Plants Can Be Classified by Their Photoperiodic Responses

Numerous plant species flower during the long days of summer, and for many years plant physiologists believed that the correlation between long days and flowering was a consequence of the accumulation of photosynthetic products synthesized during long days.

This hypothesis was shown to be incorrect by the work of Wightman Garner and Henry Allard, conducted in the 1920s at the U.S. Department of Agriculture laboratories in Beltsville, Maryland. They found that a mutant variety of tobacco, Maryland Mammoth, grew profusely to about 5 m in height but failed to flower in the prevailing conditions of summer (Figure 24.17). However, the plants flowered in the greenhouse during the winter under natural light conditions.

These results ultimately led Garner and Allard to test the effect of artificially providing short days by covering plants grown during the long days of summer with a light-tight tent from late in the afternoon until the following morning. These artificial short days also caused the plants to flower. This requirement for short days was difficult to reconcile with the idea that longer periods of radiation and the resulting increase in photosynthesis promote flowering in general. Garner and Allard concluded that the length of the day was the determining factor in flowering and were able to confirm this hypothesis in many different species and conditions. This work laid the foundations for the extensive subsequent research on photoperiodic responses.

The classification of plants according to their photoperi-odic responses is usually based on flowering, even though many other aspects of plants' development may also be affected by day length. The two main photoperiodic response categories are short-day plants and long-day plants: 1. Short-day plants (SDPs) flower only in short days (qualitative SDPs), or their flowering is accelerated by short days (quantitative SDPs).

Different Types Tobacco Plants
FIGURE 24.17 Maryland Mammoth mutant of tobacco (right) compared to wild-type tobacco (left). Both plants were grown during summer in the greenhouse. (University of Wisconsin graduate students used for scale.) (Photo courtesy of R. Amasino.)

2. Long-day plants (LDPs) flower only in long days (qualitative LDPs), or their flowering is accelerated by long days (quantitative LDPs).

The essential distinction between long-day and short-day plants is that flowering in LDPs is promoted only when the day length exceeds a certain duration, called the critical day length, in every 24-hour cycle, whereas promotion of flowering in SDPs requires a day length that is less than the critical day length. The absolute value of the critical day length varies widely among species, and only when flowering is examined for a range of day lengths can the correct photoperiodic classification be established (Figure 24.18).

Long-day plants can effectively measure the lengthening days of spring or early summer and delay flowering until the critical day length is reached. Many varieties of wheat (Triticum aestivum) behave in this way. SDPs often flower in fall, when the days shorten below the critical day length, as in many varieties of Chrysanthemum morifolium. However, day length alone is an ambiguous signal because it cannot distinguish between spring and fall.

Plants exhibit several adaptations for avoiding the ambiguity of day length signal. One is the coupling of a temperature requirement to a photoperiodic response. Certain plant species, such as winter wheat, do not respond to pho-toperiod until after a cold period (vernalization or overwintering) has occurred. (We will discuss vernalization a little later in the chapter.)

Other plants avoid seasonal ambiguity by distinguishing between shortening and lengthening days. Such "dual-day length plants" fall into two categories:

1. Long-short-day plants (LSDPs) flower only after a sequence of long days followed by short days. LSDPs, such as Bryophyllum, Kalanchoe, and Cestrum noctur-num (night-blooming jasmine), flower in the late summer and fall, when the days are shortening.

2. Short-long-day plants (SLDPs) flower only after a sequence of short days followed by long days. SLDPs, such as Trifolium repens (white clover), Campanula medium (Canterbury bells), and Echeveria harmsii (echeveria), flower in the early spring in response to lengthening days.

Finally, species that flower under any photoperiodic condition are referred to as day-neutral plants. Day-neutral plants (DNPs) are insensitive to day length. Flowering in DNPs is typically under autonomous regulation—that is, internal developmental control. Some day-neutral species,

Day length Night length

Day Neutral Plants Examples

FIGURE 24.18 The photoperi-odic response in long- and short-day plants. The critical duration varies between species: In this example, both the SDPs and the LDPs would flower in photoperiods between 12 and 14 h long.

6 18

8 16

10 14

12 12

14 10

16 8

20 4

6 18

8 16

10 14

12 12

14 10

16 8

20 4

FIGURE 24.18 The photoperi-odic response in long- and short-day plants. The critical duration varies between species: In this example, both the SDPs and the LDPs would flower in photoperiods between 12 and 14 h long.

24 h

-Light

Critical ^duration of darkness

-Flash of light Darkness

24 h

Flowers Stimulus

Short-day plants

Short-day (long-night) plants flower when night length exceeds a critical dark period. Interruption of the dark period by a brief light treatment (a night break) prevents flowering.

Short-day plants

Short-day (long-night) plants flower when night length exceeds a critical dark period. Interruption of the dark period by a brief light treatment (a night break) prevents flowering.

Lighting treatment Light Darkness

Flowering response SDP LDP

Flowering

Vegetative

Vegetative Vegetative Vegetative Vegetative

Flowering

Flowering

Flowering

Flowering

Flowering

Vegetative

24 h such as Phaseolus vulgaris (kidney bean) evolved near the equator where the daylength is constant throughout the year. Many desert annuals, such as Castilleja chromosa (desert paintbrush) and Abronia villosa (desert sand verbena), evolved to germinate, grow, and flower quickly whenever sufficient water is available. These are also DNPs.

Critical ^duration of darkness

-Flash of light Darkness

24 h

Plant Photoperiodic Response

Long-day plants

Long-day (short-night) plants flower if the night length is shorter than a critical period. In some long-day plants, shortening the night with a night break induces flowering.

Long-day plants

Long-day (short-night) plants flower if the night length is shorter than a critical period. In some long-day plants, shortening the night with a night break induces flowering.

FIGURE 24.19 The photoperiodic regulation of flowering. (A) Effects on SDPs and LDPs. (B) Effects of the duration of the dark period on flowering. Treating short- and long-day plants with different photoperiods clearly shows that the critical variable is the length of the dark period.

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