4.2.6: Chapter Summary

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Page ID: 34196

Melissa Ha, Maria Morrow, & Kammy Algiers
Yuba College, College of the Redwoods, & Ventura College via ASCCC Open Educational Resources Initiative

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Plants respond to light, gravity, water, physical contact, temperature, and other environmental factors. Tropisms are a response involve directional growth toward or away from a stimulus. Positive phototropism in stems is mediated by blue-light receptors called photoreceptors and an auxin gradient. Positive gravitropism in roots and negative gravitropism in shoots involves specialized amyloplasts called statoliths as well as an auxin gradient. Plants also change biological activities according to a 24-hour cycle maintained by an internal clock. These circadian rhythms can be entrained to the environment. Flowering is another response to environmental factors, such as temperature and photoperiod. Photoperiodism with respect to flowering involves two interconvertible forms of the red-light phytochrome with which plants can measure the length of the dark period (night). Seedlings respond to a lack of light by etiolated growth, which helps them access light. Light, temperature, rainfall, and other environmental factors can break seed dormancy (induce germination).

After completing this chapter, you should be able to...

Distinguish among phototropism, gravitropism, hydrotropism, and thigmotropism.
Discuss the adaptive value of tropisms.
Describe the mechanism of phototropism in shoots.
Describe the mechanism of gravitropism in shoots and roots.
Distinguish among thigmotropism, thigmonastic movements, and thigmomorphogenesis
Define circadian rhythms and provide examples in plants.
Describe the environmental conditions that induce flowering
Describe the mechanism of photoperiodism with respect to flowering.
Distinguish among short-day, long-day, and day-neutral plants.
Contrast etiolated seedlings with those grown in light and describe the adaptive value of etiolation.
List the environmental factors that break seed dormancy (induce germination) and define the horticultural techniques based on these factors.
Explain how phytochrome mediates light-induced germination in lettuce.