life11e_ch37

The flowering stimulus originates in a leaf

Early experiments indicated that reception of the photoperiodic stimulus occurs within the leaf. For example, in the LDP spinach, flowering occurred if the leaves were exposed to long-day periods of light, while the shoot apical meristem was masked to simulate short days. Flowering could not occur when the leaves were masked to simulate short days, while the bud was exposed to long-day periods of light.

These “masking” experiments were extended to SDP plants as well (Figure 37.10). Because the receptor of the stimulus (in the leaf) is physically separated from the tissue on which the stimulus acts (the bud meristem), the inference can be drawn that a systemic signal travels from the leaf through the plant’s tissues to the bud meristem. Other evidence that a diffusible chemical travels from the leaf to the bud meristem signal includes the following:

If a photoperiodically induced leaf is immediately removed from a plant after the inductive dark period, the plant does not flower. If, however, the induced leaf remains attached to the plant for several hours, the plant will flower. This result suggests that something is synthesized in the leaf in response to the inductive dark period, and then moves out of the leaf to induce flowering.
If two cocklebur plants are grafted together, and if only one of the plants is exposed to inductive long nights, both plants flower.
In several species, if an induced leaf from one species is grafted onto another, noninduced plant of a different species, the recipient plant flowers.

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investigating life

The Flowering Signal

experiment

Original Paper: Hendricks, S. B. and H. W. Siegelman. 1967. Phytochrome and photoperiodism in plants. Comparative Biochemistry 27: 211–235.

Knowing that plants measure night duration, the question became whether the dark hours to which a plant is exposed must be continuous. Using SDPs and LDPs as test subjects, Karl Hamner and James Bonner demonstrated that this was the case by interrupting the night with short bursts of light. Sterling Hendricks and William Siegelman repeated the experiments using light of different wavelengths to gain information about the photoreceptor involved.

Activity 37.2 Flowering and Day Length Simulation

www.life11e.com/ac37.2

work with the data

Original Paper: Borthwick, H. A., S. B. Hendricks and M. W. Parker. 1952. The reaction controlling floral initiation. Proceedings of the National Academy of Sciences USA 38: 929–934.

After it was established that the length of night, rather than day, was critical for flowering in some plants, the U.S. Department of Agriculture group that had worked on the phytochrome response of lettuce seed germination (see Figure 36.12) asked whether the signaling system that initiated lettuce seed germination was the same as that involved in flowering. Initially they showed that the most effective wavelength of light at inhibiting flowering was red light (660 nanometers [nm]). Their initial experiments were done on young plants, from which all but one leaf had been removed.

QUESTIONS

Question 1

Plants were grown as seedlings in long-night conditions (12 h dark). After exposure to red light in the middle of the night, groups of four plants were exposed to light a second time and then allowed to resume growth in short-night conditions (8 h dark). After 12 days, the possible presence of a floral meristem was examined by dissection. Table A shows the results.

Table A

	Number of plants with a floral meristem after exposure to second light
Wavelength (nm)	16 min	8 min	4 min	2 min
680	0	0	0	0
700	1	3	0	0
720	3	3	3	1

Why were the plants grown in short-night conditions prior to evaluation of flowering?
What can you conclude from the data in terms of the receptor and time needed?

The plants were grown in short-night conditions so that flowering would not be induced by having a long night. The only condition that would affect flowering would be the brief light during a long night.
The data show that a 700-nm wavelength of light was effective at inducing flowering whereas a 680-nm wavelength inhibited flowering, indicating that the receptor is phytochrome. The far-red light for 8 min at 700 nm converted phytochrome to its active form, P_r.

Question 2

Groups of four plants were exposed to red, far-red, or no light at the beginning of the dark period. That is, if there were a 7-hour dark period, the plants were exposed to light just after the day period ended (extending the day). The results are shown in Table B. Explain the data in terms of the phytochrome system.

Table B

	Number of plants with a floral meristem
Length of dark period (h)	No light	660 nm	720 nm
7.0	—	—	4
7.5	—	—	4
8.0	0	0	4
8.5	4	0	4
9.0	4	4	—
9.5	4	4	—

During the day there is more red than far-red light, and the P_r form of phytochrome gets converted to the P_fr form. At night the P_fr converts back to P_r. This takes time; in the “No light” column, the data show that it took 8.5 hours for P_r to accumulate to sufficient concentration to promote flowering. This was delayed if red light (660 nm) was administered at the beginning of the dark period, as this converted even more P_r to P_fr. However, if far-red light (720 nm) was administered at the start of the dark period, the P_fr that had accumulated during the day got converted to the active form, P_r, which allowed flowering to occur after a shorter period in the dark (7 hours).

A similar work with the data exercise may be assigned in LaunchPad.

The transmissible signal was given a name, florigen (“flower inducing”), in 1937, but its chemical nature has been described only in the past decade.

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experiment

Figure 37.10A The Flowering Signal Moves from Leaf to Bud

Original Paper: Hamner, K. C. and J. Bonner. 1938. Photoperiodism in relation to hormones as factors in floral initiation and development. Botanical Gazette 100: 388–431.

The receptors for photoperiod are in the leaf, but the transition to flowering occurs in the shoot apical meristem. To investigate whether there is a diffusible substance that travels from leaf to bud, Hamner and Bonner exposed only the leaf to the photoperiodic stimulus.

work with the data

Figure 37.10B The Flowering Signal Moves from Leaf to Bud

Original Paper: Hamner, K. C. and J. Bonner. 1938.

In 1938 Karl Hamner at the University of Chicago was working on the role of plant nutrition in flowering. The plant he studied, cocklebur, is a short-day plant that requires 16 hours of darkness to flower. When the plants were kept in 6 hours darkness (16 hours light) in a greenhouse, they did not flower. One day Hamner came to the lab to find all the plants flowering. It turned out that there had been a power outage, and the plants had received a single inductive short day (long night). Realizing that this provided a simple system to study flowering, Hamner invited a major scientist in the field, James Bonner from Caltech, to join him for the summer. The two biologists carried out a series of experiments using the single inductive period that showed that flowering is induced by night length as opposed to day length (see Figure 37.9) and that the flowering signal is received by the leaf from which it travels to a bud, inducing flowering. A Russian plant physiologist, Mikhail Chailakhyan, named this signal florigen. More recently, the molecular nature of this signal was described.

QUESTIONS

Question 1

Intact cocklebur plants (6) or plants with their leaves removed (6) were placed in the inductive (short-day) photoperiod. After 14 days, the researchers obtained the results in Table A. Based on these data, which part of the plant senses the photoperiod?

Table A

Treatment	Number of plants that flowered
No inductive period, intact plant	0
Inductive period, intact plant	6
Inductive period, leaves removed	0

The plants with their leaves removed did not flower. Therefore the leaf senses photoperiod and must be present for flowering to occur.

Question 2

Cocklebur plants were treated so that a single leaf was exposed to the inductive (short) photoperiod while the rest of the plant received the long photoperiod. The results obtained after 18 days are shown in Table B. What do these results indicate about the location of the receptor for flowering?

Table B

Treatment	Number of plants	Result
Untreated	6	Vegetative
Treated, one leaf	32	Flower

The receptor is in the leaf.

Question 3

What do the data tell you about the signal generated by the plant in response to photoperiod and that induces flowering in the apical meristem?

The environmental signal (photoperiod) is received only by the single treated leaf. The signal sets in motion a signaling pathway that ends in the bud at the shoot apex, where flowering is initiated. The data imply that an inducer of flowering is produced in leaves and is able to travel through the plant from the leaf to the shoot apex.

A similar work with the data exercise may be assigned in LaunchPad.