Plant Responses to Stimuli.docx

**[Plant Responses to Stimuli ]** [Growth Regulation\ ]Plant responses usually involve the interaction of several different growth regulators. Plants can be very sensitive to growth regulators. To be effective, growth must be regulated and coordinated. Each event must occur in the correct sequence....

**[Plant Responses to Stimuli ]** [Growth Regulation\ ]Plant responses usually involve the interaction of several different growth regulators. Plants can be very sensitive to growth regulators. To be effective, growth must be regulated and coordinated. Each event must occur in the correct sequence. Growth is controlled by two different types of factors. Factors located outside the plant are External. Factors located inside the plant are internal. [External Factors\ ]- Temperature: An unseasonably warm water may induce plants to grow. If the temperature gets too hot, plants produce heat shock proteins which protect other vital proteins from the denaturing effect of high temperatures\ Day Length: As the days get longer, plants get ready to grow and reproduce, as the conditions will be suitable for these activities. Decreasing day lengths tell deciduous trees to shed their leaves to get rid of excretory products.\ Light intensity: The more light a plant has, the more it will grow and the more productive it will be because photosynthesis increases\ Water: If water cannot obtain enough water, it will tell its stomata to close to conserve water [Internal Factors\ ]- Growth regulators: Plant hormones including Auxins, gibberellins, cytokinins, and ethylene\ - Injury and disease: Can cause changes in a plant and they mount a chemical attack against a group of chemical called phytoalexins [Growth Regulators] Growth regulators can be divided into categories;\ - Promoters: Increase growth e.g., Auxins\ - Inhibitors: Inhibit growth e.g., abscisic acid [Auxins\ ]The most common Auxin is Indole Acetic Acid (IAA). Auxin is produced in the apical meristem in young tissue and diffuses down the stem, producing many effects, including stem elongation, root, and shoot, as well as fruit development.\ Cell elongation occur when auxins cause the hydrogen bonds between cellulose fibres to loosen and also cause the cells to absorb more water. The absorption of water causes the cell to swell and the loosening of the bonds in the cellulose fibres allows the cell to elongate.\ The two main effects of auxins are phototropism and a phenomenon called apical dominance. *Phototropism\ *- Phototropism was studied by Charles Darwin and his son and they found it was controlled by the tip of the coleoptile.\ - They noticed the curving of the stem happened some distance down the stem. They hypothesised that some signal was transmitted down from the tip\ - A Danish scientist, Peter Boysen-Jensen showed that the signal was a mobile chemical. He placed a layer of gelatine between a cup tip and coleoptile, which stopped the cells coming into contact but allowed chemicals to diffuse. The signal passed through and caused bending as normal. When he used a layer of impervious mica, the signal did not pass through and there was no bending\ - When a plant is exposed to unilateral light, it causes more auxin to diffuse down the shaded side. Due to their exposure to a higher concentration of auxin, the cells on the shaded side elongate more than those on the illuminated side. This results in the tip of the shoot growing towards the light. [Apical dominance\ ]- Auxin is produced by the meristem at the tip of the stem and is transported down the stem, decreasing in concentration as it moves\ - The high auxin concentration near the growing tip inhibits the growth of lateral buds, keeping them dormant\ - As the auxin continues to drop as it goes down the stem, it reaches a concentration where it no longer inhibits the lateral buds and they begin to develop into side shoots\ - If the apical bud is removed, the IAA production stops and all the lateral buds develop, making the plant bushy [Effect of concentration\ ]- The same concentration of auxin has different effects in different parts of the plant. An auxin concentration that promotes growth in one part of a plant can inhibit growth in another part of the same plant.\ - Roots are more sensitive than shoots\ - A concentration of 10^-1^ ppm will promote the growth of the shoot and inhibit the growth of the root\ - A concentration of 100 ppm will inhibit both the shoot and the root [Uses of Growth Regulators] \- Auxins: Widely used in rooting powders. The cut end of a stem is dipped in water and then into the rooting powder before being inserted into the growing medium. The auxin promotes the growth of adventitious roots around the area of the cut, helping the cutting to become established more rapidly\ Cytokinins: Used in micropropagation to promote cell differentiation\ Ethylene: Used to ripen fruit **[Tropisms\ ]A Tropism** is the growth response of a plant to an external stimulus.\ - Tropisms ensure a plant is positioned most favourably for its growth.\ - Stems are positively phototrophic and negatively geotrophic and roots are positively geotrophic and negatively phototrophic Phototropism: The growth response of a plant to light e.g., stems grow towards light. Chemotropism: The growth response of a plant to chemicals e.g., roots grow towards minerals Thigmotropism: The growth response of a plant to touch e.g., ivy Geotropism: The growth response of a plant to gravity e.g., roots grow down into soil for water and minerals **Hydrotropism:** The growth response of a plant to water **[Adaptations that Protect Plants\ ]**- Chilies have oils that cause a burning sensation in the mouth to deter browsers\ - Nettles have stings containing formic acid which discourages animals from eating and touching the nettles\ - The Hawthorn bush has sharp thorns that it has developed to discourage animals from eating it\ - Gorse has hard, stiff, pointed leaves which deters browsers