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Tomatoes 2008
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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. |