an adverse external environment. Fungi gain access to the plant by wounds or through natural openings such as plant stomata. Interestingly, most plant diseases are caused by fungi, which are typically multicellular plants that lack chlorophyll. Fungi can be broken down into four classes: saprophytic fungi, which live only on dead tissue; parasitic fungi, which live on living tissue; obligatory fungi, which live on either dead or living tissue; and facultative fungi, which live on both living and dead tissues. Even though fungi cause most plant diseases, they are easy to control. Some examples of fungal diseases and their hosts are shown in Table 9-10.

[FIGURE 9-10 OMITTED]

Arthropods and nonarthropods. Arthropods are animals that have exoskeletons and jointed legs, whereas nonarthropods have neither. Arthropods that have horticultural significance are centipedes, millipedes, termites, ants, grubs, and a variety of other insects. An advantage of termites and ants is that they can increase drainage due to the pore spaces they produce in the soil. However, they both also produce large soil hills in lawns, which are unsightly and cause problems with mowing. Grubs cause major problems in turf because they feed on the roots of grass plants, resulting in dead patches in the lawn (Figure 9-10). Nematodes, also called roundworms and earthworms, are nonarthropods. Nematodes are appendageless, nonsegmented, worm-like invertebrates with a body cavity and complete digestive tract and are the most abundant animal found in the soil. Nematodes are parasites that attack a variety of horticultural crops, including fruit trees, turfgrasses, vegetable crops, and ornamentals. One of the advantages of some types of nematodes is that they feed on the mole cricket, thereby acting as a form of biological control. However, the disadvantage of nematodes is that they feed on the roots of plants and create an entry for disease organisms. In addition, nematodes enter and inhabit the roots, which creates nodules on the roots and reduces the growth of the plant. Earthworms are very beneficial to the soil they inhabit. They do extremely well in areas that are moist and have high organic matter, so they are typically not found in dry, acidic soils. They improve the movement of water into the soil and soil aeration by producing holes in the soil. In addition, earthworms feed on thatch and dead material found in the soil, which increases organic matter and promotes better soil structure.

Vertebrate animals. Vertebrate animals that inhabit the edaphic environment are rodents and small animals such as mice, gophers, chipmunks, ground squirrels, and rabbits. Although the holes produced by these animals can increase soil drainage, these holes are generally too big and unsightly. Turf that the rodents have burrowed under also presents a hazard to people walking on it. In addition, rodents cause heavy economic losses to the grower annually because when they come out of their underground holes, they feed on crops and then retreat to their underground hiding place.

SUMMARY

You now understand how plants are affected by the environment. Abiotic atmospheric environmental factors--including temperature, moisture, light, and wind--together with biotic factors affect plant growth and development significantly. Abiotic edaphic factors include water movement into the soil, soil water availability, soil aeration, soil pH, and saline conditions. The biotic edaphic environment also has a profound effect on the plant. The biotic edaphic environment is rich in a variety of organisms, including microorganisms, arthropods, nonarthropods, and vertebrate animals.

Review Questions for Chapter 9

Short Answer

1. List two main areas of the plant environment and define each.

2. List five abiotic atmospheric conditions that have an effect on plant growth.

3. What are the effects of red/yellow light, green/blue light, high-intensity light, and low-intensity light on plant growth?

4. Why does excessive water applied to the roots kill plants?

5. What are two benefits and two problems associated with wind?

6. What are two problems caused by impeded water movement into the soil?

7. Why is it important to aerate the soil?

8. What are four factors that affect soil temperature?

9. Provide one way to control soil temperature in the field.

10. What are two ways salt builds up in the soil and causes problems with plant growth?

11. List the four main categories of soil organisms and provide examples of each.

Define

Define the following terms:

biotic

abiotic

weather

climate

light intensity

etiolation

light quality

photoperiod

short-day plants

long-day plants

day-neutral plants

hardy plant

tender plant

hardened-off

humidity

relative humidity

transpiration

turgid

water stress

cuticle

pathogen

wear

saturated soil

gravitational water

thatch

hardpan

available water

field moisture capacity

evapotranspiration

wilting point

soil aeration

epinasty

soil salinity

ECe number

pathogenic

mutualism

fungi

saprophytic fungi

parasitic fungi

obligatory fungi

facultative fungi

arthropods

nonarthropods

nematodes

True or False

1. Atmospheric conditions such as temperature, moisture, light, and wind are all influenced by the sun.

2. When plants are grown under high-light intensities, they are generally shorter and darker green than plants grown under low-light intensities.

3. Light color influences plant growth. Red/yellow light promotes elongation growth whereas green/blue promotes shorter plants.

4. Photoperiod is the length of the light period that affects plant growth.

5. One of the benefits of wind is that it promotes shorter plants with tougher cuticles, thereby making them more resistant to stress.

6. The biotic environment is the soil and area where plant roots are located.

7. Soil temperature is not effected by air temperature.

Multiple Choice

1. Excessive watering kills plants

A. due to elevated levels of oxygen in the root zone.

B. due to reduced levels of oxygen in the root zone.

C. due to elevated atmospheric nitrogen levels in the root zone.

D. None of the above

2. Field moisture capacity is

A. the amount of water retained by the soil that plants can absorb.

B. when the water content of the soil fills the small pore spaces.

C. when the water content of the soil fills the large pore spaces.

D. None of the above

3. Saline soils are caused by

A. acid rain.

B. salt buildup from irrigation water.

C. improper crop rotations.

D. All of the above

Fill in the Blanks

1. -- keeps plant cells turgid.

2. Excessive water kills plants due to reduced -- in the roots.

3. Water movement into the soil can be impeded by -- , which is the layer of organic residue above the soil surface and just below the green leaves of the host plant.

Activities

Now that you have learned how a plant's environment has a profound effect on its growth, you will have the opportunity to explore in more detail how the environment affects plants. In this activity, you will report on specific abiotic and biotic atmospheric and edaphic environmental conditions and show how they affect plant growth and development. Your activity is to search the Internet for two sites that present information on the effects of different environmental conditions on the plant. For each site that you find, answer the following questions:

1. How do environmental factors affect plant growth and development?

2. Provide potential or existing methods used for modifying environmental factor(s) under field and greenhouse conditions.

3. Provide examples of topics in this area that were exciting and less than exciting; be sure to explain why you came to these conclusions.

4. What is the URL for the Web site where you found your information?
TABLE 9-1 SHORT-DAY PLANTS

Plant                  Scientific Name

Poinsettia             Euphorbia pulcherrima
Chrysanthemum          Chrysanthemum x morifolium
Kalanchoe              Kalancho blossfeldiana
Strawberry             Fragaria x ananasia
Violet                 Viola papilionaceae

TABLE 9-2 LONG-DAY PLANTS

Plant                  Scientific Name

Spider plant           Chlorophytum comosum
Baby's breath          Gypsophila paniculata
Evening primrose       Oenothera spp.
Fuchsia                Fuchsia x hybrida
Rex begonia            Begonia rex

TABLE 9-3 DAY-NEUTRAL PLANTS

Plant                  Scientific Name

Cucumber               Cucumis sativus
Corn                   Zea mays
Pea                    Pisum sativum
Tomato                 Lycopersicon esculentum
Kidney bean            Phaseolus vulgaris

TABLE 9-4 HARDY PLANTS

Plant                  Scientific Name

Geranium               Pelargonium spp.
Impatiens              Impatiens spp.
Primrose               Primula spp.
Hydrangea              Hydrangea spp.
Lettuce                Lactuca sativa
Carrot                 Daucus carota
Broccoli               Brassica spp.
Radish                 Raphanus sativa

TABLE 9-5 TENDER PLANTS

Plant                  Scientific Name

African violet         Saintpaulia spp.
Begonia                Begonia spp.
Coleus                 Coleus blumei
Pineapple              Ananas comosus
Petunia                Petunia spp.
Cucumber               Cucurbita sativus
Tomato                 Lycopersicon esculentum
Sweet potato           Ipomea batatas

TABLE 9-6 PLANTS WITH DIFFERENT pH REQUIREMENTS

pH 4.5 to 5.5     pH 5.5 to 6.5     pH 6.5 to 7.5

Potato            Tomato            Pea
Strawberry        Corn              Broccoli
Blueberry         Watermelon        Cabbage
Dandelion         Zinnia            Poinsettia
Gardenia          Pansy             Gladiolus
Azalea            Boston fern       Crocus
Rhododendron      Dogwood           Apple

TABLE 9-7 CROPS THAT EXHIBIT A 50 PERCENT DECREASE
IN YIELD WHEN SOIL SALINITY VALUES ARE BETWEEN 5 AND
10 (EXPRESSED AS (a) ECe)

Plant                  Scientific Name

Tomato                 Lycopersicon esculentum
Cucumber               Cucumis sativus
Potato                 Solanum tuberosum
Bell pepper            Capsicum annuum
Chrysanthemun          Chrysanthemum x morifolium
Lily                   Lilium longiflorum
Geranium               Pelargonium x hortorum
Grape                  Vitis spp.
Apple                  Malus pumila

(a) Units of electrical conductivity (ECe)
are milliSiemens/cm at 25[degrees]C.

TABLE 9-8 SALT-TOLERANT AND SALT-INTOLERANT PLANTS

GENERALLY SALT TOLERANT

Plant                  Scientific Name

Bermudagrass           Cynodon dactylon
Beet                   Beta vulgaris
Broccoli               Brassica oleraceae
Muskmelon              Cucumis melo
Rose                   Rosa ordorata

GENERALLY SALT INTOLERANT

Plant                  Scientific Name

Kentucky bluegrass     Poa prantensis
Strawberry             Fragaria spp.
Geranium               Pelargonium x hortorum
Azalea                 Rhododendron spp.
Pepper                 Capsicum annum

TABLE 9-9 BACTERIAL DISEASES THAT AFFECT HORTICULTURAL
CROPS

            Pathogen

Common Name          Scientific Name              Host Plant

Crown gall           Agrobacterium tumifasciens   Tree fruits and
                                                    woody plants
Hairy root           Agrobacterium rhizogenis     Tree fruits and
                                                    woody plants
Bacterial wilt       Erwinia tracheiphila         Cucurbits spp.
Bacterial canker     Pseudomonas syringae         Cherry, peach,
                                                    and plum
Bacterial soft rot   Erwinia carotovora           Vegetables, tubers,
                                                    and fruits
Common blight        Xanthomonas phaseoli         Beans

TABLE 9-10 FUNGAL DISEASES THAT AFFECT HORTICULTURAL
CROPS

         Pathogen

Common Name     Scientific Name         Host Plant

Dutch elm       Ceratocystis ulmi       Elm trees
Fusarium wilt   Fusarium oxysporum      Pea, tomato, and others
Damping-off     Rhizoctonia spp.        Commom problem in seedlings
Late blight     Phytophoria infestans   Tomato and potato
Rust            Puccinia graminis       Turfgrasses
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Author:Arteca, Richard N.
Publication:Introduction to Horticultural Science
Geographic Code:1USA
Date:Jan 1, 2006
Words:6284
Previous Article:Chapter 8 media, nutrients, and fertilizers.
Next Article:Chapter 10 plant growth regulators.
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