recap

37.2 recap

Flowering of some angiosperms is controlled by night length, a phenomenon called photoperiodism. Low temperatures can induce flowering in some species (vernalization). Some species flower when their stems have grown by a certain amount, independent of environmental cues. All pathways to flowering converge on the meristem identity genes. The photoperiodic flowering stimulus originates in the leaf, and a signaling pathway ends up in the expression of floral identity genes in the bud.

learning outcomes

You should be able to:

  • Give evidence showing that night length, rather than day length, is the cue that triggers flowering.

  • Describe evidence that a diffusible chemical travels from the leaf to the bud meristem to initiate flowering.

  • Describe the three genes involved in florigen production and action, including where they are active, their functions, and their interactions.

  • List factors other than photoperiodism and genetic triggering of florigen that can initiate flowering, and give evidence for each.

Question 1

Describe the proteins and mutations that could be involved in the following observations:

  1. A mutant plant flowers without its normal inductive dark period. When a leaf from the mutant plant is grafted onto an unexposed wild-type plant, the recipient plant flowers.

  2. A mutant plant does not flower when exposed to the normal inductive dark period. When a leaf from a mutant plant that has been exposed to the inductive dark period is grafted onto an unexposed wild-type plant, the recipient plant flowers.

  3. A plant flowers only after exposure to cold.

  4. If a gene is experimentally overexpressed in the leaf, flowering is induced. Overexpression of the gene in the shoot apical meristem, however, does not induce flowering.

  1. The mutation stabilized the CO protein.
  2. The mutation caused nonfunction of the FD protein.
  3. The mutation increased expression of the FLC protein.
  4. The mutation caused constitutive expression of the CO protein.

Question 2

A horticulturist provides cut flowers for many occasions and often requires large numbers of flowers all blooming at the same time. Many of her flowers are normally summer-blooming, or long-day plants. How might she force these flowers to bloom during the winter?

Long-day plants typically bloom when days are long and nights are short, as in the summer. But in a greenhouse setting, plants can be tricked into responding as though the nights are short by turning on the lights for a short time in the middle of the night (night interruption lighting). This breaks up the long night, so the plants act as though the night is short—that is, they begin to flower.

Question 3

Give at least two pieces of evidence supporting the presence of a transmissible substance that moves from leaves to bud meristems to initiate flowering.


Examples of evidence include:

  1. Spinach will not flower if the shoot is masked to imitate short days but will flower if the bud (and not the shoot) is masked.
  2. If a photoperiodically induced leaf is immediately removed from a plant, flowering will not occur. But if the leaf remains on the plant for several hours (allowing time for the substance to move to the bud), flowering will occur.
  3. In grafting experiments, if only one plant (or plant part) is induced to flower, the other will flower as well.

You have seen how environmental factors interact with genes to control flowering in angiosperms. The function of flowers is sexual reproduction, which maintains beneficial genetic variation in a population. Many angiosperms, however, also benefit from being able to reproduce asexually.