Plants respond to two aspects of light: (1) its quality—that is, the wavelengths of light that can be absorbed by molecules in the plant; and (2) its quantity—that is, the intensity and duration of light exposure.
In Chapter 10 you learned about the role of light in photosynthesis: how chlorophyll and other pigments absorb light at certain wavelengths (quality), and how light intensity affects photosynthetic rate (quantity). Here you’ll learn how light affects plant development. Earlier in this chapter we described phototropism and how auxin mediates a plant stem’s bending toward light. In addition to phototropism, light influences seed germination, shoot elongation, the initiation of flowering, and many other important aspects of plant development. Several photoreceptors take part in these processes. Blue-