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Understanding Plant Hormones and Responses: Control Mechanisms and Adaptations

This chapter explores plant hormones as chemical messengers that regulate growth and responses to environmental stimuli. Key hormones discussed include auxins, gibberellins, cytokinins, ethylene, and abscisic acid, each with specific functions such as promoting growth, influencing cell division, and regulating dormancy. The chapter also highlights plant responses like tropisms, circadian rhythms, and photoperiodism that dictate growth direction and flowering based on light changes. Additionally, defensive strategies against diseases and herbivores are examined, showcasing plants' adaptation mechanisms.

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Understanding Plant Hormones and Responses: Control Mechanisms and Adaptations

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  1. Control Systems in Plants Chapter 26

  2. Control of Plant Development • plant hormones • chemical messengers within organisms • each has a very specific function • auxins • promote growth at apical meristems • cause growth in different directions • gibberellins • promote stem elongation • cytokinins • promote cell division • prevent premature aging (senescence) • ethylene • causes dropping of leaves and fruit (abscission) • ripens fruit • abscisic acid • promotes seed and bud dormancy • closes stomata

  3. Plant Responses to the Environment • tropisms • growth toward or away from a variable • positive vs. negative • causes growth in different directions • phototropism, chemo-, thigmo-, gravi- Fig. 26.11 (a) Negative gravitropism in stem growth. (b) Positive gravitropism in root growth. An example of positive phototropism

  4. circadian rhythms • activities that occur in 24-hour cycles • biological clocks • internal (genetic) mechanism that maintains circadian rhythms • strongly influenced by photoperiod • sleep movements Fig. 26.15 Examples of sleep movements in prayer plants and morning glories. Fig. 26.13 Coiling response is an example of thigmotropism.

  5. photoperiodism • responses caused by changes in day-length • controlled by the pigment phytochrome • acts as a photoreceptor • best example is flowering • short-day vs. long-day vs. day-neutral plants Fig. 26.5 Graph of circadian rhythms in morning glories. The influence of photoperiod on flowering in long-day and short-day plants.

  6. Plant Defenses • like animals, plants have defenses against disease • bacterial, viral, and fungal infections • much of their defense is based on genetics • chemical attacks against pathogens • often localized in more vulnerable areas • phytoalexins, alkaloids, glycosides, etc. • plants also protect themselves from herbivores • structural defenses • spines, thorns, hair layers, tough bark, etc. • chemical defenses • toxins, inedible substances, sticky substances, etc. Fig. 26.19 Plant fungal infection Fig. 26.19 Examples of plant herbivores

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