At some point, the apical and lateral buds that grew while forming leaves during the vegetative growth stage become floral buds that form floral organs as it enters the reproductive growth stage. This transition is called flowering. The flowering stage is an important event for plants as it determines the success or failure of sexual reproduction. In many plants, the floral bud differentiates in a particular season, and these plants make use of day-length information to determine this season. Plants can be differentiated according to the relationship between day-length and flowering into long-day plants, in which flowering is induced when the day-length becomes longer; short-day plants, in which flowering is induced when the day-length becomes shorter; and neutral plants, in which flowering has no direct association with day-length. The response to periodic changes in light conditions such as day-length is called a photoperiodic response. A typical example is the flowering of long-day and short-day plants.

As its name implies “day-length” is the length of daylight; however, the length of the night is actually more important to plants. Long-day plants start forming floral buds when the length of the night becomes shorter than a predetermined period, called the critical dark period, while short-day plants do so when night becomes longer than the critical dark period. Here, the length of the night is the length of the continuous period of darkness, and not the total amount of darkness in one day. For example, if light is applied (light breaking) for 1 hour during an 11-hour dark period to break the dark period into 5 hours each, the effective length of the dark period is neither 11 hours, nor 10 hours, but 5 hours. Under such light conditions, short-day plants with a critical dark period of 8 hours will not flower, but long-day plants with the same critical dark period will.

How plants actually differentiate day and night and verify their lengths against the critical dark period in photoperiodic flowering is an interesting yet perplexing topic. Existing studies have revealed that the clock mechanism of plants depends on a biological clock based on a circadian rhythm, and that phytochromes and cryptochromes are involved in light sensing by plants. It is thought that plants process this information on sensed light along with the time according to the biological clock to determine the day-length (whether there is light during specific times determined by the biological clock is important). Photoreception by phytochromes and cryptochromes also play a role in adjusting the biological clock to the light and dark cycles. Consequently, light plays a dual role in photoperiodic flowering (Figure 22-8A).

Figure 22-8 Photoperiodic flowering

A) Role of light in photoperiodic flowering. B) Genetic route of flowering in Arabidopsis and rice

The identification and analysis of key regulator genes in model plants is gradually bringing more information to light about the gene networks related to photoperiodic flowering. Figure 22-8B shows the core genetic path. In Arabidopsis, a long-day plant, the gene GIGANTEA (GI) serves as the contact point between the biological clock and flowering. GI controls the expression of CONSTANS (CO) positively while CO controls the expression of FLOWERING LOCUS T (FT) positively. Under long-day conditions, the expression of FT is heightened due to this pathway, inducing flowering. Short-day plants like rice also have the same genes. The flowering genes corresponding to GI, CO, and FT are OsGI, Heading date1 (Hd1), and Heading date3a (Hd3a), respectively. These flowering genes also construct a similar genetic pathway as in Arabidopsis. However, control of Hd3a by Hd1 is not positive but negative. For this reason, long-day conditions inhibit flowering in rice plants. Temperature also influences flowering. For most plants that germinate in the autumn and form floral buds after passing the winter, experiencing a long period of low temperatures is a precondition for flowering. Vernalization is a process which uses this plant trait to artificially promote flowering using a low temperature treatment. It is generally only effective for plants with an active shoot apical meristem, and it differs from the low-temperature treatment sometimes required for germination.

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