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Tomatoes 2008
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The sexual plant parts include the flowers and fruits. These parts are involved in the reproductive processes of the plant. Flowers The flower, generally the showiest part of the plant, has sexual reproduction as its sole function. Its attractiveness and fragrance have not evolved to please man but to ensure the continuance of the species. Fragrance and color are devices to attract pollinators (insects that play an important role in the reproductive process). Binomial nomenclature is the scientific system of giving a double name to each plant or animal. The first (genus) name is followed by a descriptive (species) name. Modern plant classification, or taxonomy, is based on the system of binomial nomenclature developed by a Swedish physician, Carl von Linné (Linnaeus). Prior to Linnaeus, people based classification on leaf shape, plant size, flower color, etc. However, none of these systems proved workable. Linnaeus’s revolutionary approach based classification on the flowers or reproductive parts of a plant and gave plants a genus and species name. This has proven to be the best system since flowers are the plant part least influenced by environmental changes. For this reason, a knowledge of the flower and its parts is essential for anyone who is interested in plant identification (Figure 1.20). Parts of the flower. As the reproductive part of the plant, the flower contains either the male pollen or the female ovule plus accessory parts such as petals, sepals and nectar glands. Sepals are small, green, leaf-like structures on the base of the flower that protect the flower bud. The sepals are collectively called the calyx. Petals are the brightly colored portions of the flower. They may contain perfume and nectar glands. The number of petals on a flower is often used in the identification of plant families and genera. Petals are collectively called the corolla. Flowers of dicots typically have sepals and/or petals in numbers of four, five or multiples thereof. Monocots typically have these floral parts in threes or multiples of three. The pistil is the female part of the plant. It is generally shaped like a bowling pin and is located in the center of the flower. It consists of the stigma, style and ovary. The stigma is located at the top and is connected by the style to the ovary. The ovary contains the eggs which reside in the ovules. After the egg is fertilized, the ovule develops into a seed. The stamen is the male reproductive organ. It consists of a pollen sac called the anther, as long as a long supporting filament, which holds the anther in position. The pollen contained in the anther may be dispersed by wind or carried to the stigma by pollinating insect, birds, or other animals. Types of flowers. If a flower has stamens, pistils, petals and sepals, it is called a complete flower. If one of these parts is missing, the flower is designated incomplete. If a flower contains functional stamens and pistils, it is called a perfect flower. These are considered the essential parts of a flower and are involved in the seed producing process. If either of the essential parts is lacking, the flower is imperfect. Pistillate (female) flowers are those that possess a functional pistil or pistils but lack stamens. Staminate (male) flowers contain stamens but no pistils. Because cross fertilization combines different genetic material and produces stronger seed, cross-pollinated plants are usually more successful than self-pollinated plants. More plants reproduce by cross-pollination than by self-pollination. There are plants that bear only male flowers (staminate plants) or bear only female flowers (pistillate plants). Species in which the sexes are separated into staminate and pistillate plants are called dioecious. Most holly trees are either male or female plants. Therefore, in order for a female tree to produce berries, it is necessary to have a male tree nearby to provide pollen. Monecious plants are those that have separate male and female flowers on the same plant, such as corn plants and pecan trees. Some plants, like cucumbers and squash, bear only male flowers at the beginning of the growing season, but later develop both sexes. How seeds form. Pollination is the transfer of pollen from an anther to a stigma. It may occur by wind or by pollinators. Wind-pollinated flowers lack showy floral parts and nectar since they don’t need to attract pollinators. Flowers are brightly colored or patterned and contain a fragrance or nectar when they must attract pollinators, such as insects, animals or birds. In the process of searching for nectar, these pollinators will transfer pollen from flower to flower. The stigma contains a chemical that excites the pollen, causing it to grow a long tube down the inside of the style to the ovules inside the ovary. The sperm is released by the pollen grain and fertilization typically occurs. Fertilization is the union of the male sperm nucleus from the pollen grain and the female egg found in the ovary. If fertilization is successful, the ovule will develop into a seed. Types of inflorescence. Some plants bear only one flower per stem and are called solitary flowers. Other plants produce an inflorescence, a term that refers to a cluster of flowers and how they are arranged on a floral stem. Most inflorescences may be classified into two groups, racemes and cymes (Figure 1.21). In the racemose group, florets, which are individual flowers in an inflorescence, bloom from the bottom of the stem and progress toward the top. Some examples of racemose inflorescence include spike, raceme, umbel, corymb and head. A spike is an inflorescence in which many stemless florets are attached to an elongated flower stem or peduncle. An example is gladiolus. A raceme is similar to a spike except the florets are borne on small stems attached to the peduncle. An example of a raceme inflorescence is the snapdragon. A corymb is made up of florets whose stalks or pedicels are randomly arranged along the peduncle in such a way that the florets create a flat, round top. Yarrow has a corm inflorescence. An umbel is similar except that the pedicels all arise from one point on the peduncle, such as dill. A head or composite inflorescence is made up of numerous, stemless florets, which is characteristic of daisy inflorescence. The second group of inflorescences is called a cyme. In this case, the top floret opens first and blooms downward along the peduncle. A dischasium cyme has florets opposite each other along the peduncle, such as Baby’s-breath inflorescence. A helicoid cyme is one in which the lower florets are all on the same side of the peduncle, examples being freesia and statice inflorescences. A scorpioid cyme is one in which the florets are opposite each other along the peduncle, examples being tomato and potato inflorescences. Fruits Parts of fruit. Fruit consists of the fertilized and mature ovules (seeds) and the ovary wall. The wall may be fleshy, as in an apple, or dry and hard, as in a maple fruit. The only parts of the fruit which are genetically representative of both the male flower and female flower are the seeds (mature ovules). The rest of the fruit arises from the maternal plant and is, therefore, genetically identical to that parent plant. Some fruits have seeds enclosed within the ovary, such as apples, peaches, oranges, squash and cucumbers. Others have seeds that are situated on the periphery of fruit tissue, such as corn and strawberries. Types of fruit. Fruits can be classified as simple fruits, aggregate fruits or multiple fruits (Figure 1.22). Simple fruits develop from a single ovary, such as cherries, peaches (drupe), pears, apples (pome) and tomatoes (berries). Tomatoes are a botanical fruit since they develop from the flower, as do squash, cucumbers and eggplant. All of these fruits develop from a single ovary. Other types of simple fruit are dry. The fruit wall in these fruits becomes papery or leathery and hard as opposed to the fleshy examples just mentioned. Examples are peanut (legume), poppy (capsule), maple (samara) and walnut (nut). An aggregate fruit comes from a single flower that has many ovaries. The flower appears as a simple flower with one corolla, one calyx and one stem but with many pistils or ovaries. The ovaries are fertilized separately and independently. Examples are strawberry, raspberry and blackberry. If ovules are not pollinated successfully, the fruit will be misshapen and imperfect. Multiple fruits are derived from a tight cluster of separate, independent flowers borne on a single structure. Each flower will have its own calyx and corolla. Pineapple, fig and the beet seed are examples of multiple fruits. The seed or matured ovule is made up of three parts (Figure 1.23). The embryo is a miniature plant in an arrested state of development. Most seeds contain a built-in food supply called the endosperm (the orchid is an exception). The endosperm can be made up of proteins, carbohydrates or fats. The third part is the hard outer covering, called a seed coat, which protects the seed from disease and insects. The seed coat also prevents water from entering the seed and initiating the germination process before the proper time. Plant Processes The three major processes in plant growth and development are photosynthesis, respiration and transpiration (Figure 1.24). Photosynthesis Photosynthesis literally means “to put together with light.” One of the major differences between plants and animals is the plant’s ability to manufacture its own food. To produce food for itself, a plant requires energy from sunlight, water from soil and carbon dioxide from air. If any of these ingredients is lacking, photosynthesis (or food production) will stop. If any factor is removed for a long period of time, the plant will die. The photosynthetic reaction is shown in Equation 1. Plants can store the energy from light in carbohydrates such as sugars and starches for use during days when light is limited, or they can transport these chemicals to the roots. Sugars and starches are converted back to water and carbon dioxide (CO2), and the stored energy is released to perform activities necessary for growth in a process called respiration. Only cells in the mesophyll layer of the plant leaves and stems can manufacture energy. These cells, which contain chloroplasts, are located between and protected by the upper and lower epidermis of the leaf (Figure 1.25). The green pigment, chlorophyll, is found in the chloroplasts of these cells and traps light energy so it can be used to manufacture sugar and starches. Photosynthesis is dependent on the availability of light. Generally speaking, as sunlight increases in intensity, photosynthesis increases. This means greater food production. Many garden crops such as tomatoes respond best to maximum sunlight. As light intensity decreases, tomato production is drastically reduced. Only two or three varieties of tomatoes will produce any fruit at all in greenhouses in late fall and early spring months when sunlight is minimal. Water plays several important roles in photosynthesis. First, it maintains a plant’s turgor, or the firmness or fullness of plant tissue. Turgor pressure in a cell can be compared to air in an inflated balloon. Water pressure or turgor is needed in plant cells to maintain shape and ensure cell growth. Second, water is split into hydrogen and oxygen by the energy of the sun that has been absorbed by the chlorophyll in the plant leaves. The oxygen is released to the atmosphere and the hydrogen is used in the manufacture of carbohydrates. Third, water dissolves minerals from the soil and transports them from the roots throughout the plant. These minerals serve as raw materials in the growth of new plant tissues. The soil surrounding a plant should be moist, but not too wet or too dry. Water is pulled through the plant by evaporation of water through the leaves, a process known as transpiration. When the guard cells in leaves inflate, thus opening the stomata, water is lost from the leaf. Photosynthesis also requires carbon dioxide, which enters the plant through the stomata. Carbon dioxide is split into carbon and oxygen, which are used in the manufacture of carbohydrates. Carbon dioxide in the air is plentiful enough so that it is not a limiting factor in plant growth. However, since carbon dioxide is consumed in making sugars and is not replenished by plants at a rapid rate, a tightly closed greenhouse in midwinter may not allow enough outside air to enter the greenhouse to maintain an adequate carbon dioxide level. Under these conditions, improved crops of roses, carnations, tomatoes and certain other crops are produced if the carbon dioxide level is raised with carbon dioxide generators or, in small greenhouses, with dry ice. Although not a direct component in photosynthesis, temperature is an important factor. Photosynthesis occurs at its highest rate at temperatures ranging from 65E to 85EF (18E to 27EC), and decreases when temperatures fall above or below this range. Respiration Carbohydrates made during photosynthesis are of value to the plant when they are converted to energy. This energy is used in the building of new tissues or in plant growth. Oxidation is the chemical process by which sugars and starches produced by photosynthesis are converted to energy. It is similar to burning wood or coal to produce heat. Controlled oxidation in a living cell is known as respiration and is shown by the Equation 2. This equation is the opposite of that used to illustrate photosynthesis. Therefore, photosynthesis maybe called a building process, while respiration is a breaking-down process. By now it should be clear that respiration is the reverse of photosynthesis (Table 1.1). Unlike photosynthesis, respiration occurs at night as well as during the day. Respiration occurs in all life forms and in all cells. The release of accumulated carbon dioxide and the uptake of oxygen occurs at the cell level. In animals, blood carries both carbon dioxide and oxygen to and from the atmosphere by means of the lungs or gills. In plants, there is simple diffusion into the open spaces within the leaf and exchange through the stomates. Transpiration Transpiration is the process by which a plant loses water primarily from leaf stomata. Transpiration is a necessary process that uses about 90 percent of the water that enters the plant through the roots. The other 10 percent of water is used in chemical reactions and in plant tissues. Transpiration is necessary for mineral transport from the soil to the plant parts, for cooling plant parts through evaporation, to move sugars and plant chemicals, and to maintain turgor pressure. The amount of water lost from the plant depends on several environmental factors, such as temperature, humidity and wind or air movement. As temperatures or air movement increase, transpiration increases. As humidity decreases, transpiration increases. Plant Hormones Plant hormones are naturally occurring chemicals produce in one part of the plant and translocated to another part of the plant where it performs its physiological function. The function of many hormones may be mimicked by synthetically produced plant growth regulators. The class of plant hormone/plant growth regulator most commonly used by gardeners are the auxins. The hormone indole acetic acid is produced in plant buds and translocates to the roots where it stimulates root growth. Indole butyric acid and Napthelene acetic acids are among several commonly produced auxin plant growth regulators available in nurseries as a powder applied to the base of stem cuttings to stimulate the formation of roots. Natural auxins are also responsible for the phenomenon called apical dominance which refers to the fact that the auxin produced in a terminal bud inhibits the growth of lateral buds below the terminal bud. When the terminal bud is lost due to pruning or injury, the lower buds are released from this control and begin to grow. Gibberillins are another important hormone, but are rarely used by home gardeners as a product applied to plants. Its natural function within the plant is to stimulate flowering in some plants and to cause stem elongation observed in bolting of lettuce and other biennial plants. It is used commercially to cause enlargement of some varieties of seedless grapes. Ethylene gas also has hormonal properties and is produced in some plants. It causes flowering in some plants and causes fruit ripening in some plants. Environmental Factors Affecting Plant Growth Plant growth and distribution are limited by the environment. If any single environmental factor is less than ideal, it becomes a limiting factor in plant growth. Limiting factors are also responsible for the geography of plant distribution. For example, only plants adapted to limited amounts of water can live in deserts. Most plant problems are caused by environmental stress, either directly or indirectly. Therefore, it is important to understand the environmental aspects that affect plant growth. These factors are light, temperature, water, humidity and nutrition. Light Light has three principal characteristics that affect plant growth. These are light quality, light quantity and light duration. Light quality refers to the color or wavelength reaching the plant surface. Sunlight can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors, producing a rainbow. Red and blue light have the greatest effect on plant growth. Green light is least effective to plants because it is reflected, not absorbed. It is this reflected light that makes them appear green to us. Blue light is primarily responsible for vegetative growth or leaf growth. Red light, when combined with blue light, encourages flowering in plants. Fluorescent light or cool white light is high in the blue range of light quality and is used to encourage leafy growth. Such light is excellent for starting seedlings. Incandescent light is high in the red or orange range, but generally, it produces too much heat to be a valuable light source. Fluorescent grow lights have a mixture of red and blue colors that attempt to imitate sunlight as closely as possible, but they are costly and are generally not of any greater value than regular fluorescent lights. Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. The maximum is present in summer and the minimum in winter. The more sunlight a plant receives, up to a point, the better capacity it has to produce plant food through photosynthesis. As the quantity of sunlight decreases, the photosynthetic process decreases. Light quantity can be decreased in a garden or greenhouse by using cheesecloth shading above the plants. It can be increased by surrounding plants with reflective material, white backgrounds or supplemental lights. In addition to season variations in light intensity, global latitude directly affects the intensity of sunlight. Light intensity is greatest near the equator and lessens with distances both north and south of the equator. For example, the increased light intensity in New Mexico versus Kansas may explain why some plants (i.e., bluegrass) thrive in Kansas’ summers and burn up in New Mexico's summers, even though air temperatures are equal. The dry air, higher elevation, and more southerly latitude result in higher light levels. Even in New Mexico, there are differences in light intensity and climate. According to the USDA Plant Hardiness Zone Map, New Mexico has five distinct hardiness zones. The hardiness values assigned to these zones are also accompanied by varying degrees of light intensity. However, elevation and cloudiness will have the greatest influence on light intensity in New Mexico. Light duration refers to the amount of time a plant is exposed to sunlight and is designated as the photoperiod. When the photoperiod was first recognized, it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short day and long day). It was then discovered that it is not the length of the light period but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by the photoperiod. Plants can be classified into three categories depending upon their flowering response to the duration of light or darkness. These are short-day, long-day or day-neutral plants. Short-day plants form their flowers only when day length is less than about 12 hours in duration. Short-day plants include many spring and fall flowering plants, such as chrysanthemum and poinsettia. Long-day plants form flowers only at day lengths exceeding 12 hours (short nights). They include almost all of the summer flowering plants such as rudbeckia and California poppy, as well as many vegetables including beet, radish, lettuce, spinach and potato. Day-neutral plants form flowers regardless of day length. Some plants do not fit into any category but may be responsive to combinations of day lengths. The petunia will flower regardless of day length, but it will flower earlier and more profusely under long daylight. Since chrysanthemums flower under the short days of spring or fall, the method of manipulating the plant into experiencing short days is very simple. If long days are predominant, a shade cloth is drawn over the chrysanthemum for 12 hours daily to block out light until flower buds are initiated. To bring a long-day plant into flower when sunlight is not longer than 12 hours, artificial light is added until flower buds are initiated. Temperature Temperature affects the growth and productivity of a plant. The degree of this effect depends upon whether the plant is a warm or cool season crop. If temperatures are high and day length is long, cool season crops such as spinach will flower. Temperatures that are too low for a warm season crop such as tomato will prevent fruit set. Adverse temperatures also cause stunted growth and poor quality vegetable production. For example, bitterness in lettuce is caused by high temperatures. Sometimes, temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daylight temperatures are 59EF (15EC). The Christmas cactus forms flowers as a result of short days and low temperatures. Temperatures alone also can influence flowering. Daffodils are forced to flower by putting the bulbs in cold storage at 35E to 40EF (2E to 4EC) in October. Cold temperatures allow the bulb to mature, and then the bulbs are transferred to the greenhouse where growth begins in midwinter. The flowers are then ready for cutting in 3 to 4 weeks. Thermoperiod refers to a daily temperature change. Plants respond and produce maximum growth when exposed to a day temperature that is about 10 to 16 degrees higher than a night temperature. This allows the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature and to curtail the rate of respiration during a cooler night. Temperatures higher than needed cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur, photosynthesis must be greater than respiration. Referring back to the blue grass example used for the effect of light intensity on plants, bluegrass may also thrive in Taos and not in Las Cruces because of an imbalance of photosynthesis and respiration. With relatively hot nighttime temperatures in Las Cruces compared to Taos, respiration of the bluegrass plant may exceed photosynthesis and thus lead to unhealthy or dead grass. Low temperatures can produce poor growth. Photosynthesis is slowed by low temperatures. Since photosynthesis is slowed, growth is slowed, resulting in lower yields. Not all plants grow best under the same temperature range. For example, snapdragons grow best at nighttime temperatures of 66EF (12EC) and the poinsettia at 62EF (17EC). Florist cyclamen does very well under very cool conditions, while many bedding plants prefer a higher temperature. Recently it has been found that roses can tolerate much lower nighttime temperatures than previously believed. This has resulted in energy conservation for greenhouse growers. In some cases, however, a certain number of days of low temperature are needed by plants in order to grow properly. This is especially true of crops growing in cold regions of the country. Peaches are a prime example. Most varieties require 700 to 1,000 hours below 45EF (7EC) and above 32EF (0EC) before they break their rest period and begin growth. Lilies need 6 weeks of temperatures at 33EF (1EC) before blooming. Plants can be classified as either hardy or nonhardy depending upon their ability to withstand cold temperatures. This is the basis of the USDA Plant Hardiness Zone Map (Figure 1.26). It should be mentioned that the Hardiness Zone Map does not consider the plant’s ability to withstand various soil types (i.e., alkaline versus acidic, clay versus sand). Winter injury can occur to nonhardy plants if temperatures are too low or if unseasonably low temperatures occur early in fall or late in spring. Winter injury may also occur because of desiccation or drying out. Plants need water during winter. When the soil is frozen, the movement of water into the plant is severely restricted. On a windy winter day, broadleaf evergreens can become water-deficient after a few minutes; then the leaves or needles turn brown. Wide variations in temperatures can cause premature bud break in some plants and consequent fruit bud freezing damage. Late spring frosts can ruin entire peach crops. If temperatures drop too low during the winter, entire trees of some species are killed by freezing and splitting plant cells and tissue. Table 1.2 provides a review of the effect of temperature on plant growth. Water As mentioned earlier, water is a primary component in photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of stomates, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of solvent for the plant' nutrients and for moving carbohydrates to their site of use or storage. Water is important in the chemical reactions of photosynthesis and respiration. By its gradual evaporation (transpiration) from the leaf surface near the stomate, water helps stabilize plant temperature. Relative humidity
greatly affects the rate of transpiration and water use by the plant.
Relative humidity, expressed as a percentage, is the ratio of water vapor
in the air at a given temperature and pressure to the amount of water the
air could hold at that temperature and pressure. For example, if a pound
of air at 75EF could hold 4 grams of water vapor and there are only 3
grams of water in the air, then the relative humidity (RH) is: Warm air holds more water vapor than cold air; therefore, if the amount of water in the air stays the same and the temperature increases, the relative humidity decreases. Water vapor will move from an area of high relative humidity to one of low relative humidity. The greater the difference in humidity, the faster water will move. The relative humidity in the air space between the cells within the leaf approaches 100 percent; therefore, when the stomate is open water vapor rushes out. As water moves out, a bubble of high humidity is formed around the stomate (Figure 1.27). This bubble of humidity helps slow down transpiration and cools the leaf. If winds blow the humidity bubble away, transpiration will increase. Movement of water through the plant. The cohesion theory best explains how water moves into and through a plant. It is through this theory that one can begin to understand how water moves from the root system of a California redwood through the vascular system and ultimately to the tips of the leaves some 350 feet above ground. There are three basic elements of the cohesion theory — the driving force, hydration of the pathway, and the cohesion of water. The driving force for the movement of water through the plant is the tremendous affinity for water that dry air has. Discussed earlier in terms of relative humidity, water moves from an area of high water concentration to an area of lower concentration. For example, air at a relative humidity of 50 percent will pull (suck) moisture from plant tissue, which is near 100 percent saturation. This process was discussed earlier and is known as transpiration. The hydration component refers to water’s ability to adhere with great strength to the surface of cell walls. As water is sucked through the plant by transpiration, hydration keeps the water moving upward, preventing it from receding back down the plant due to gravity forces. Cohesion of water is the key component of the theory. Water is highly resistant to changes in volume and can be subjected to strong suction or tension. The driving force of transpiration can pull water from the soil into the roots and up into the plant. Through the properties of hydration and cohesion, water can then be pulled to the top of even a 350 foot redwood tree. Plant response to lack of water. When plants experience a lack of water in the soil, several responses can occur. The most common sign of drought stress is wilting. However, plants also show other signs, including leaf rolling, color changes, leaf burning and loss of leaves. Most of the turf grasses show stress by wilting, as indicated when footprints are seen after a walk across the lawn. Turf grasses with wider leaves will roll their leaves lengthwise in an attempt to reduce the leaf area and water loss. Lawn grasses often show a dullness versus the shiny green of a healthy plant. Many vegetables, flowers and shrubs will show these signs and/or burning of the leaf edges or margins. The crispy margins occur when less than adequate supplies of water are flowing through the plant. Some plants in the landscape and garden will also drop leaves or fruit during drought stress. The plant is simply attempting to lighten the demand for water and increasing its ability to survive drought. Two examples of this response are: 1) ocotillo (Fouquieria splendens), a desert plant that drops its leaves under water stress and 2) the common fig, which drops its fruit at the first sign of water shortage. Managing plant water stress. The goal of the home gardener is to reduce plant water stress in order to maintain a quality landscape and/or a productive garden. When adequate moisture is available to the plant, a continuous flow of water exists from the root hairs up to the leaves. If inadequate moisture is present in the soil or if the rate of evaporation from the leaves exceeds the rate at which water can be moved upwards by the plant, then water stress ensues. During hot and dry summer months, moderate stress can be tolerated by most plants on a daily basis as long as moisture is replenished during the low-stress night period. However, severe or prolonged moisture stress will result in permanent wilting and damage to the plant. Plants differ greatly in their ability to extract water from the soil and in the absolute amount of water required for normal plant growth and development. Some plants, in fact, are classified as “drought tolerant” because they can function with “dry” soil conditions. Drought tolerance can be due to several physical features:
Too much water in the root zone can also be damaging to the plant due to a reduction in oxygen in the area around the root hairs. This can occur when irrigation is too frequent or too much for the plant to remove and use from the root zone. Thus, the objective of a proper irrigation schedule is to supply the right amount of water before harmful stress occurs and to supply enough water at that time to replenish the amount of water used since the last irrigation. |