A plant's response to changes in the length of days and nights is called photoperiodism. Photoperiodism affects many plant processes, including the formation of storage organs and bud dormancy. However, the most-studied photoperiodic process is flowering. Some plants require a particular night length to flower. In other species, a particular night length merely makes the plant flower sooner than it otherwise would.

Day Length and Night Length

Researchers have found that the important factor in flowering is the amount of darkness, or night length, that a plant receives. Many plant species have specific requirement for darkness, called the critical night length. Although scientists now know that night length, and not day length, regulates flowering, the terms short-day plant and long-day plant are still used. A short-day plant flowers when the days are short and the nights are longer than a certain length. Conversely, a long-day plant flowers when the days are long and the nights are shorter than a certain length.

Plants can be divided into three groups, depending on their response to the photoperiod, which acts as a season indicator. The three groups of plants are summarized in Table 31-3. The largest group, made up of day-neutral plants (DNPs), is not affected by day length. Plants that are DNPs for flowering include tomatoes, dandelions, roses, corn, cotton, and beans.

TABLE 31-3 Flowering Photoperiodism

Type of plant

Conditions needed for flowering

Seasons of flowering

Day-neutral plant (DNP)

not affected by day-night cycle

spring to fall

Short-day plant (SDP)

short days (long nights)

spring, fall

Long-day plant (LDP)

long days (short nights)


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