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Chapter 7: Plant physiology and growth.

Key Concepts

* Photosynthesis is the process by which plants convert energy from sunlight to chemical energy.

* Respiration is the conversion of sugars and starches to energy for use in metabolism of living cells.

* Transpiration is the loss of water from stomata of plants. It has several important functions.

* Symbiotic nitrogen fixation is a process that makes atmospheric nitrogen available to plants of the legume family (Fabaceae).

* Photoperiodism is the reaction of plants to changing lengths of darkness. Reactions include flowering, dormancy, and leaf abscission.

* Vernalization is a photoperiod response during the winter that is important for triggering flowering in the spring.

* Plants are categorized as annual, biennial, or perennial based on their life cycle.

* The general developmental stages of annual plants include seed, seedling, vegetative, flowering, fruit, seed, and senescence.

Key Terms

adenosine triphosphate (ATP)

annual

biennial

[C.sub.3] plant

[C.sub.4] plant

Calvin cycle

CAM plant

carbon-fixation reaction

chloroplast

chlorophyll

day-neutral

development

dry matter

evapotranspiration (ET)

Feekes scale

growth

light reaction

long-day

mesophyll

metabolism

nicotinamide adenine dinucleotide phosphate (NADPH)

nodule

perennial

photoperiod

photoperiodism

photosynthesis

phytochrome

pseudo-annual

respiration

rhizobia

short-day

transpiration

vernalization

Introduction

The foods and fibers that we use from plants are the product of complex physiological and metabolic reactions that occur at a microscopic level within plant cells (Figure 7-1). What we cannot see, we often take for granted. Metabolism is the group of vital biochemical reactions that occurs in the cells of all living organisms including plants. Plant growth and development require many essential metabolic processes. Metabolic energy transformations are critical for plants' survival and are the foundation for the human food source. In this regard, the processes of photosynthesis and respiration require special consideration.

Another specialized metabolic reaction important to agriculture is biological nitrogen fixation. Biological nitrogen fixation is a symbiotic relationship between a legume and a soil bacterium that enables plants to use atmospheric nitrogen for growth. Because many crop plants contain from 80 to 95% water, this chapter also discusses the process of water movement in plants, as well as transpiration. Metabolic reactions fuel the growth and development of plants.

[FIGURE 7-1 OMITTED]

Photosynthesis

Whereas animals consume food derived from plants to acquire energy, plants use photosynthesis to acquire energy. Photosynthesis is the process by which plants convert energy from sunlight to chemical energy. Plants use this energy for all other metabolic processes. Photosynthesis from plants, directly or indirectly, is the fundamental source of food and energy for all other complex life on Earth. Photosynthesis performed by a plant during the growing season is directly related to its yield of food and fiber at harvest. The overall chemical reaction of photosynthesis is

6 C[O.sub.2] + 12 [H.sub.2]O + Sunlight [right arrow] [C.sub.6] [H.sub.12] [O.sub.6] + 6 [O.sub.2] + 6 [H.sub.2] O

The final product is [C.sub.6] [H.sub.12] [O.sub.6] (glucose), a simple sugar that is used in metabolism and converted to other more complex compounds (see Chapter 5). Though this overall reaction seems simple, a number of intermediate steps occur.

Photosynthesis takes place in the chloroplast, an organelle located in the mesophyll cells of leaves (see Chapter 6). Chloroplasts are small, with as many as 200 per cell. Inside the chloroplasts are special molecules called pigments. These include the green chlorophylls a and b as well as yellow-orange carotenoids. These pigments absorb visible light in the range of 400-700 nanometers (Figure 7-2). The chlorophylls are most effective in absorbing the blue and red wavelengths of light and least effective in absorbing the green. The green portion of the spectrum is reflected, and this is why humans perceive plants as green. Chlorophyll converts the photons of light energy into chemical energy. It is important to note that photosynthesis uses only about 1-2% of the total solar radiation that reaches a crop field.

[FIGURE 7-2 OMITTED]

Light and Carbon-Fixation Reactions

Photosynthesis can be divided into two types of reactions: the light reactions and the carbon-fixation (or dark) reactions. The light (or photochemical) reactions are the first part of photosynthesis and the only ones actually requiring light. In this step, the photons of light energy strike the chlorophyll in chloroplasts and set off a chain of reactions caused by electron transfers. Water is split into hydrogen and oxygen, and two energy compounds--adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH)--are produced. This energy is used in subsequent reactions.

The second part of the photosynthetic process is the carbon-fixation (or dark) reactions. For these reactions to occur, C[O.sub.2] in the air diffuses through open stomata into the chloroplast within the mesophyll cells of the leaf. Then, NADPH and ATP made in the light reaction power the reduction of C[O.sub.2] to simple sugars. The collective process that involves many intermediary compounds is called the Calvin cycle or [C.sub.3] cycle. As shown in Figure 7-3, C[O.sub.2] is added to a five-carbon compound called ribulose diphosphate (RuBP). The resulting six-carbon compound is then split into two three-carbon molecules (phosphoglycerate, PGA). This cycle is called the [C.sub.3] cycle because 3-PGA is the first stable compound. PGA is converted to glucose, which is used in metabolism and converted to disaccharides or polysaccharides for transport or storage. Plants that use the [C.sub.3] cycle are called [C.sub.3] plants.

The majority of world's plants originating in temperate regions of the world (e.g., alfalfa, wheat, soybean, smooth bromegrass) and a few subtropical species (e.g., cotton, tobacco, peanut) are [C.sub.3] plants. Photosynthesis in [C.sub.3] plants is most effective at temperatures from 10-25[degrees]C and decreases thereafter. [C.sub.3] photosynthesis is inefficient in warm high tropical climates and when C[O.sub.2] levels in the leaf are low such as occurs during drought when stomata are partially closed. This is because [C.sub.3] species have a version of respiration called photorespiration that can use up a significant portion of the carbon energy fixed during photosynthesis.

[FIGURE 7-3 OMITTED]

Variations in Carbon Fixation: [C.sub.4] and CAM

Plants adapted to tropical environments of high air temperature, drought, and high light intensity have alternative systems of carbon fixation. [C.sub.4] plants have a different leaf anatomy and an enzymatic system that first fixes C[O.sub.2] into a three-carbon compound (phosphoenol pyruvate; Table 7-1). The first compounds formed are the four-carbon compounds malate and aspartate (Figure 7-4). These compounds are then transferred to bundle sheath cells that surround the vascular system. In these cells, C[O.sub.2] is released to the Calvin cycle that then generates glucose and other sugars. Only about 10% of plant species are [C.sub.4] plants, but some are very significant for world agriculture. Examples of [C.sub.4] plants include corn, sorghum, amaranths, and millet. Many prairie grasses also have the [C.sub.4] cycle including big bluestem, indian-grass, and switchgrass.

Crassulacean acid metabolism (CAM) plants have yet another type of carbon fixation. CAM plants, like [C.sub.4] plants, are adapted to hot temperatures. They use the [C.sub.3] and [C.sub.4] pathways like [C.sub.4] plants, but they use the [C.sub.4] pathway at night and the [C.sub.3] pathway in the day. This enables these plants to preserve water because they open their stomata only at night. Examples of CAM plants include cacti and pineapple. About 3-4% of plant species are CAM plants, but the most significant types of plants for crop production are [C.sub.3] and [C.sub.4] plants.

[FIGURE 7-4 OMITTED]

Respiration

The conversion of sugars formed through photosynthesis to energy (ATP) for use in metabolism of living cells is called respiration. Plants use this energy for cell maintenance, growth, and building new tissues. One molecule of glucose formed during photosynthesis results in the production of 36 ATP molecules. The overall process of respiration occurs in both the cytoplasm and the mitochondria, small microscopic organelles within the cytoplasm of living plant cells. The overall reaction that occurs in respiration is

[C.sub.6] [H.sub.12] [O.sub.6] + 6 [O.sub.2] [right arrow] 6 C[O.sub.2] + 6 [H.sub.2] O + Energy (ATP and heat)

Respiration, like photosynthesis, is a complex process with many steps. In summary, glucose and oxygen are transformed into carbon dioxide, water, and energy. The four primary processes of respiration include

1. Glycolysis, which occurs in the cell's cytoplasm. Glucose is split into two three-carbon molecules (i.e., pyruvate). This reaction also produces ATP, NADH, and water. ATP and NADH are both high energy compounds.

2. Pyruvate enters the mitochondria and is split into CO2 and acetyl coenzyme A (acetyl CoA). This reaction also produces NADH.

3. The Krebs cycle uses acetyl CoA to produce ATP and NADH, and it releases CO2 and water. The NADH will be used in the next step to generate more ATP.

4. Oxidative phosphorylation is the final step of energy formation and occurs within the mitochondrial membrane. NADH is used to synthesize additional ATP.

[FIGURE 7-5 OMITTED]

For an overview of how photosynthesis and respiration are related, see Figure 7-5. A comparison of photosynthesis and respiration is shown in Table 7-2.

Transpiration

Transpiration is the process of water loss from stomata of plants. Plants vary in their transpiration rates. Individual crop plants such as corn and sunflower can transpire from 0.5 gallon to more than 1 gallon (2 to 5 liters) per day. Large trees can transpire more than 50 gallons (200 liters) per day. Transpiration has several important functions including cooling of the plant, movement of nutrients within the plant, and uptake of mineral nutrients. Transpiration is driven by the sun's energy and a moisture concentration gradient. Water moves along a gradient from the roots, where there is the most water (or a high concentration) to the leaves, where there is the least water (or a low concentration). Transpirational pull creates suction that pulls available water from the soil.

Root hairs that develop behind the meristematic region of a root tip (see Chapter 6) absorb water from the soil. Water molecules and any dissolved minerals pass through the epidermal cell membrane of the root hair. As it moves from the epidermal cells to the xylem, water passes through the root cortex, endodermis, and pericycle.

Water travels through the xylem tissue to the veins of the leaves and finally into the mesophyll (Figure 7-6). The plant will close its stomata and wilt if the available soil moisture is inadequate to support transpiration. Transpiration is affected by soil moisture content, high air temperatures, low water concentration in the air, and air movement. Therefore, on hot, dry, windy days with low relative humidity, we can expect plants to use more water. Factors that affect transpiration include the following:

* The process of transpiration is dependent on the presence of soil moisture. Therefore, climate and local weather that provide water and agronomic factors (e.g., weed control, crop residues, or fallowing) that minimize water loss enhance transpiration.

* Water evaporation within the leaf and transpiration are nearly doubled for every 10[degrees]C rise in air temperatures within an air temperature range of 10-30[degrees]C.

* Air moisture content (or humidity) influences water loss because water molecule change from liquid to gas is dependent on the concentration of water molecules in the air. A greater concentration of water molecules in the air suppresses the escape of additional water molecules to the air.

* Wind or air movement pulls water vapor away from the leaf surface and increases the transpiration pull.

[FIGURE 7-6 OMITTED]

Transpiration facilitates the absorption of many essential minerals that are soluble in water. Minerals are moved throughout the plant within the water stream.

Evapotranspiration

Evapotranspiration (ET) is the plant's total water use. As the name suggests, ET includes the actual water loss in transpiration as well as water that is evaporated from the surface of the leaves or from the soil surrounding the plant. A plant's total water use varies greatly. The ET ratio (kilograms of water required to produce a kilogram of crop dry matter) depends on several factors including type of photosynthesis (e.g., [C.sub.3] or [C.sub.4]) and various adaptations (e.g., number of stomata, presence of hairs on leaves, and so on). [C.sub.4] plants are the most efficient, requiring the least water to produce a kilogram of dry matter. [C.sub.3] legumes are the least efficient. For example, alfalfa requires 858 kilograms of water to produce 1 kilogram of dry matter, whereas proso millet requires only 267 kilograms of water.
How Plants Control Water Loss

Plants control water loss by regulating the opening and closing of the
stomata. Stomata is derived from the Greek term stoma, which means
mouth. Stomata are very small openings (typically less than 40
micrometers long) in the plant epidermis that regulate exchange of
water vapor, carbon dioxide, and oxygen. Leaves of crops can have as
many as 50,000 stomata per square centimeter. Stomata are also on the
stems of plants. They consist of pores surrounded by a pair of guard
cells that regulate the opening and closing of the pore by changing
shape in response to an increase in water pressure (or turgidity) (see
Chapter 6). Fully turgid cells bow apart and open the pore. When
deflated, the guard cells close the pore. Guard cell water content is
regulated by potassium content. Potassium accumulation in guard cells
causes water accumulation and opens the pore. Stomata of most plants
close during water deficits to prevent water loss and at night when
photosynthesis is not occurring.

Plants adapted to hot and dry environments have other specific
morphological adaptations that affect transpiration.

* Stomata are located primarily on the underside of the leaf, where
they are protected from direct exposure to sunlight. Lower underside
leaf temperatures lead to less transpiration. For example, corn has
about 5000 stomata per square centimeter on the upper leaf surface but
more than 10,000 on the lower leaf surface.

* Leaf surfaces are covered with pubescence that shades the stomata and
reduces leaf temperature as well as reduces water vapor loss from the
leaf surface. Corn and sunflower have considerable leaf and stem
pubescence.

* Leaf surfaces are covered with a thick waxy coating (the cuticle)
that decreases water movement directly from epidermal cells. Sorghum, a
crop especially adapted to low moisture regions, has a whitish waxy
coating (called glaucous) on stems and leaves.

* Stomata are sunken on the leaf surface. This protects the stomata
from wind.

* Leaf curling. During moisture stress, leaves can fold and roll to
decrease surface area and reduce stomata exposure to sun and air. The
rolling of corn leaves is a typical symptom of moisture stress.


Symbiotic Nitrogen Fixation

Nitrogen is one of the most important elements for plants and is the most limiting nutrient in terrestrial environments. Much of the world's nitrogen is in the atmosphere, where it is the most abundant gas (see Chapter 9). Symbiotic nitrogen fixation is a process that makes atmospheric nitrogen available to plants. Plants from the legume family (Fabaceae) commonly engage in symbiotic nitrogen fixation. However, some nonlegume species (mostly trees and shrubs) and free-living algae are also able to conduct biological fixation. This mutually beneficial (symbiotic) partnership occurs between legumes and bacteria collectively known as rhizobia. The legume plant supplies nutrients and energy to the bacteria that reside in root nodules. A bacterial enzyme, nitrogenase, converts nitrogen from the soil into ammonia (N[H.sub.3]), which is reduced to ammonium (N[H.sub.4]) that is used by the legume plant to form amino acids and protein. This process is costly for the plant, and if nitrogen levels in the soil are high, plants may reduce [N.sub.2] fixation levels. The overall reaction that involves the nitrogenase bacterial enzyme is

[N.sub.2] + 8[H.sup.+] + 16 ATP (energy)[right arrow] 2N[H.sub.3] + [H.sub.2] + 16ADP

Rhizobia that are present in the soil, or supplied in inocula to the seed, infect plant root hairs and stimulate development of tumor-like nodules on the roots. A specific rhizobial species is required for a given legume species. For example, bacteria infecting and nodulating white clover will not effectively nodulate soybean. The amount of fixed nitrogen varies depending on the symbiosis (Table 7-3). Some of the fixed nitrogen can be transferred to nonfixing plants grown in mixtures or may be used by subsequent crops in crop rotations.

Nodule Formation

The following is the process of nodule formation, as shown in Figure 7-7:

1. Root hairs grow. They release root exudates (flavonoids, sugars, amino acids, and so on), which attract specific rhizobia to the root.

2. The rhizobia attach to the root hair surface.

3. The root hair curls, entrapping the rhizobia.

4. The rhizobia digest the cell wall and form an infection thread into the center of the root. Within the thread, the bacteria divide and increase in number.

5. The rhizobia induce division of the root cells.

6. A nodule is formed from the protrusion of the root cells to the surface of the root.

7. Within the individual root cells, the bacteria become devoid of a cell wall and become bacteroids, which develop the nitrogenase enzyme and fix atmospheric nitrogen.

Nodule shapes vary and can be the elongated lobes found in roots of alfalfa and the clovers, or they can be round like those found on birdsfoot trefoil and soybean. Lobed nodules are perennial. They overwinter and fix nitrogen for more than one growing season, whereas round nodules die and reform on roots each year (Figure 7-8). Upon dissection, active regions of nodules will be observed to contain a pink pigment, leghemoglobin, that is responsible for oxygen regulation in the nodule. Nitrogen that is fixed by the bacteroids enters the plant's vascular system and can be transported throughout the plant.

[FIGURE 7-7 OMITTED]

[FIGURE 7-8 OMITTED]

Photoperiodism

Photoperiodism is a plant's reaction to changing durations of darkness. This response involves the pigment phytochrome that senses changes in the proportion of red (absorption peak at a wavelength of 660 nanometers) and far red light (absorption peak at a wavelength of 730 nanometers). Phytochrome has two interconvertible forms, [P.sub.r] and [P.sub.fr]. [P.sub.fr] is the active form of the pigment and is located in vegetative portions of plants and in seeds. There are several types of photoperiodic response. These include flowering, dormancy reactions, and leaf abscission. In addition, photoperiod affects germination of some seeds.

Flowering in many plants is affected by photoperiod. Plants can be categorized by their flowering responses as short-day, long-day, or day-neutral. Some plants flower at the same time of year every year. These are either short-day or long-day plants (Figure 7-9). Short-day plants flower when the days are shorter than a certain time called a photoperiod. Long-day plants flower when the photoperiod is longer than a certain period. The main differentiation between "shorter than" and "longer than" is the length of the dark period or night.

Flowering Response

Spring or fall-flowering plants are short-day plants, and summer-flowering plants are long-day plants. Short-day plants generally do not flower in the longest days of the summer. For example, to flower, strawberries must have a day not longer than 10 hours. Short-day crops include corn, soybean, cotton, and rice. Day-neutral plants are not dependent on a certain photoperiod to flower. Day-neutral plants flower after a certain age or when a certain level of growth has been achieved rather than in response to a photoperiod. An example is the garden petunia, which have been bred to flower constantly. Day-neutral crops include ever-bearing strawberries, cucumbers, and tomatoes. Long-day plants generally flower in spring or early summer. Long-day crops include smooth bromegrass, wheat, flax, and mustards.

[FIGURE 7-9 OMITTED]

Dormancy Response

In northern latitudes, perennial plants respond to shortening days and decreasing temperatures by undergoing a dormancy reaction. In alfalfa and perennial grasses, the dormancy reaction involves reducing herbage growth, storing energy in the roots and crowns, increasing sugar concentration in the cells, and forming crown buds for regrowth in the spring. In many perennial trees, leaves drop off (leaf abscission) in response to short days.

Vernalization

Vernalization is a response during the winter that is important for triggering flowering in the spring. To flower in the spring, winter annual small grains such as rye and wheat, along with perennial grasses such as smooth bromegrass and reed canarygrass require exposure to short photoperiods and low temperatures during winter. Winter wheat and winter rye that are planted in the spring without vernalization will not flower.

Growth and Development

The cell uses the carbon compounds produced during photosynthesis for metabolic processes including respiration and enzyme formation. A portion of that carbon is also used for growth of the plant. Growth is the irreversible increase in weight or size of an organism due to an increase in number and size of cells. For example, growth would be the development of leaves and increase in height while the plant is vegetative. Growth is also associated with development. Development is the process of differentiation when plants transition from germination, seedling, vegetative, flowering, and seed maturation states.

Dry Matter Accumulation

Plant growth can be measured in terms of fresh weight and dry weight, as linear dimensions such as height, or by leaf area. Because of fluctuations in water content caused by the environment, dry weight is the most commonly used measure of growth. Dry matter is the weight of all the components of a plant minus the water. Dry matter accumulation changes with developmental stage (Figure 7-10). During early stages of plant development, the dry matter in leaves and stems increases linearly and then begins to plateau as plant energy is partitioned into seed development. At this point, the dry matter increases only in the seed. In examining a typical pattern of dry matter accumulation throughout a growing season, it is apparent that the proportion of dry matter in the root, stem, leaf, or fruit varies as the plant matures. The overall dry matter accumulation increases throughout the season until near maturity, when a slight drop in overall biomass occurs. Individual crops have a unique dry matter accumulation curve throughout the season.

[FIGURE 7-10 OMITTED]

Measuring Development

As discussed in Chapter 6, the plant body is composed of shoots, roots, leaves, flowers, fruits, and seeds. The development of a plant starts from the seed (see Chapter 5). Once the seed germinates, the primary plant organs--the shoots and roots--develop first. From the shoots, the stem and leaves form and, after a certain time, the flowers, fruits, and seeds develop. The general developmental stages are seed, seedling, vegetative, flowering, fruit, seed, and senescence.

We can further break down the developmental stages of some crop species. A system called the Feekes scale assesses development in cereals. The Feekes scale is a numerical system for assigning developmental stages. The scale goes from 1 (the first leaf stage) to 11 (the grain ripening stage). Other systems include the Zadoks and Haun scales. Corn and soybean growth stages are shown in Figures 7-11 and 7-12. The corn and soybean growth scales shown are from Illinois State University. This system divides plant development into two stages: vegetative and reproductive. Vegetative stages are denoted by V, whereas reproductive stages are denoted by R. Alfalfa developmental stages are shown in Table 7-4.

[FIGURE 7-11 OMITTED]

Scientists and farmers use crop growth stages to describe crop development and as a guide to when to apply management treatments. Alfalfa, for example, has different nutrient concentrations depending on the growth stage at which it is harvested. Some herbicides, pesticides, or mechanical weed control operations can be conducted only at certain crop development stages. Finally, farmers must harvest at the proper stage of maturity to maximize yields. To develop sound management strategies, especially of inputs, producers thus should understand development and its relationship to timing of procedures such as fertilization, irrigation, and harvest.

The Life Cycles of Plants

All plants are categorized as annual, biennial, or perennial (Figure 7-13). Life cycles are important in the production of crops and in the management of weeds.

Annuals

Annuals are plants that complete their life cycle in one year or one growing season. The termination of their life cycle is seed production. Many of our most important grain crops are annuals. Annuals are dependent on regeneration of new plants from seed to start the next growth cycle. Transportation of seed to new environments is the primary approach to colonizing new environments. Two categories of annuals are summer annuals and winter annuals.

[FIGURE 13 OMITTED]

* Summer annuals are planted in the spring months, grow during the summer months, and mature during the late summer to fall. Crops include corn, soybean, spring wheat, and rice. Weeds include pigweed, velvetleaf, foxtails, and ragweed.

* Winter annuals are planted in the late summer to early fall, grow vegetatively before winter, overwinter in a dormant state, and flower in the spring. Winter annuals typically flower in response to the lengthening days in late spring. Crops include winter wheat, winter rye, and some canola varieties. Weeds include chickweed and wild mustard.

Biennials

Biennials are plants that complete their life cycle in two growing seasons. In the first year, biennials are vegetative and accumulate significant storage reserves in the fall of the seeding year. After overwintering, biennials flower, form seed, and die. Crops include sugar beet and sweet clover. Weeds include wild carrot and bull thistle. When harvested for processing to sugar, sugarbeet roots are harvested in the fall of the seeding year and are not allowed to overwinter. Carrots, a horticultural crop, are also biennial.

Perennials

Perennials are crops that persist for three or more years or growing seasons. They typically flower each year but may remain vegetative throughout their lives. In northern temperate climates, perennials typically undergo a dormant period in the winter and resume growth in the spring. To survive periods of dormancy, perennials often have some type of specialized energy storage structure such as rhizomes in quackgrass, fleshy taproots in alfalfa, or corms in timothy. Crops include alfalfa, white clover, smooth bromegrass, big bluestem, switchgrass, and hybrid bermudagrass. Weeds include quackgrass, johnsongrass, dandelion, and Jerusalem artichoke.

Pseudo-Annuals

Sometimes perennial and biennial plants are managed as annuals and could be considered pseudo-annuals. For example, sugar beet is a biennial plant that is harvested for sugar production after only one growing season. Alfalfa and red clovers are perennial plants that can be grown as an annual for use as a green manure crop.

Review Questions

1. How do plants manufacture energy?

2. Explain the process of photosynthesis and identify its products.

3. What is the overall chemical reaction of photosynthesis?

4. What are the light and carbon-fixation reactions of photosysnthesis?

5. Explain the differences between [C.sub.3] and [C.sub.4] plants.

6. Why is cellular respiration important for plants? What is the overall chemical reaction that describes the process?

7. What is the process of transpiration?

8. Identify three important functions of transpiration.

9. How do plants regulate transpiration?

10. What is evapotranspiration?

11. Explain the process of symbiotic nitrogen fixation. Identify some crop plants that can conduct symbiotic nitrogen fixation.

12. What is the pigment responsible for photoperiodism in plants? Give some examples of photoperiodic responses by plants.

13. As grass and legumes grow, they go through several general growth stages. Identify these.

14. Describe the terms annual, biennial, and perennial and give examples of crops in each category.

Critical Thinking

1. Explain why plants appear green to humans.

2. In previous chapters, we discussed the subject of global warming. How will increases in atmospheric CO2 concentration and air temperature affect important processes such as photosynthesis and transpiration? Will crop yields increase?

3. Plants are uniquely adapted to our world. Using photosynthesis and photoperiodism as an example, explain how plants have adapted to the light received on the earth and the seasonal variation in light on the earth.

4. Why is it important to characterize the exact stage of a crop's development?

References

Capon, B. (1990). Botany for gardeners. Portland, OR: Timber Press, Inc.

Fick, G. W. & Mueller, S. C. (1989). Alfalfa quality, maturity, and mean stage of development. Cooperative Extension Bulletin 217. Ithaca, NY: Cornell University Cooperative Extension.

Graham, L. E., Graham, J. M., & Wilcox, L. W. (2006). Plant biology (2nd ed.). Upper Saddle River, NJ: Prentice-Hall, Inc.

Levetin, E., & McMahon, K. (1996). Plants and society. Dubuque, IA: William C. Brown Publishers.

Moore, R., Clark, W. D., Stern, K. R., & Voldopich, D. (1995). Botany. Dubuque, IA: William C. Brown Publishers.

Northington, D. K. & Schneider, E. L. (1996). The botanical world (2nd ed.). Dubuque, IA: William C. Brown Publishers.

Opik, H., & Rolfe, S. A. (2005). The physiology of flowering plants (4th ed.). Cambridge, UK: Cambridge University Press.

Powers, L. E., & McSorley, R. (2000). Ecological principles of agriculture. Albany, NY: Delmar Thomson Learning.

Salisbury, F. B. & Ross, C. W. (1992). Plant physiology (4th ed.). Belmont, CA: Wadsworth, Inc.

Sheaffer, C. C., Ehlke, N. J., Albrecht, K. A., & Peterson, P. R. (2003). Forage legumes: Clovers, birdsfoot trefoil, cicer milkvetch, crownvetch and alfalfa (2nd ed.). Station Bulletin 608-2003. St. Paul, MN: Minnesota Agricultural Experiment Station, University of Minnesota.

Taiz, L., & Zeiger, E. (2006). Plant physiology (4th ed.). Saunderland, MA: Sinauer Associates, Inc.

University of Illinois Extension (2007). Pocket guide to crop development: illustrated growth timelines for corn, sorghum, soybean, and wheat. Champaign-Urbana, IL: University of Illinois Extension. <http://weeds.cropsci.uiuc.edu/> Accessed 26 October 2007.

University of Nebraska, Lincoln (2007). Growth stages: corn, sorghum, soybean and wheat. Lincoln, NE: University of Nebraska Extension. <http://www.unl.edu/> Accessed 24 October 2007.
Table 7-1
Major differences between [C.sub.3] and [C.sub.4] plants.
Adapted from Salisbury and Ross (1992).

Characteristics        [C.sub.3] plants     [C.sub.4] plants

Leaf anatomy           No distinct bundle   Well-organized
                         sheath of            bundle sheath,
                         photosynthetic       rich in
                         cells                chloroplasts
Carboxylating enzyme   Rubisco              Phosphoenolpyruvate
                                              (PEP) carboxylase,
                                              followed by Rubisco
Transpiration ratio    450-950              250-350
  (kg [H.sub.2]O per
  kg dry matter
  produced)
Photorespiration       Yes                  No
Optimum temperature    15-25[degrees]C      30-47[degrees]C
  for photosynthesis
Minimum ppm            30-70                0-10
  C[O.sub.2]
  required for
  photosynthesis

Table 7-2
A comparison of photosynthesis and respiration.

Photosynthesis       Respiration

Occurs in light      Occurs in dark and light
Produces sugars      Uses sugars
Stores energy        Releases energy
Uses [H.sub.2]O      Produces [H.sub.2]O
Uses C[O.sub.2]      Produces C[O.sub.2]
Releases [O.sub.2]   Uses [O.sub.2]

Table 7-3
Quantities of nitrogen fixed by various legumes. Legumes vary in the
amount of atmospheric nitrogen they can fix. This variation is in part
due to the relative effectiveness of the symbiosis between plants and
the bacteria. Sheaffer et al. (2003).

                         Nitrogen fixed
Legume              (kilograms/hectare/year)

Alfalfa                      13-36
Birdsfoot trefoil             8-27
Crownvetch                     18
Cicer milkvetch                25
Crimson clover                 10
Hairy vetch                    18
Kura clover                   3-29
Lentil                       27-31
Red clover                    11-1
Soybean                       4-1
Sub clover                    9-30
Sweetclover                    22
White clover                 21-33

Table 7-4
Developmental stages of alfalfa. Adapted from Fick and Mueller (1989).

Stage                           Description

Vegetative phase: stems and
leaves; no buds, flowers, or
seedpods
  Early vegetative              Stem length [less than or equal to]
                                  15 centimeters
  Mid-vegetative                Stem length 16-30 centimeters
  Late vegetative               Stem length [greater than or equal
                                  to] 31 centimeters
Flower bud development: stems
with flower buds
  Early bud                     1-2 nodes with buds
  Late bud                      [greater than or equal to] 3 nodes
                                  with buds
Flowering phase: stems with
open flowers
  Early flower                  1 node with 1 open flower
  Late flower                   [greater than or equal to] 2 nodes
                                  with open flowers
Seed production: stems with
flowers and seedpods
  Early seedpod                 1-3 nodes with green seedpods
  Late seedpod                  [greater than or equal to] 4 nodes
                                  with green seedpods
  Ripe seedpod                  Nodes with mostly brown
                                  mature seedpods

Figure 7-11 Some of the developmental stages of corn.
Adapted from University of Illinois Extension (2007).

Vegetative Stages

Stage   Description

VE      Emergence
V1      First leaf
V2      Second leaf
V3      Third leaf
V6      Sixth leaf
V12     Twelfth leaf
VT      Tasseling

Reproductive Stages

Stage   Description

R1      Silking
R2      Blister
R3      Milk
R4      Dough
R5      Dent
R6      Physiological maturity

Figure 7-12 Some of the growth stages of soybean.
Adapted from University of Illinois Extension (2007).

Vegetative Stages

Stage   Description

VE      Emergence
VC      Cotyledon growth stage
V1      Unifoliate leaves completely unrolled
V2      Leaves at node above unifoliate leaves unrolled
V3      Three nodes with unrolled leaves

Reproductive Stages

Stage   Description

R1      One flower at any node
R3      Pod is 1/2 cm long with unrolled leaf
R5      Seeds beginning to develop
R8      Physiological maturity
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Publication:Introduction to Agronomy, Food, Crops, and Environment
Geographic Code:1USA
Date:Jan 1, 2009
Words:5457
Previous Article:Chapter 6: Plant anatomy and morphology.
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