Module Plant Responses By Ms Cullen

Responding to stimuli

Plants increase their chances of survival if they respond to……

Responses to herbivory Chemical defences – producing toxic chemicals when they are eaten eg tannins – bitter chemicals, in cattle & sheep they bind to proteins in the gut making them hard to digest. Can be found in oak alkaloids – bitter tastes, poisonous characteristics or noxious smells that deter or kill herbivores. Tobacco plants produce nicotine when their tissue is damaged.

Responses to herbivory Releasing pheromones– signalling chemicals that produce a response in other organisms eg corn plants eaten by caterpillars can produce pheromones which attract parasitic wasps. Folding when touched – the plant Mimosa pudica will fold all it’s leaves if it is touched. This can protect the plant by scaring off herbivores or knock small insects off it’s leaves

Responses to abiotic stress Abiotic stress is anything non-living which is harmful to the plant. eg drought - the quiver tree in Africa self amputates in extreme drought. Carrots produce antifreeze proteins in cases of low temperatures, these bind to ice crystals and prevent more forming.

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

Nastic Movements Nastic movements are plant movements that occur in response to environmental stimuli but unlike tropic movements, the direction of the response is not dependent on the direction of the stimulus. Some of the most spectacular plant movements are nastic movements. These include the closing of the carnivorous Venus Flytrap leaf when it captures prey or the folding of the mimosa leaf when it is disturbed.

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.

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 and amplify their effect (synergism) Other hormones can oppose the effects of others (antagonism)

Plant Growth Hormones HormoneWhere are they produced?Effects Auxins ( Indoleacetic acid - 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.

Plant hormones and seed germination When a seed absorbs water it will begin to produce giberellins. Giberellins switch on the genes that produce amylase and proteases which begin to break down the cotyledons or endosperm (food stores) of the seed. The embryo produces ATP so it can begin to grow and break through the seed coat. There is also evidence that another plant hormone abscisic acid (ABA) acts as an antagonist to giberellin.

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.

The leaf petiole showing the abscission zone

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?