Agriculture Seeks to Maximize Photosynthesis

Miracle Farm Blueprint

Organic Farming Manual

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Recall from Chapter 2 that plants, along with algae and some bacteria, transform light energy into chemical energy via the process of photosynthesis. Photosynthesis converts the energy from sunlight, carbon dioxide from the atmosphere, and water into energy-rich molecules called carbohydrates. Plants then use carbohydrates to run the activities of their cells. In other words, plants make their own food. In fact, plants are so effective at converting light energy into chemical energy that they produce enough energy to store and to grow larger. Other organisms, such as humans, harvest this excess energy. Our agricultural practices are intended to maximize the photosynthesis of crop plants so they will produce a surplus of carbohydrates we can harvest.

If we look only at the chemical reaction that occurs during photosynthesis, it is clear that plants require carbon dioxide, water, and light to produce carbohy-drate—but the effective production of chemical energy from light requires a few more components. For instance, photosynthesis requires nitrogen-containing proteins in the choloroplast and magnesium atoms that are present in the green pigment called chlorophyll. Elements such as nitrogen and magnesium, typically called nutrients, must be obtained from the plant's environment for

Figure 15.10 Plant growth requirements. Plants require the basic ingredients for photosynthesis— water, carbon dioxide, and light—as well as nutrients from the soil and freedom from damage to produce excess carbohydrate for human consumption.

Plant growth requirements:

Figure 15.10 Plant growth requirements. Plants require the basic ingredients for photosynthesis— water, carbon dioxide, and light—as well as nutrients from the soil and freedom from damage to produce excess carbohydrate for human consumption.

1. Solar energy (sunlight)

2. Carbon dioxide

3. Freedom from damage

' 4. Water i 5. Inorganic nutrients

1. Solar energy (sunlight)

2. Carbon dioxide

3. Freedom from damage

' 4. Water i 5. Inorganic nutrients photosynthesis to occur (Figure 15.10). In addition, plants must be able to support themselves against the pull of gravity; surprisingly, they use water as one of their main methods of support. Thus, plants require much more water than just that needed for the chemical reactions of photosynthesis. Modern farming practices attempt to maximize the amount of light, water, carbon dioxide, and nutrients devoted to crop production.

Maximizing Exposure to Light and Water There are two components to maximizing a plant's exposure to the sun. One is to ensure that the plant can effectively orient its leaves to intercept incoming light. The other is to maximize the amount of light that the plant can potentially intercept.

To hold its leaves perpendicular to the sun's rays, a plant's cells must be full of water. Unlike an animal cell, the membrane of a plant cell is surrounded by a stiff cell wall. The cell wall is tough, but it is also elastic, meaning that

Turgid cells

(a) Turgid plant

(b) Wilted plant

Flaccid cells

Turgid cells

(a) Turgid plant

(b) Wilted plant

Flaccid cells

Cell membrane water

Cell membrane water

Figure 15.11 Water in plant cells. (a) When a plant is fully hydrated, the cell's vacuoles balloon with water and the cells press against each other, so the plant remains "crisp." (b) As the plant dries out, the vacuoles lose water and the cells no longer support each other, so the plant wilts.

Cell membrane water

Cell membrane water

Figure 15.11 Water in plant cells. (a) When a plant is fully hydrated, the cell's vacuoles balloon with water and the cells press against each other, so the plant remains "crisp." (b) As the plant dries out, the vacuoles lose water and the cells no longer support each other, so the plant wilts.

it can stretch. As a plant cell's vacuole fills with water, the cell wall balloons slightly, becoming turgid—it is the pressure of adjoining ballooned cell walls that holds the leaves upright (Figure 15.11a). When water levels drop and the cells begin to lose water, the pressure of cells pushing against each other decreases, the cells become flaccid, and the plant wilts (Figure 15.11b). Thus, part of maximizing a plant's exposure to light is ensuring that it has plenty of water.

Plants are continually losing water, especially during hot, sunny weather. To minimize the chance of wilting during these conditions, farmers try to provide an optimal amount of water to their crops. In many regions, adding water through irrigation is necessary. The enormous production of corn, wheat, and soybeans in the former Dust Bowl region is a result of an equally enormous amount of irrigation. The water for irrigation in this rather dry region comes from a huge underground reservoir called the Ogallala aquifer (Figure 15.12).

Another way to maximize the amount of water reaching a preferred plant—that is, one that a farmer or gardener desires to grow—is to reduce the amount of competition for water. Farmers do this by trying to minimize the number of nonpreferred plants, commonly called weeds, in agricultural fields. Any plant can be a weed if it grows in the wrong place—just ask a gardener who maintains a turf lawn and must continually pull turf grass out of the flower bed! Minimizing the number of weeds growing with a crop has the additional benefit of minimizing the chance that a weed will physically block sunlight from the preferred plants. Thus, reducing the number of weeds benefits preferred crops by allowing them access to the maximum amount of light as well as water.

Farmers have several techniques for controlling weed growth. One is to remove competitors before the crop is planted. This process is referred to as tilling and involves turning over the soil to kill weeds that have sprouted from seed but are still small and vulnerable to damage. After a crop begins growing, it is possible to reduce the growth of competitors, and in addition keep soil from losing water, by mulching the crop. To mulch, farmers spread straw or other dead plant material, or sometimes dark-colored plastic, around the base of preferred plants. A thick layer of mulch keeps sunlight from penetrating

Figure 15.12 Irrigation. (a) The bright green circles in this aerial photo are formed by lush plant growth. The circle pattern derives from center-pivot irrigation sprayers, which spray water in a complete arc from the center point. (b) The Ogallala aquifer is the largest single pool of freshwater in the world; it underlies and supplies the United States' "bread basket" region.

Figure 15.12 Irrigation. (a) The bright green circles in this aerial photo are formed by lush plant growth. The circle pattern derives from center-pivot irrigation sprayers, which spray water in a complete arc from the center point. (b) The Ogallala aquifer is the largest single pool of freshwater in the world; it underlies and supplies the United States' "bread basket" region.

the soil and stimulating weed seeds to germinate. Mulching is impractical for most large crop producers, but it is common in smaller gardens and for certain crops.

Although tilling sets back the growth of weeds, it cannot kill all of the weeds, and some spring back to life quickly. In these cases farmers use an herbicide, a chemical that kills plants, to eliminate weeds. Until recently, the use of herbicides was limited to relatively few crops because it is difficult to target just the weeds. Flowering plants can be divided into two large groups—dicots, also known as broad-leaved plants, and monocots, or narrow-leaved plants. In the 1940s, scientists studying a growth hormone in plants (first discovered by Charles Darwin in the 1880s) found that certain forms of this hormone were deadly to broad-leaved plants but harmless to narrow-leaved plants. They developed herbicides that are used to control weeds in narrow-leaved crops, such as corn, wheat, and rye. You may be familiar with this class of herbicides as well—many homeowners use it to kill dicot dandelions on their monocot grass lawns. However, these herbicides are ineffective against narrow-leaved weeds and cannot be used to control weeds in broad-leaved crops, and other herbicides are deadly to all plants and thus kill the crop along with the weeds.

The development of genetically modified organisms (GMOs) has increased the applicability of herbicides to many more crop-and-weed combinations in the past decade. As we described in Chapter 7, some crops, such as soybeans and corn, have had a gene inserted in them that makes them resistant to a broad-spectrum herbicide—that is, one that kills all plants. Thus, a field planted with a resistant crop could be sprayed with this herbicide, which would kill everything except the crop.

When water is abundant:

When water is scarce:

When water is abundant:

Guard cells swell Stoma (pore) is large High CO2 uptake High water loss

When water is scarce:

Guard cells shrink Stoma (pore) is small Low CO2 uptake Low water loss

Figure 15.13 Guard cells. Guard cells help regulate the size of stomata on a plant's surface, thus minimizing water loss.

Maximizing Uptake of Carbon Dioxide Carbon dioxide is a gas in the atmosphere. For plant cells to obtain this gas for photosynthesis, the cells must be exposed to the atmosphere. The photosynthetic surfaces of plants are covered with tiny pores called stomata that allow air into the internal structure of leaves and green stems. However, the porosity of leaves and stems comes with a price—stomata also provide portals through which water can escape.

In most plants, each stoma is encircled by a pair of guard cells, which serve to regulate the size of the pore (Figure 15.13). When the guard cells have abundant water, the pore is a large space. When the guard cells are water-deprived, the pore is tiny. The guard cells thus help minimize water loss under dry conditions and maximize carbon dioxide uptake under wet conditions. Therefore, maximizing the carbon dioxide uptake of crop plants requires the same inputs as maximizing light exposure—abundant water and minimal competition from weeds.

Maximizing the Nutrients Needed for Plant Growth Aside from carbon dioxide, all of the nutrients required for plant growth are available in the soil. A plant's roots take up these nutrients, and the less competition between a crop-plant's roots and a weed-plant's roots, the better the crop will grow. Thus, techniques that reduce competition for light, water, and carbon dioxide between crops and weeds also reduce nutrient competition.

In most environments, nutrients become available to plants through nutrient cycling. When materials pass through a food web, the pathway that biotic energy follows in an environment (Figure 15.14a), the nutrients are generally not lost from the environment—hence the term nutrient cycling.

You should note as you review Figure 15.14a that plants absorb simple molecules from the soil and make them into more complex molecules. These complex molecules move through the food web with relatively minor changes until they reach the soil. Here, complex molecules are broken down into simpler

(a) Nitrogen cycle in a natural environment

(a) Nitrogen cycle in a natural environment

Figure 15.14 A nutrient cycle. The movement of nitrogen in (a) a natural system, and (b) an agricultural system, is illustrated here. As energy in the form of food is transferred from one organism to another in a food web, the nutrients are recycled. Notice that nutrients are not recycled to the soil in an agricultural system; they are lost because plants are removed from the system. Fertilizer is added to soil to make up the loss.

Figure 15.14 A nutrient cycle. The movement of nitrogen in (a) a natural system, and (b) an agricultural system, is illustrated here. As energy in the form of food is transferred from one organism to another in a food web, the nutrients are recycled. Notice that nutrients are not recycled to the soil in an agricultural system; they are lost because plants are removed from the system. Fertilizer is added to soil to make up the loss.

ones by the action of decomposers, typically bacteria and fungi. Nutrient cycling in a natural environment relies on a healthy community of these organisms in the soil.

In agricultural systems, the nutrient cycle is often broken, or at least disconnected. Instead of plants being eaten on-site by animals, which are then eaten in the same location by other animals and where their wastes and bodies decompose, the plants from agricultural fields are removed and trucked to other locations where they will be consumed and their nutrients released. In the case of food for human consumption, most of the nutrients that are consumed end up in human waste, which often ends up in a waterway flowing into lakes or oceans. Therefore, in agricultural systems nutrients are not recycled back into the soil, but are essentially mined from it (Figure 15.14b).

Farmers add nutrients to replace what is lost from the soil in the form of fertilizer. Fertilizer that is applied to the soil can be organic—that is, complex molecules made up of the partially decomposed waste products of plants and animals—or inorganic—meaning simple molecules produced by an industrial process. Plants require nutrients in simple, inorganic form—thus organic fertilizer must first be further decomposed before it is available for plant growth. The vast majority of farmers in the United States fertilize primarily with industrially produced inorganic nitrogen, but also may apply large amounts of inorganic phosphorus (an essential component of DNA), or potassium (which is important for maintaining the water balance of plant cells), as well as occasionally other nutrients such as calcium, an important component of cell walls.

(b) In an agricultural system, the nitrogen cycle is disrupted.

Plant protein

Fertilizer (usually inorganic) added to soil to replace nitrogen

Plant protein

Fertilizer (usually inorganic) added to soil to replace nitrogen

Figure 15.14 (continued)

Farmers choose to use inorganic fertilizer because it is more concentrated and thus much easier to transport, store, and apply than most organic fertilizers and also because plants can utilize it immediately.

Farmers can also replace some nutrients in soil through crop rotation; that is, by not putting the same crop in the same field year after year. Surprisingly, some plants actually increase levels of nutrients in the soil. Nitrogen is among the most important nutrients for plants in terrestrial systems, because it is the fourth most abundant chemical in plant cells (after carbon, hydrogen, and oxygen, all of which are components of carbon dioxide and water). Plants that can add nitrogen to the soil do this by having a mutualistic relationship with nitrogen-fixing bacteria (Figure 15.15). These bacteria have the ability to convert nitrogen gas, an abundant component of the atmosphere, into a form that can be taken up by plant roots. Legumes, plants that produce seeds in pods, such as soybeans, alfalfa, clover, and peanuts, have a complex symbiosis with nitrogen-fixing bacteria. These plants develop structures called nodules on their roots in response to chemical signals from certain species of nitrogen-fixing bacteria. The nodules house the bacteria and supply them with energy in the form of carbohydrate. In return, these plants receive the excess fixed nitrogen the bacteria produce. When a legume is growing well, the bacteria produce more fixed nitrogen than the plant requires, and this excess is released into the surrounding soil. If legumes are planted in alternating years with another crop that uses up soil nitrogen (such as corn or wheat), the need for nitrogen fertilizer can be greatly reduced.

Figure 15.15 Nitrogen-fixing crop. The nodules on the roots of this alfalfa plant contain nitrogen-fixing bacteria. The plant provides a home and a food source for the bacteria, and the bacteria convert nitrogen from the air into a form that is easily taken up by the plant.

Figure 15.15 Nitrogen-fixing crop. The nodules on the roots of this alfalfa plant contain nitrogen-fixing bacteria. The plant provides a home and a food source for the bacteria, and the bacteria convert nitrogen from the air into a form that is easily taken up by the plant.

Minimizing Loss to Pests Wheat grains, corn kernels, rice grains, and millet— four of the major staple crops supporting human populations—are all fruits. In fact, most of the calories we consume from plants are from their fruits or seeds (such as beans). The major exceptions are potatoes, which are tubers (a thickened underground stem); cassava, a thickened root; and sugar, the product of either sugar beets (a root) or sugar cane (a stem). Of course, our diet is made up of other fruits as well, such as apples and cucumbers; and we also consume other plant parts, such as the leaves of lettuce or the flowers of broccoli (Figure 15.16). Seeds are small packages containing a plant embryo and an energy-rich food source to support the growth of that embryo. Most fruits function as carriers so the seeds can disperse far from the parent plant. Some fruits help seeds disperse on the wind or water, while others help seeds disperse by using animals to carry the fruit and dropping it, or its seeds, elsewhere (Figure 15.17). Since the seeds and fruits we eat are so packed with energy, they are attractive to many different consumers—including other mammals, birds, insects, fungi, and bacteria. Competing consumers of agricultural products are generally referred to as pests.

Some crop pests directly consume the fruits and seeds we seek to harvest. Other pests reduce the growth of plants, limiting the amount of energy the plant can put into seeds, fruits, and tubers; these pests include insects, fungi, and bacteria that damage the leaves, stems, and roots of plants. Some studies estimate that 42% of potential food production in major crops is lost to pests every year. Thus, farmers have a strong incentive to reduce the number and effect of pests to improve their crop yields.

The primary tools farmers use to reduce pest impact are pesticides, chemicals designed to kill or reduce the growth of a target pest. (Herbicides are also commonly considered to be a class of pesticides. In this book, we use pesticide to refer only to chemicals that control insect, fungal, and bacterial pests.) Pesticide use became widespread in the years following World War II, when the scientists who were developing chemical weapons for the war effort turned these weapons into potent insect killers. Today, millions of tons of these chemicals are applied to crops all over the world (Figure 15.18).

Surprisingly, pesticide use has not reduced the overall loss of crops to pests much. Instead, these chemicals have had a dramatic impact on farming methods—for instance, allowing farmers to plant a single crop over a

1. Flowers attract insects that move pollen from one flower to another, helping fertilization to occur (other flowers are wind pollinated).

Flower petals

Male reproductive structure produces pollen (containing sperm)

Female reproductive structure (ovary) contains eggs

2. Fruits package seeds in a structure that aids their dispersal, such as a tasty fruit or a parachute.

3. Seeds contain an embryo and endosperm (a food source). The seed coat enables some seeds to survive the digestive tracts of some animals.

Flower petals

Male reproductive structure produces pollen (containing sperm)

Female reproductive structure (ovary) contains eggs

Figure 15.17 Flowers, fruits, and seeds. Fruits are the ripened ovary of a flower and contain seeds. From the plant's perspective, fruits serve as a delivery system for seeds.

Seed coat

Embryo

Endosperm

Seed coat

Embryo

Endosperm

Figure 15.17 Flowers, fruits, and seeds. Fruits are the ripened ovary of a flower and contain seeds. From the plant's perspective, fruits serve as a delivery system for seeds.

Figure 15.18 Pesticides. Farmers reduce insect damage to crop plants by spraying pesticides, chemicals that may be toxic to both insects and people.

Figure 15.18 Pesticides. Farmers reduce insect damage to crop plants by spraying pesticides, chemicals that may be toxic to both insects and people.

wide acreage. This practice is called monoculture. Monocultural production of crops is very efficient. Farmers have one planting, fertilizing, and pest-control schedule; they only require the planting and harvesting equipment specific to their crop; and they can use enormous mechanical harvesters to quickly collect enormous amounts of product (Figure 15.19). Monoculture and its accompanying mechanization has greatly reduced the cost of food to consumers by decreasing the amount of labor that is required to produce it. The percentage of the population working on farms in the United States has dropped from 25% to less than 2% in the past 50 years due to this change in farming practice.

Before the availability of pesticides, farmers used cultural control to minimize pest populations. Since most pests have evolved to attack a single crop, the cultural control of crop rotation moves plants away from their pests. A small number of pests in a population of plants can quickly grow into a large population of pests in a single growing season. After the growing season, the pest population spends the winter in the soil or on plant waste left in the field. If the same crop is planted in this field, the pest population has easy access to more resources and continues to grow. However, if a different crop is planted, the pests from the previous crop must disperse in an attempt to find their preferred crop. Only some of these dispersers will find their host, and the population of pests in the new site will be relatively small.

Another form of cultural control is polyculture: planting many different crop plants over a single farm's acreage. This minimizes crop loss and the risk to farmers by ensuring that a pest outbreak on a single crop does not destroy all of the farm's production for that year. Polyculture planting also keeps pest populations relatively small, because after the pests have consumed a patch of plants, they must disperse widely to find another source of food—in a monoculture, another appropriate plant is right next door. Thus, while monoculture increases the efficiency of agricultural production, it results in increased pest populations and an increased need for pesticides just to keep pest damage at historical levels.

Figure 15.19 Monoculture. Planting the same crop over many hundreds of acres allows for the efficient use of massive planting and harvesting equipment.

Designing Better Plants In addition to providing resources to crops to maximize production, farmers have turned to scientists to provide crop plants that respond strongly to these resources. Traditionally, agricultural scientists have used plant breeding to produce better crops. Plant breeding generally consists of creating hybrids, the offspring of two different varieties, or races, of an agricultural crop. For instance, one of the first hybrids produced on a large scale was a wheat plant. One parent variety was a high-producing standard-sized wheat that was difficult to harvest because it tended to fall over when the wheat grains were ripe. The other parent variety was a lower-producing dwarf plant. The hybrid of these two varieties was high producing but short, making it much easier to harvest with the large machines necessary to effectively work on very large farms. Widespread planting of the hybrids sold by seed companies, rather than the numerous varieties that had been maintained on different farms, also ensures that the fruits produced on different farms are relatively uniform in character—this is a benefit especially in the case of wheat, where similar levels of starch makes refining the wheat grains into flour much more efficient. The production of hybrid varieties that produced well and were easy to harvest and process was the foundation of the Green Revolution described in Essay 15.2.

More recently, agricultural scientists have designed plants, called genetically modified organisms, or GMOs, using DNA technology (inserting genes from one organism into the genetic instructions of another organism, see Chapter 7). DNA technology has resulted in the production of genetically modified corn

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