Transpiration 2018-19 Plant Physiology PDF

STT 1043 PLANT PHYSIOLOGY LU 5: TRANSPIRATION  Transpiration Loss of water from plants in the form of vapour Transpiration can be divided into 2 phases: Evaporation of water in mesophyll cells bordering intercellular spaces Diffusion. Temperature h...

STT 1043 PLANT PHYSIOLOGY LU 5: TRANSPIRATION  Transpiration Loss of water from plants in the form of vapour Transpiration can be divided into 2 phases: Evaporation of water in mesophyll cells bordering intercellular spaces Diffusion. Temperature high, water evaporates via diffusion.  Transpiration Transpiration is a necessary process and uses about 90% of the water that enters a plant's roots. The other 10% of water is used in chemical reactions and in plant tissues.  Importance of transpiration It controls water movement. When water is taken up from the plant is depending on transpiration. Supply photosynthesis. Transport minerals from the roots for biosynthesis within the leaf. Cools the plant through evaporation.  Evaporating surfaces Major area where this occur is in the mesophyll cells bordering intercellular spaces. Inner surfaces of epidermal cells. Inner surfaces of guard cells (constituting the stomatal apparatus). Leaf Structure  General Mechanism of Water Movement Plants depend on a vascular system connecting root to shoot. Water is lost through transpiration. A control feedback mechanism (stomata) keeps absorption and transpiration in balance.  General Mechanism of Water Movement Water losses from leaf when stomata open & this process is called transpiration. Transpiration develops pressure gradient. Differences in osmotic pressure drives movement of water into cells. Transpiration may occur through the cuticle, lenticels or stomata.  Cuticular Transpiration Cuticle is a layer of wax-like covering on the epidermis of leaves and herbaceous stems. It is meant to regulate transpiration. However, some water may be lost through it. Cuticular transpiration accounts nearly 20% of the total water loss by a plant.  Lenticular transpiration: Lenticel Loss of water in the form of water vapour taking place through the lenticels present in woody stem and fruits. It contributes 1-5% of the total water loss by the plant.  Stomatal transpiration Stomata are minute pores present on the epidermis of green leaves. Opening and closing of stomata are controlled by guard cells. Maximum loss of water (80-90%) from the plant tissues via stomatal opening.  Stomatal transpiration The rate of transpiration largely runs in tandem with the rate of photosynthesis. For plants to absorb carbon dioxide for photosynthesis, they must open their stomata and thereby expose themselves to water loss through the stomata.  Control of water movement Stomata Stomatal opening causes evaporation and loss of water and induce water to flow Root Control is also through root absorption: if root temperature is cold or roots lack aeration, transpiration is reduced because lack of absorption produce water deficit that causes stomatal closure  Mechanism of stomatal action Each stomate is flanked by a pair of guard cells that are capable of changing shape, thereby widening or narrowing the gap between the two cells. When guard cells take in water by osmosis, they become turgid and swell. Guard cells are not uniformly thick. this, along with a series of radically oriented cellulose microfibrils in the cell wall, cause the guard cell to bend outward.  Mechanism of stomatal action As they swell, the gap between the guard cells widens. If the plant loses water, the guard cells become flaccid and the gap closes. Mechanism of stomatal action  Mechanism of Transpiration Turgor pressure inside mesophyll cells of the leaf forces water outwards through the cell wall. Water is collected in the intercellular spaces. From intercellular spaces, water diffuses out of the stomata into the atmosphere. When soil water is in short supply, plants can respond by closing their stomata to decrease water loss from transpiration, but this simultaneously decreases photosynthetic rate.  How much water do plants transpire? During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons (11,400- 15,100 litres) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 litres) per year.  Factors affecting transpiration 1. Temperature 2. Relative humidity 3. Wind and air movement 4. Soil moisture availability 5. Type of plant  Temperature Transpiration rates increase as the temperature rises, especially during the growing season, when the air is warmer due to intense sunlight and warmer air masses. High temperature (30 – 35 oC) usually causes stomatal closure.  Relative humidity As the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air.  Wind and air movement Increased movement of the air around a plant will result in a higher transpiration rate. If there is no wind, the air around the leaf may not move very much, raising the humidity of the air around the leaf. Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air.  Soil-moisture availability 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.  Types of plant Different plants transpire water at different rates. Some plants which grow in arid regions, such as cacti and succulents, conserve precious water by transpiring less water than other plants.