Response to Seasons

How do plants respond to seasonal changes?

“To every thing there is a season.” Nowhere is this more evident than in the regular cycles of plant growth. Year after year, some plants flower in the spring, others in summer, and still others in the fall. Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants. Plants such as spinach and irises flower when days are long and are therefore known as long-day plants.

Photoperiod and Flowering How do all these plants manage to time their flowering so precisely? In the early 1920s, scientists discovered that tobacco plants flower according to their photoperiod, the number of hours of light and darkness they receive. Additional research showed that many other plants also respond to changing photoperiods, a response called photoperiodism. This type of response is summarized in Figure 24–19. Photoperiodism is a major factor in the timing of seasonal activities such as flowering and growth.

It was later discovered that a plant pigment called phytochrome (FYT oh krohm) is responsible for plant responses to photoperiod. Phytochrome absorbs red light and activates a number of signaling pathways within plant cells. By mechanisms that are still not understood completely, plants respond to regular changes in these pathways. These changes determine the patterns of a variety of plant responses.

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FIGURE 24–19 Effects of Photoperiod Changes in the photoperiod can affect the seasonal timing of flowering. Form an Opinion Are “short-day plant” and “long-day plant” the best names for categorizing these plants, or would it be better to name plants after their responses to night length? Explain your reasoning.

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