Light and Photoreceptors

The length of the night determines the onset of winter dormancy in many plant species. As summer wears on, the days become shorter (that is, the nights become longer). Leaves have a mechanism for measuring the length of the night, as we will see in the next chapter. Measuring night length is an accurate way to determine the season of the year. If a plant determined the season only by the temperature, it might be fooled by a winter warm spell or by unseasonably cold weather in the summer. The length of the night, on the other hand, is determined by Earth's rotation around the sun and does not vary. Plants use the environmental cue of night length to time several aspects of their growth and development.

Night length is one of several environmental cues detected by plants, or by individual organs such as leaves. By its presence or absence, its intensity, its spectral properties (specific wavelengths), and its duration, light provides cues to various environmental conditions. In spite of the reservation just mentioned, temperature, too, provides important environmental cues, both by its value at any particular time and by the distribution of warmer and colder stretches over a period of time. The plant senses these environmental cues and then responds, often by stepping up or decreasing its production of hormones. We'll discuss an example of a temperature cue in the next chapter. In this section, we'll see how certain photorecep-tors sense light, its duration, and its wavelength distribution.

Light regulates many aspects of plant development in addition to phototropism. The affected processes range from seed germination to shoot elongation to the initiation of flowering. Several photoreceptors take part in these and other processes. Five phytochromes mediate the effects of red and dim blue light. Four or more types of blue-light receptors, discovered more recently, mediate the effects of higher-intensity blue light.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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