Section 3

objectives

• Define photoperiodism.

• Describe the role of critical night length in flowering.

• Explain the process of vernalization.

• Explain changing fall colors in leaves.

vocabulary photoperiodism critical night length short-day plant long-day plant day-neutral plant phytochrome vernalization biennial bolting fall color

Short-day plants (SDPs) flower in the spring or fall, when the day length is short. For example, ragweed flowers when the days are shorter than 14 hours, and poinsettias flower when the days are shorter than 12 hours. Chrysanthemums, goldenrods, strawberries, tulips, and soybeans are typically SDPs for flowering.

Long-day plants (LDPs) flower when days are long, usually in summer. For example, wheat flowers only when the days are longer than about 10 hours. Radishes, asters, irises, spinach, and beets are typically LDPs for flowering.

Adjusting the Flowering Cycles of Plants

Examine Figure 31-10, which compares the flowering of a poinset-tia (an SDP) with that of an iris (LDP). With an 8-hour night, the LDP flowers but the SDP does not. With a 16-hour night, the SDP flowers and the LDP does not. However, if a 16-hour night is interrupted in the middle by one hour of light, the LDP flowers and the SDP does not. This response shows that the length of uninterrupted darkness is the important factor. Even though there is a daily total of 15 hours of darkness, the SDP does not flower because of that one hour of light. Flower growers who want to obtain winter flowering of LDPs simply expose them to a low level of incandescent light in the middle of the night. Summer flowering of SDPs is obtained by covering the plants in the late afternoon with an opaque cloth so that the SDPs receive enough darkness.

Regulation by Phytochrome

Plants monitor changes in day length with a bluish, light-sensitive pigment called phytochrome (FIET-uh-KROHM). Phytochrome exists in two forms, based on the wavelength of the light that it absorbs. The form that absorbs red rays is called Pr, and the form that absorbs far red (infrared) rays is called Pr. Daylight converts Pr to Pfr. In the dark, Pfr is converted to Pr. Phytochrome plays a role in bud dormancy and seed germination as well as in flowering.

www.scilinks.org Topic: Photoperiodism Keyword: HM61139

Maintained by the ,i j National Science I. \ J\ .3. Teachers Association

www.scilinks.org Topic: Photoperiodism Keyword: HM61139

Maintained by the ,i j National Science I. \ J\ .3. Teachers Association figure 31-10

The figure compares a short-day plant (SDP) with a long-day plant (LDP) in three variations of night length. The SDP has a critical night length of 14 hours. The LDP has a critical night length of 10 hours.

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