F Plant Responses By Ms Cullen

Responding to stimuli

Tropisms – directional growth responses Phototropism Geotropism Chemotropism – pollen tubes Thigmotropism – climbing plants

How is plant growth different to animal growth? Plant growth is continuous throughout the plant’s life. Cell walls around the plant cells inhibit cell division, so it only occurs in particular areas of a plant. These areas are called meristems, these are groups of immature cells that are capable of dividing and specialising.

Week 27 Meristems in a plant

Longitudinal section of a plant shoot

What controls plant responses? Hormones! (sometimes called plant growth regulators) Chemical messengers which are transported from area they are produced to a target cell, tissue or organ. In plants there are some hormones which stay in the cell they are produced and have an effect there. Unlike animal hormones they are not produced by endocrine glands. Q: Explain how hormones are specific.

How are hormones transported around the plant? Some hormones can increase the effects of other hormones (synergy) Other hormones cancel out the effects of others (antagonism)

Tropisms HormoneWhere are they produced?Effects Auxins (IAA)In shoot apex and young leaves. Promote cell elongation, inhibit growth of side- shoots and leaf fall. CytokininsWherever mitosis is occuring. Promote cell division GibberellinsIn chloroplasts of young leaves, buds, seeds & root tips. Promote seed germination and growth of seeds. Abscisic acidIn chloroplastsInhibits seed germination & growth; causes stomatal closure when the plant is stressed by low water availability. EtheneProduced by all plant organs Promotes food ripening.

Week 27 Mechanism of the phototropic response in shoots

Senescing Cytokinins usually prevent this by making sure the leaf is a sink for translocation, ensuring the leaf has a good supply of nutrients. If cytokinin production drops, the supply of nutrients drops and senescence begins. Senescence is followed by abscission when leaves are shed.

Week 27 The leaf petiole showing the abscission zone

Task: Apical Dominance, Giberellins and Stem Elongation. Using the textbook P and Internet to help, write up the experiments of Thiman, Skoog and Gocal as if you had carried them out in class. Make sure you are clear about how plants respond to growth substances and how their experiments provided evidence. Explain how giberellins cause stem elongation. Include any refs for sources you have used.

Commercial uses of plant hormones - Auxins Sprayed on developing fruits to prevent them dropping too early. Sprayed on flowers to start formation of fruit even if flower has not been fertilised. This is called parthenocarpy. Why is this useful? Used in rooting powders to stimulate root growth in cuttings. Broad leaved plant weedkillers.

Commercial uses of plant hormones - Gibberellins Sprayed on plants to promote parthenocarpy eg seedless grapes. Can make apples elongate to improve their appearance. Used in brewing industry, stimulates barley to produce α-amylase which converts starch into maltose sugar. ‘malting’. Can promote seed production in the first year of biennial plants (usually only flower in second year of life.)

Commercial uses of plant hormones - Cytokinins Used in tissue-culture to help mass produce plants as they promote bud and shoot growth. To prevent leaves discolouring especially in lettuce and cabbage crops once they have been picked. Can be sprayed on fresh flowers, or added to their water, so that they last longer. Prevents abscission.

Commercial uses of plant hormones - Ethene Speeds up ripening of fruit. Can prevent fruits ripening by keeping in cold, oxygen-deficient storage and then use ethene to ripen fruit when ready. Promotes lateral growth in some plants. Promotes female sex expression in cucumbers. This prevents self-pollination which can cause bitter tasting cucumbers and will also increase yield.

Commercial uses of plant hormones – Abscisic Acid Can be sprayed on fruit trees to regulate the fruit drop. Why is this useful?