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Plant behaviour Plant BEHAVIOUR?! Plants are also influenced by external factors/stimuli. Behaviour = reaction to stimulus Tropisms The growth of a plant in response to a stimulus Light Water Gravity Touch Light tropism Also called PHOTOTROPISM Plants will face the light, usually sunlight If plant FACES light, it is a POSITIVE phototropism. If it turns away from the light, it is a NEGATIVE phototropism Phototropism experiments Tips of growing grass were covered Tips no longer bent toward light Therefore, region of receiving light messages must be in the tip We now know that AUXIN, a plant hormone, is produced in the plant tip, and creates new cells for growth of the tip Auxin moves away from light If the plant is bathed in even light, there is an even amount of auxin throughout the entire tip. If the light is concentrated on one side, the auxin moves to the “dark side”, and produces greater growth on that side, elongating one side of the tip, creating a bend. The action of auxin Gravity tropism Geotropism Roots head down into the ground, while the tip of the plant grows upwards Again – auxin is the culprit! Why have a geotropism? Gain sunlight through green parts • Gain water through root system from ground water Climbing plants Thigmotropism – change in growth in response to contact with another object Twisting comes from elongation of cells that are not touching the object Where do you think auxin is concentrated? Rhythmic behaviours in plants Rhythmic behaviours: behaviours that happen daily, seasonally, yearly Time stimulus Solar tracking Time as stimulus for rhythmic behaviour Plants have a biological clock Leaves horizontal in day, “sleep” at night (why?) 24 hour cycle – circadian rhythm Opening and closing of flowers is also circadian Solar tracking Heliotropism Leaves and flowers move throughout the day so they are facing sun Eg. Sunflowers (Helianthus annuus) have the ability to face the sun all day. What is the benefit of this? Timing of flowering Photoperiodism – response of plants to different periods of light (relative length of day and night) Some plants (eg. Chrysanthemum) only flower when day is shorter than night – short-day plants. Some (eg. Carnations) only flower when day is longer than night – long-day plants If they flower regardless of day/night length (eg. Dandelions), they are dayneutral plants Short and long day plants If the “dark” period is interrupted: Short-day: no flowering Long-day: will flower Short and long day plants During light hours, hormones are produced These influence flower growth The right amount of hormone must be produced for the specific plant Activities Glossary: tropism, phototropism, auxin, geotropism, thigmotropism, biological clock, circadian rhythm, heliotropism, photoperiodism, short-day plant, long-day plant, day-neutral plant Quick Check Questions: pg 367 Biochallenge pg 368, Chapter Review Q 2, 3, 4, 5, 6, 7, 8, 9, 10.