Transpiration Mr. West AP Biology 1. Definition • Transpiration is the evaporation of water from the aerial parts of plants. Of all the water plant absorbs, over 95-99% is transpired to the air as water vapor. 4. From where water is transpired? • Aerial parts of whole young plant • Lenticels (lenticular transpiration) 0.1% • Cutin (cuticular transpiration) 3%~10% Stomatum (stomatal transpiration) ~ 90% What is most likely leaving through the stomata of the leaf picture here? • Water (H2O) What is this process called? Stomatal Transpiration Stomatal transpiration Cuticle Prevents water loss Mesophyll Site of photosynthesis Cuticle Stomata Guard cells Openings allow gases and water to move in and out of leaf Open and close the stomata Transport in plants • H2O & minerals – transport in xylem – transpiration • evaporation, adhesion & cohesion • negative pressure • Sugars – transport in phloem – bulk flow • Calvin cycle in leaves loads sucrose into phloem • positive pressure • Gas exchange – photosynthesis • CO2 in; O2 out • stomates – respiration • O2 in; CO2 out • roots exchange gases within air spaces in soil Why does over-watering kill a plant? Water & mineral absorption • Water absorption from soil – osmosis – aquaporins • Mineral absorption – active transport – proton pumps • active transport of H+ aquaporin root hair proton pumps H2O Control of transpiration • Balancing stomate function – always a compromise between photosynthesis & transpiration • leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis Importance of transpiration Guard Cells What goes O2 out? What process involves Guard Cells using CO2 and H2O releasing O2 as a waste H2O product? • Photosynthesis What goes in? Stoma CO2 Stoma Open What is the plant using this process to make? Stoma Closed • Carbohydrates-glucose If the plant needs water for photosynthesis, why is water coming out of the stoma? Function of Stomata • These stomata (leaf openings) naturally allow water to evaporate out. Why would the plant close stomata with guard cells? • Prevent excess water loss throughStoma transpiration. Open (conserve water) So what is the point of having stomata? • Allows gas exchange for photosynthesis Guard Cells Stoma Closed Function of Guard Cells How do the guard cells react to the availability of water? Dry – guard cells CLOSE lots of H2O – guard cells OPEN http://www.ualr.edu/~botany/images.html 4. Guard cells: • cells that open and close the stoma 5. Stomata: openings in leaf’s surface; when open: • • GAS EXCHANGE: Allows CO2 in & O2 out of leaf TRANSPIRATION: Guard Cells Stomata 5. Characteristics of guard cells Guard cell properties and their relationship with stomatal control • Thickness of CW varies in the ventral and dorsal part of the guard cells. • Contains chloroplast and can perform light reaction. (not dark reaction for the lack of key enzymes) • Structurally isolated from epidermal cells for the lack of plasmodesmata (water and ions transmit only through cellular pathway, thus helps to build up water gradient) • Little volume, little amount of water absorption or loss controls stomtal aperture. 6. Factors influencing stomatal aperture • • • • • Light Temp. CO2 Water content Plant hormone (1). Light • Stomata of most plant open in the day and close at night, while CAM plants are just the opposite. • Stomata opening are sensitive to red light and blue light, and blue light is more effective, it stimulates opening by a bluelight receptor: zeaxanthin. (2) Temperature • Stomatal aperture increase with Temp, within 20- 30℃ (the optimal). (3). CO2 • Low CO2 conc. promotes stomatal opening, while high CO2 conc. inhibits stomatal opening through its acidification of the guard cell thus inhibits PM hyperpolarization. (4) Water content • Stomta open when the leaf contain enough water. When there is a water shortage, they close. (6) Plant hormones • CTK promotes opening • ABA inhibits Factors that influence transpiration Transpiration from the leaf depends on two major factors: 1. Difference in water vapor gradient 2. Diffusional resistance The driving force of transpiration is the “vapor pressure gradient.” This is the difference in vapor pressure between the internal spaces in the leaf and the atmosphere around the leaf Diffusional resistance comprises stomatal resistance and boundary layer resistance Transpiration rate=Driving force/resistance water vapor inside the leaf - water vapor of the air = stomatal resistance + boundary layer resistance 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 speeds up transpiration by warming the leaf . 2. Temperature Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. 3. Humidity When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly. • 4. Wind When a breeze is present, the humid air is carried away and replaced by drier air. • 5. Soil water 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 the leaf and stem, the plant wilts. If you were an aquatic plant where would your stomata be? Fringed Water-lily Stomata are found only on the upper epidermis because the lower epidermis is submerged in water. If the stomata were to be on the underside, they wouldn't be able to perform their function (i.e to allow water to evaporate and thus contribute to transpiration). Reviewing Concepts: Water Transport • Movement of water and minerals in a plant involves entry into roots, xylem, and leaves. • 3 processes: 1. Osmosis 2. Capillary Action (Adhesion) 3. Cohesion-Tension Theory Water Transport 1. Osmosis - Water entering root cells creates a positive pressure called root pressure. a) Root pressure (primarily at night) tends to push xylem sap upward in plant. b) Guttation is appearance of drops of water along the edge of leaves, it is result of root pressure. Root pressure is not a sufficient mechanism for water to rise to the tops of trees Water Transport 2. Capillary Action – is the rise of liquids in narrow tubes. a) Adhesion – Molecular attraction between UNLIKE substances. Capillary Action is also not a sufficient mechanism for water to rise to the tops of trees Water Transport 3. Cohesion-Tension Theory a) Transpiration – evaporation of water from plants b) Cohesion – water molecules attracted to other water molecules. (polarity & hydrogen bonds) c) Bulk Flow – water movement from roots to leaves as water molecules evaporate from the leaf surface. Opening and Closing of Stomates • Each stomate has two guard cells with a pore between them. – Stomates OPEN - when guard cells take up water = increase in turgor pressure – Stomates CLOSE - when guard cells lose water = decreases in turgor pressure . • Guard cells are attached to each other at their ends; inner walls are thicker than outer walls. • As they take up water, they buckle out, thereby creating an opening between cells.