TRANSPIRATION

TRANSPIRATION

Introduction:

·         Transpiration explains how water moves up the plant against gravities in tubes made of dead xylem cells without the use of pump.

·         Water on the surface of surface of spongy and palisade cells (inside the leaf) evaporate and then diffuses amount of the leaf. This is called transpiration.

·         More water is drawn out of the xylem cells inside the leaf to replace what’s lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this act like a drink of straw, producing a flow of water and dissolved minerals from roots to leaves.

·         Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. Plants absorb soil water through their roots and this water can originate from deep in the soil. For example, corn plants have roots that are 2.5m deep, which some desert plants have roots that extend 20m into the ground.). Plants pump the water up from the soil to deliver nutrients to their leaves.

·         This pumping is driven by the evaporation of water through small pores called “stomates”, which are found on the underside of leaves.

·         Transpiration accounts for approximately 10% of all evaporating water.

·         It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis.

·         Transpiration also includes a process called guttation, which is the loss of water in liquid form from the uninjured leaf or stem of the plant, principally through stomata.

·         Studies have revealed that about 10% of the moisture found in the atmosphere is released by plants through transpiration. The remaining 90% is mainly supplied by evaporation from oceans, seas and other bodies of water (lakes, rivers, streams).

·         Plant transpiration is pretty much an invisible process, since the water is evaporating from the leaf surfaces. An acre of corn gives off about 3,000-4,000 gallons (11,000-151,000) litre of water each day and a large oak tree can transpire 40,000 gallons (151,000) litre/year.

·         Plants need to cool themselves for several reasons. When temperature are too high, energy systems (metabolic function), growth and flowering slows and stops. In extreme heat, plants are severely stressed and can decline. Sometimes heat will cause bubbles to form that block the flow of water, leading to dehydration. Transpiration is an evaporative cooling system that brings down the temperature of plants.

Definition:

Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it change to vapour and is released to the atmosphere.

Environmental factors that affect the rate of transpiration:

1.       Light: Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata. Light also speed up transpirate by warming the leaf.

2.       Temperature: Plants transpire more rapidly at higher temperature because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.

3.       Humidity:The rate of diffusion of any substances increases as the difference in concentration of all substances in the two region increases. When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

4.       Wind: When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.

5.       Soil water:Water move from soil into plant roots, up through the A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs and the stomata close. This immediately reduces the rate of transpiration. If the loss of turgor extends to the rest of leaf and stem, the plant wilts.

6.       Soil moisture availability: When moisture is lacking, plants can begin to senesce (premature ageing, which can results in leaf loss) and transpire loss water.

7.       Types of plant: Plants transpire water at different rates. Some plants will grow in arid regions such as cacti and succulents. Conserve precious water by transpiring less water than other plants.

Transpiration rate:

A number of factors affect transpiration rates for plants and regulating the amount of water loss while still exchange the necessary amount of gas through stomata.  Plants naturally transpire at different rates but there are some factors that affect the rate of water loss:

1.       Stomata can regulate the rate of transpiration via guard cells.

2.       Having more stomata will allow for more transpiration

3.       Stomata that are open wider will facilitate a greater rate of transpiration while stomata that are more narrowly spaced will decrease the rate of transpiration.

4.       Since stomata open during photosynthesis to take in CO2, lighter will signal the stomata to open which in turn will increase the rate of transpiration.

5.       As the temperature increases transpiration rate will increases.

6.       Warmer temperature is usually associated with sunlight and the growing season. So, higher temperature causes guard cells to open the stomata while cold weather signals the guard cell to close the stomata.

Importance of transpiration:

·         It is the “engine” that pulls water up from the roots.

·         Transpiration supply photosynthesis (1-2% of the total).

·         Transpiration brings minerals from the roots for biosynthesis within the leaf.

·         Transpiration helps in cooling of leaf.

Interesting facts about transpiration:

·         A leaf transpire about 90% of the water evaporates from the water surface of same area, even though the combined area of stomatal pores is only 1-2% of the total leaf area.

·         Transpiration rates are highest in leaves that are stiff with turgor when leaves wilt, they offer less surface area to sun exposure, and thus will transpire less and saving water.

·         Succulents save water by opening the stomatal pores at night to reduce transpiration and to take in CO2 which is stored in their leaves until the next day when they can photosynthesize.

·         Cacti don’t have leaves; they have few stomata in their green stems. Due to less stomata they transpire very little.

·         Many xeric plants have small silvery reflective leaves or produce essential oils which are the strategies to reduce transpiration by reducing evaporation.

·         In the summer, a large maple tree can transpire 50-60 gallons of water/hour. Into the atmosphere. This adds to the humidity which in the west helps us to feel more comfortable, is less drying for our skin and reduces bronchial problems in our lungs. It also helps in cooling our environment.

·         When there are large areas of trees, the combined effect of their transpiration can create a super saturated condition in the clouds, which can result in rain or snow.                                                          For example, snows of Kilimanjaro have disappeared primarily because deforestation then reduced local transpiration resulting in less snowfall.

·         Anti- transpiration products like wilt- Preef can reduce transpiration by covering the stomata pores, but also limits photosynthesis. They can be valuable sprayed on evergreens to get then through their first winter in our landscape.

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