Transpiration

Stomatal Mechanism

Stomata

Stomatal Mechanism pores or gaps in the lower epidermis through which gaseous exchange and water loss take place Found in leaves and stems Surrounded by two guard cells that contain chloroplast and can photosynthesise Guard cells control the size of the stoma by changes in turgidity

Stomatal Mechanism guard cells consist of cellulose fibres arranged radially around each cell Cell walls unevenly thickened inner walls (ventral wall) much more packed with inelastic fibers, thicker, less elastic Ventral walls expand less during turgidity

Stomatal Mechanism outer walls (dorsal walls) are less packed with these fibres, thinner they expand more during turgidity. open during the day to allow the diffusion of CO 2 into the plant needed for photosynthesis. This depends on the structure and arrangement of the cellulose fibres around the guard cells of the stomata.

Stomatal Mechanism during the day the guard cells acquire a lower water potential as ions are secreted into them water subsequently is lost from them by transpiration This contributes to a lower potential in the guard cells water rushes into guard cells from the spongy mesophyll cells allowing for their expansion

Stomatal Mechanism The inner walls will expand less than the outer walls the inner portion increases in length expansion outwards is prevented because of the close stacking of the inelastic cellulose fibres Ends of guard cells joined-dorsal wall strectches more than ventral wall making each guard cell semi-circular Hence, stoma appears between guard cells

Stomatal Mechanism When the guard cells lose water by transpiration the pore closes Transpiration is the main driving force for the opening and closing of the stoma Opening of a stoma is a result in changes in pressure potential of the guard cells resulting from changes in water potential Stomata open in light and close in darkness In the light K+ ions and Cl- ions enter guard cells

Stomatal Mechanism This lowers their water potential, water enters by osmosis Hence, stoma opens In darkness K+ ions move out of the guard cells into surrounding epidermal cells Water potential of guard cells increases Water moves out of the cells Loss of pressure makes the guard cells change shape and the stoma closes

Measuring Transpiration

Use of potometer-measures the rate of water uptake by a cut shoot or young seedling Does not measure transpiration directly but since most water taken up is lost by transpiration, the two processes are closely linked As the shoot transpires the water vapour is replaced by water drawn in from the potometer via the xylem of the stem

Procedure Cut a leafy shoot under water if possible to avoid air entering the xylem Submerge the potometer in water, filling it with water Fit shoot onto rubber tube ensuring a tight fit Remove apparatus from water and shake off excess water Seal joints around tube with vasaline to keep apparatus watertight

Procedure Introduce an air bubble into the water column by using the syringe to push the water almost to the end of the capillary tube Leave a small air space Place open end of capillary tube in water Draw up more water behind the air space Measure the distance moved by the air space in a given time Calculate the water uptake

Procedure Repeat experiment under different conditions eg. light, temperature, still and moving air

Factors affecting Transpiration Rate 1.Temperature 2.Humidity and Vapour Pressure 3.Air Movement 4.Light

Temperature a measure of the degree of heat in the environment Temp affects transpiration because an increased temperature allows for a higher rate of evaporation from the leaves of the plant thus increasing the transpiration rate temperature decreases the relative humidity of an environment which again increases transpiration rate

Humidity and Vapour Pressure a measure of the concentration of water vapour/vapour pressure high humidity - greater concentration of water molecules in the atmosphere just outside the leaves This reduces the steepness of the diffusion gradient of water from inside to outside so that transpiration rate is significantly reduced as humidity increases transpiration rate decreases and as humidity decreases transpiration rate increases

Air movement allows for the displacement of the saturated water vapour around the leaves this decreases the relative humidity and subsequently increases the transpiration rate

Light affects the opening and the closing of the stomata affects the movement of water into and out of the leaves by transpiration. in the night when the stomata are closed the humidity outside decreases and in the morning the transpiration rate would increase