34 Photoperiodism in Plants

Key Idea: Photoperiodism is the response of a plant to the relative lengths of light and dark. It is controlled by the pigment phytochrome, which occurs in two forms Pr and Pfr. Flowering is a photoperiodic activity that is dependent on the species' response to light. It is controlled through the action of a pigment called phytochrome. Phytochrome acts as a signal for some biological clocks in plants and exists in two forms, Pr and Pfr. It is important in initiating flowering in longday and short-day plants, but is also involved in other light initiated responses, such as germination and shoot growth.

Phytochrome

Phytochrome is a blue-green pigment that acts as a photoreceptor for detection of night and day in plants and is universal in vascular plants. It has two forms: Pr (inactive) and Pfr (active). Pr is readily converted to Pfr under natural light. Pfr converts back to Pr in the dark but more slowly. Pfr predominates in daylight. The plant measures daylength (or rather night length) by the amount of phytochrome in each form.

In the dark or in far red light (730 nm) Pfr reverts slowly, but spontaneously, back to the inactive form of phytochrome Pr.

Sunlight

Pr

S l o wl y in darkness Rapid conversi o n

Pfr

Physiologically active

“Clock genes”

In daylight or red light (660 nm), Pr converts rapidly, but reversibly, to Pfr.

Pfr is the physiologically active form of phytochrome. It promotes flowering in long-day plants and inhibits flowering in short-day plants.

Phytochrome interacts with genes collectively called "clock genes" that maintain the plant's biological clock.

There is still uncertainty over what the flowering hormone (commonly called florigen) is. Recent studies suggested it may be the protein product of the gene FLOWERING LOCUS T (FT) (in long day plants at least) which appears to influence gene expression that includes the gene LEAFY (LFY) in the apical meristem and causes flowering.

Flowering hormone

The hormone is transported to the apical meristem where it causes a change in gene expression that leads to flowering.

1. (a) Identify the two forms of phytochrome and the wavelengths of light they absorb:

(b) Identify the biologically active form of phytochrome and how it behaves in long day plants and short day plants with respect to flowering:

2. (a) Discuss the role of phytochrome in a plant's ability to measure daylength:

(b) Explain how this helps to coordinate flower production in a plant species?

1. Long-day plants (LDP) flower when the photoperiod is greater than a critical day length. Short-day plants (SDP) flower when the photoperiod is less than a critical day length. 2. Interruption of the long dark period inhibits flowering in SDP but promotes flowering in LDP. 3. Dark must be continuous in SDP but not in LDP. 4. Interruption of the light period inhibits flowering in LDP but not in SDP. 5. Alternating cycles of short light and short dark inhibit flowering in SDP. 6. Plants that do not use daylength to initiate flowering are called dayneutral (e.g. cucumber, tomato). Chrysanthemums

Long-day plants

When subjected to the light regimes on the right, the 'long-day' plants below flowered as indicated:

Flowering Controlling the light-dark régime has allowed flower growers and horticulturists to produce flowers out of season or to coincide flowering with specific dates. Plants kept in greenhouses can be subjected to artificial lighting or covered to control the amount of light they receive. To be totally effective at controlling flowering, temperature must also be controlled, as this is also an important flowering cue. For the example of the Chrysanthemum, a shortday plant, flowering is can be controlled under the following conditions. The temperature is kept between 16 - 25 oC. The light-dark regime is controlled at 13 hours of light and 11 hours of dark for 4-5 weeks from planting to ensure vegetative growth. Then the regime changes to 10 hours light and 14 hours darkness to induce flowering.

Photoperiodism in plants

An experiment was carried out to determine the environmental cue that triggers flowering in 'longday' and 'short-day' plants. The diagram below shows 3 different light regimes to which a variety of long-day and short-day plants were exposed.

Short-day plants

When subjected to the light regimes on the left, the 'short-day' plants below flowered as indicated:

0 2 hours 4

Long-d Long-day Short night

No flowering

No flowering

Shor Short-day Long night

Flowering

Examples: lettuce, clover, delphinium, gladiolus, beets, corn, coreopsis

Short-day Long night

Long night interrupted by a short period exposed to light

3. (a) What is the environmental cue that synchronises flowering in plants?

(b) What is a biological advantage of this synchronisation to the plants?

4. Study the three light regimes above and the responses of short-day and long-day flowering plants to that light. From this observation, describe the most important factor controlling the onset of flowering in:

(a) Short-day plants:

(b) Long-day plants:

5. What evidence is there for the idea that short-day plants are best described as "long-night plants":

Flowering

No flowering

Examples: potatoes, asters, dahlias, cosmos, chrysanthemums, pointsettias