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Botany |
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KEY CONCEPTS
I. Plant Parts
II. Plant Processes
III. Environmental Factors
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III. ENVIRONMENTAL FACTORS
Temperature can affect plant growth in the following ways:
- increasing or decreasing photosynthesis
- increasing or decreasing respiration
- increasing or decreasing transpiration
- increasing or decreasing food storage
- inducing or breaking DORMANCY
- killing plant tissue
Temperature is the factor that dramatically influences plant growth. The gardener must be able to distinguish COOL-SEASON CROPS from WARM-SEASON CROPS. Spinach or leaf lettuce, cool-season crops, will bolt and become bitter if temperatures are too high. For warm-season crops, such as tomatoes and peppers, cool temperatures will delay growth and prevent fruit set.
Peach crops ruined by late frost, are an example of plant tissues killed by cold temperatures once dormancy is broken and growth begins. Peaches must experience 500 to 750 hours below 40o F to end their dormancy. Once growth begins, buds no longer protect cambial tissue, and late frosts and cold temperatures will damage these tender tissues. Lilies and tulips are other examples of plants that require a certain number of weeks of low temperatures before blooming.
The plant's ability to withstand cold temperature is called hardiness. However, hardiness or "survivability" of plants also includes summer heat tolerance, water requirements and humidity needs. Knowledge of a plant's hardiness limitations is essential for success in growing.
Hardiness maps are in the reference section, Reference Bot.1, 2 and 3. Ohio is usually considered to be in Zone 5. There is a region within a few miles of Lake Erie where winter and summer temperatures are less severe due to the moderating effects of a large body of water. In the fall the water maintains warmer temperatures on the nearby land. In spring the same area is kept cooler by these same waters.
Within each garden site there may be different MICROCLIMATES due to exposure, soil conditions and nearby plants. Some plants, such as broadleaf evergreens, are particularly sensitive to temperature or drying winter winds. Examples of protected sites include those bordered by walls, buildings, or evergreen screens, and those where the ground changes slope. These sites offer plants protection from full exposure to harsh weather conditions.
FROST POCKETS are another example of the differences created by microclimates. Weather conditions are often quite different in large cities compared to the surrounding countryside. Large expanses of heat-absorbing and heat-retaining pavement and the effects of large buildings cause these differences.
Water affects plant growth by the following:
- being necessary for photosynthesis
- keeping plants firm or TURGID
- carrying nutrients throughout the plant
- cooling the plant during warm weather
Water acts as a SOLVENT for minerals and stored food that must be moved through the plant. Water evaporation on the leaf surface helps cool the plant during very hot weather.
Plants even require water during winter. Water is absorbed through and stored in roots for winter. When soil is frozen, water movement is very restricted. On windy winter days, leaves of broadleaf evergreens, such as rhododendrons, may curl to reduce water loss. Curling reduces the amount of leaf surface exposed to drying winds, reducing evaporation.
Continued water loss may cause tissue to die and turn brown. This kind of plant injury is called desiccation. It is critical that broadleaf evergreens have an adequate supply of soil moisture going into winter.
The three aspects of light that affect plant growth are quality, quantity and duration.
Light is made up of wavelengths. Sunlight consists of a wide range of wavelengths, some of which cannot be seen by the human eye. Artificial light can come close to duplicating the full range of wavelengths found in sunlight. One warm white and one cool fluorescent bulb used together will come close to producing the full spectrum of light wavelengths for indoor growing. Special grow lights also produce this quality of light. Incandescent lights produce an adequate quality light, but their heat may damage plant tissues.
Quantity describes intensity or concentration. The quantity of light decreases with distance and angle from the source. Light quantity is measured in foot candles.
Plants receive maximum quantity of sunlight in summer and minimum in winter because of the angle at which the wavelengths of light hit the plant surface. Plants must be positioned 2 to 6 inches from an artificial light source for the light to be effective.
Duration is the time that a plant is exposed to light, critical to growth and flowering. In indoor growing situations, lights must be on for 12 to 16 hours in most instances. Outdoors the duration of light varies with the season. Summer days have a longer duration of light than winter days.
Some plants flower in response to the length of darkness they experience. This response to light and dark periods is called photoperiodism. Some plants are short-day types that form flowers
when day length is 12 hours or less. Poinsettia and chrysanthemum are popular plants that require short day lengths in order to flower.
Long-day plants form flowers during day lengths of more than 12 hours. Long-day plants include those plants that bloom outdoors in summer. Spinach and lettuce are examples of long-day plants.
Day-neutral plants do not depend on day length to form flowers, as long as there is enough light for normal growth. Everbearing strawberries are an example of day-neutral plants.
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