Water is one of the substances needed for photosynthesis and must be pumped from the roots of the plant. The "engine" pulling water and nutrients up the plant is transpiration. Nutrients are absorbed from the soil and moved throughout the plant's cells by way of transpiration. The minerals distributed during this process are necessary for biosynthesis in the leaves. There are many environmental factors that can affect the rate of transpiration. I will address five of the most important here; light, temperature, humidity, wind, and soil water.
Light stimulates the opening of the stomata at daybreak. As the stomata opens to allow photosynthesis to occur, the transpiration rate increases. With light comes heat. The leaf can be heated by the temperature of the environment and also by the heat released during photosynthesis. Transpiration provides a cooling mechanism for the plant to release excess heat in the leaves and maintain internal temperature necessary for biological and chemical processes to occur. Transpiration occurs more quickly at higher temperatures due to increased evaporation.
Summer tends to be a time of decreased transpiration in plants because of increased temperature. In dry climates transpiration is increased. Water is forced to diffuse more rapidly into the air due to the concentration difference between the environments outside and inside the plant.
Low humidity creates a vapor gradient between the plant and the air. In dry air, there is a lack of water, forcing water to be pulled from the plant to the atmosphere increasing transpiration.
Therefore, in humid climates, transpiration is less effected by diffusion. On windy days the moisture present in the air is swept away from the leaf causing it to transpire more. On calmer days, the humidity rate can rise causing a decrease in transpiration. The amount of wate rin the soil also plays a major role in the rate of transpiration.
The plant must have a continuous supply of water to be able to transpire. If adequate water cannot be absorbed by the roots and carried up the xylem, the rate of transpiration will decrease. A lack of water supply will also decrease the rate of photosynthesis and the overall health of the plant.
Transpiration can be hazardous to plants if there is a higher rate of transpiration than rate of moisture absorption through the roots. This is called moisture stress or plant stress. This often happens to houseplants in the winter months when we increase the ambient temperature. Furnaces typically create dry heat which results in a warm, dry environment. Even well watered plants may wilt if the plant cannot adapt it transpiration rate. Plant Adaptations.
As discussed above, environmental factors can play a large role in the rate of transpiration. Plants in hot arid environments have found ways of limiting their water loss to avoid dehydration. Some of the adaptations desert plants use are: the absence of leaves, stomata that can open and close or that only open at night, C4 photosynthesis, special water storage capabilities, alternative root structures, and periods of dormancy.
Xerophytes are plants that have adapted by altering their physical structure. These plants exhibit several adaptations which allow them to survive in harsh climates.
Xerophytic plants, such as cacti, do not have leaves but instead depend on chlorophyll in the outer tissue of their skin to conduct photosynthesis. By eliminating leaves or greatly reducing leaf size, transpiration is reduced.
The waxy surface of their skin seals in moisture and produces food for the plant. In general, evapotranspiration is the sum of evaporation and transpiration. Some definitions include evaporation from surface-water bodies , even the oceans. But, since we have a Web page just about evaporation, our definition of evapotranspiration will not include evaporation from surface water. Here, evapotranspiration is defined as the water lost to the atmosphere from the ground surface, evaporation from the capillary fringe of the groundwater table , and the transpiration of groundwater by plants whose roots tap the capillary fringe of the groundwater table.
The banner at the top of this page offers an even more simple definition. The transpiration aspect of evapotranspiration is essentially evaporation of water from plant leaves. Studies have revealed that transpiration accounts for about 10 percent of the moisture in the atmosphere, with oceans, seas, and other bodies of water lakes , rivers, streams providing nearly 90 percent, and a tiny amount coming from sublimation ice changing into water vapor without first becoming liquid.
Just as you release water vapor when you breathe, plants do, too — although the term "transpire" is more appropriate than "breathe. If the bag had been wrapped around the soil below it, too, then even more water vapor would have been released, as water also evaporates from the soil. Plants put down roots into the soil to draw water and nutrients up into the stems and leaves.
Some of this water is returned to the air by transpiration. Transpiration rates vary widely depending on weather conditions, such as temperature, humidity, sunlight availability and intensity, precipitation, soil type and saturation, wind, and land slope.
During dry periods, transpiration can contribute to the loss of moisture in the upper soil zone, which can have an effect on vegetation and food-crop fields.
After a plastic bag is wrapped around part of a plant, the inside of the bag becomes misty with transpired water vapor. Plant transpiration is pretty much an invisible process. Since the water is evaporating from the leaf surfaces, you don't just go out and see the leaves "breathing". Just because you can't see the water doesn't mean it is not being put into the air, though. One way to visualize transpiration is to put a plastic bag around some plant leaves. As this picture shows, transpired water will condense on the inside of the bag.
During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,, gallons 11,, liters of water each day, and a large oak tree can transpire 40, gallons , liters per year. The amount of water that plants transpire varies greatly geographically and over time. There are a number of factors that determine transpiration rates:. In many places, the top layer of the soil where plant roots are located is above the water table and thus is often wet to some extent, but is not totally saturated, as is soil below the water table.
The soil above the water table gets wet when it rains as water infiltrates into it from the surface, But, it will dry out without additional precipitation.
Temperature Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. Humidity The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases. When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly. Wind When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration.
Evaporative cooling: As water evaporates or converts from a liquid to a gas at the leaf cell and atmosphere interface, energy is released. This exothermic process uses energy to break the strong hydrogen bonds between liquid water molecules; the energy used to do so is taken from the leaf and given to the water molecules that have converted to highly energetic gas molecules.
These gas molecules and their associated energy are released into the atmosphere, cooling the plant. Accessing nutrients from the soil: The water that enters the root contains dissolved nutrients vital to plant growth. It is thought that transpiration enhances nutrient uptake into plants.
Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf.
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