FACTORS AFFECTING TRANSPIRAT Archives

The factors that affect transpiration are grouped into two. Environmental and Structural Factors Cuticle Environmental factors Temperature High temperature increases the internal temperature of the leaf. This in turn increases kinetic energy of water molecules which increases evaporation. High temperatures dry the air around the leaf surface maintaining a high concentration gradient. More water vapour is therefore lost from the leaf to the air. Humidity The higher the humidity of the air around the leaf, the lower the rate of transpiration. The humidity difference between the inside of the leaf and the outside is called the saturation deficit. In dry atmosphere, the saturation deficit is high. At such times, transpiration rate is high. Wind Wind carries away water vapour as fast as it diffuses out of the leaves. This prevents the air around the leaves from becoming saturated with vapour. On a windy day, the rate of transpiration is high. Light Intensity When light intensity is high; more stomata open hence high rate of transpiration. Atmospheric Pressure The lower the atmospheric pressure the higher the kinetic energy of water molecules hence more evaporation. Most of the plants at higher altitudes where atmospheric pressure is very low have adaptations to prevent excessive water-loss. Availability of Water The more water there is in the soil, the more is absorbed by the plant and hence a lot of water is lost by transpiration. Structural Factors Cuticle Plants growing in arid or semi-arid areas have leaves covered with a thick waxy cuticle. Stomata The more the stomata, the higher the rate of transpiration. Xerophytes have few stomata which reduce water-loss. Some have sunken stomata which reduces the rate of transpiration as the water vapour accumulates in the pits. Others have stomata on the lower leaf surface hence reducing the rate of water-loss. Some plants have reversed stomatal rhythm whereby stomata close during the day and open at night. This helps to reduce water-loss. Leaf size and shape Plants in wet areas have large surface area for transpiration. Xerophytes have small narrow leaves to reduce water-loss. The photometer can be used to determine transpiration in different environmental conditions. Translocation of organic compounds Translocation of soluble organic products of photosynthesis within a plant is called translocation. It occurs in phloem in sieve tubes. Substances translocated include glucose, amino acids, vitamins. These are translocated to the growing regions like stem, root apex, storage organs e.g. corms, bulbs and secretory organs such as nectar glands. Phloem Phloem is made up of: Sieve tubes Companion cells Parenchyma, a packing tissue Schlerenchyma, a strengthening tissue Sieve Tubes These are elongated cells arranged end to end along the vertical axis. The cross walls are perforated by many pores to make a sieve plate. Most organelles disappear and those that remain are pushed to the sides of the sieve tube. Cytoplasmic strands pass through the pores in the plate into adjacent cells. Food substances are translocated through cytoplasmic strands. Companion Cells Companion cells are small cells with large nuclei and many mitochondria. They are found alongside each sieve element. The companion cell is connected to the tube through plasmodesmata. The mitochondria generate energy required for translocation. Phloem Parenchyma These are parenchyma cells between sieve elements. They act as packing tissue. See also: arteries DETERMINATION OF THE GENOTYPE OF A DOMINANT PHENOTYPE VARIATION IN POPULATION BIOLOGY PRACTICAL ACTIVITIES JOINTS AND MOVEMENT