Plant Responses A stimulus is anything that causes a reaction in an organism. Examples: light, gravity and temperature A response is the activity of an organism as a result of a stimulus. Examples: Growth, flowering, production of plant enzymes. NOTE: plant responses involve growth and changes in growth. Their movement is much slower than that of animals. Responses in flowering plants Flowering plants growth is controlled by internal and external factors. External factors: Light: affects plant growth by providing the energy needed for photosynthesis. Plants normally Grow towards light. Gravity: allows roots to grow downwards into the soil and shoots to grow upwards away from gravity. Temperature: affect the growth of plants by affecting the rate of enzyme reactions. Plants grow faster at higher temperatures. Day length: plays a role in causing plants to flower and allow fruit and seeds to form.
Internal factors: Plants produce growth regulators in the meristematic regions (shoot tip and root tip) of the plant. Tropism Tropism is a change in the growth of a plant in response to an external stimulus. Positive tropism = Negative tropism = Advantages of tropism: Allows plants to grow in more favourable conditions. Stems grow toward the light so they can make more food. Roots grow downwards due to gravity so they can anchor into the soil and absorb nutrients.
Main types of tropism: i) Phtotropism ii) Geotropism iii) Thigmotropism iv) Hydrotropism v) Chemtropism Phototropism Phototropism is the change in growth of a plant in response to light. Stems are positively phototropic ( ). Many roots are negatively phototropic. These roots grow away from the light. Example: Geotropism Geotropism is the change in growth of a plant in response to gravity. Root grows towards gravity (positively geotropic) so they anchor themselves into the soil. Stems grow away from gravity (negatively geotropic) Thigmotropism Thigmotropism is the change in growth of a plant in response to touch. Climbing plants (ivy, vines and peas) have tendrils which wrap around supporting structures.
Hydrotropism Hydrotropism is a change in growth of a plant in response to chemicals. Roots and pollen tubes grow towards the water. Chemotropism Chemotropism is a change in growth of a plant in response to chemicals. Roots grow towards minerals (nitrogen, phosphorus and potassium) in the soil ( ) Pollen tubes grow towards chemicals released by the ovule. Note: Negatively chemotrophic to acids or lead. Growth regulators Growth regulators are chemicals that control the growth of a plant. Most growth regulators are produced in small amounts in the meristem of plants and transported around the plant (hormones) in the vascular tissue. Why is it so hard to establish the exact role of growth regulators: Active in very small amounts. Effects depend on their concentration ( can have opposite effects at high or low concenctrations) Effects depend on the location in the plant where they are acting. Different regulators interact in different ways. Some can combine to produce a greater effect while others can combine to have no effect. Example of growth regulators: Auxins are growth promoters to increase the rate of growth of a plant. Abscisic acid and ethylene are regulators that slow down the growth of a plant (growth inhibitors)
Growth promoters Auxins A common known auxin is called IAA (indoleacetic acid) Made in the shoot tips, young leaves and seeds. Auxins cause stem and root growth, fruit formation Where auxins are produced: Produced in the meristematic tissue in tips of shoots, young leaves and developing seeds. Functions of auxin: Stimulating stem elongation and root growth. Developing fruit Inhibiting side branches in stems Causing phototropism and geotropism Effects of auxins: Tropisms: Cause cell elongation and growth or bending Apical dominance: Auxin produced in the tip will pass down the stem and inhibit the lateral buds which means the apical bud will grow at the expense of the side branches. Example: cacti and conifers If apical bud is removed the side branches develop more, and the plant creates a bushy form. Fruit formation: IAA is made in developing seeds. Causes food to form around the seed.
Root growth At low concentrations, IAA causes roots to grow. IAA can be applied artificially to stimulate rooting. Auxin and cell elongation How plants respond to light (phototropism) Auxin loosens cell walls which allows them to expand and elongate. Role of auxin (IAA) in phototropism: 1) IAA (auxin) is produced in the growth tips ( ) of the stem. 2) If one side of the plant is exposed to light, IAA will difuse down the shaded side. 3) This causes the cells on the shaded side of stem to elongate more than the cells on the bright side of the stem. 4) As a result, the stem bends towards the light.
Growth inhibitors Ethene (ethylene) Ethene is a gas and is made by plants in ripe fruit and decaying leaves. Plays a major role in ripening fruits and causing leaves to fall in autumn. Causes fruit colour and fruit flavor to form. Used commercially to ripen bananas. Abscisic acid Produced in leaves, stems and root caps. Causes plants to respond to harmful conditions. Example: causes stomata to close in dry conditions. Presence in seeds inhibits germination, which allows the seeds to remain dormant in winter NOTE: one rooting apple in a bag causes other apples to rot. WHY? Commercially prepared growth regulators: Examples of artificially prepared growth regulators: Rooting powder: NAA is a growth regulator which stimulates rapid root formation on stem cuttings. Tissue culturing (a piece of plant material is used to form new plants):
Plant tissue grown in high auxin concentration, will develop into a mass of similar cells ( ). Using different concentrations of auxins, the callus can form roots and shoots. Ethene Ripens bananas. Plant adaptations for protection (four methods required) Anatomical (structural) features: Have a physical barrier consisting of epidermis or bark. These prevent pathogens from entrying. The epidermis can also adapt to form thorns for protection (blackberry bushes). Stomata can close when there is a shortage of water with the guard cells shriveling. Abscisic acid causes the stomata to close and help to conserve water. Chemical protective features To much heat can cause plant enzymes to lose shape and become denatured. Plants can form special heat shock proteins that surround other proteins (enzymes) and help them to maintain their shape. Plants that are infected by micro organisms can produce a stress protein called phytoalexins. These stress proteins help damage the harmful micro organisms.