What is transpiration?

a) What is transpiration?



Process by which plants lose water to the atmosphere; in form of vapour (through lenticels, stomata and cuticle); 2 mks


b) Explain how the various environmental factors affect the rate of transpiration

Temperature; an increase in temperature increase the water vapour holding capacity of air in the spaces between mesophyll cells due to increased evaporation; therefore more water diffuses from the cells increasing the water vapour pressure; this causes an increase in the diffusion gradient between the intercellular spaces and the atmosphere; hence increasing the rate of transpiration; low temperature decreases water vapour pressure and the diffusion gradient hence; lowering the rate of transpiration; Light intensity; high light intensity increases the rate of photosynthesis in the guard cells; causing the opening of stomata; leading to increased water loss; it also increases the internal temperature of the leaf; that increases the evaporation rate in the intercellular spaces; leading to a higher rate of transpiration; low light intensity; reduces the rate of water loss; Humidity; a humid atmosphere lowers the water vapour diffusion gradient; hence lowering the rate of water loss; in a less humid/dry atmosphere, water diffusion gradient is high/steep; hence the rate of transpiration increases; Availability of water in the soil; more water will diffuse to the atmosphere when there is adequate or excess water; as more will be absorbed; increasing the rate of water loss; the guard cell will also remain turgid; hence stomata are open; leading to more water loss; however, less water in the soil leads to a reduced diffusion gradient between the mesophyll cells and the atmosphere; thereby reducing the rate of transpiration; Wind/air currents; wind blowing over a leaf surface carries pockets of moisture away from the leaf; creating a steep diffusion gradient between the atmosphere and the leaf; increasing the rate of water loss; in still air/on a calm day however, water vapour at the leaf area becomes saturated; and the diffusion gradient is lowered; reducing the rate of transpiration; Atmospheric pressure; low atmospheric pressure leads to a high rate of diffusion of water vapour; since air molecules move at a faster rate; and this increases the rate of water loss; in high atmospheric pressure conditions however, there is low rate of diffusion of water vapour; hence the rate of water loss is lowered;