recap

36.4 recap

Light controls several physiological and developmental events in plants, a process called photomorphogenesis. Pigment photoreceptors such as phototropin, cryptochromes, and phytochrome mediate the effects of light on plant growth and development. Phytochrome exists in two interconvertible states; conversion from one state to the other is controlled by the ratio of red to far-red light. Circadian rhythms are influenced by light reception.

learning outcomes

You should be able to:

  • Describe and evaluate evidence for the existence of blue-light receptors in plants.

  • Explain the process and conditions under which conversion of the Pr and Pfr forms of phytochrome occur.

Question 1

What is the evidence for blue-light receptors in plants?

An action spectrum for the bending of coleoptiles toward light shows that blue light is most effective. Genetic screens for blue-light insensitivity have shown that there is a blue-light receptor present on normal plants that is not present in plants unable to respond to blue light.

Question 2

Why does red light affect seed germination differently than far-red light does?

Red light causes phytochrome to convert from the Pr to the Pfr form. The latter is active in promoting seed germination. Far-red light reduces Pfr by causing it to covert to Pr, which is inactive in promoting seed germination.