How do plants monitor day length? Discuss the various categories of plants based on their photoperiodic responses. What happens when the dark period is interrupted by night break and why?

How Plants Monitor Day Length

Plants utilize a photoreversible pigment called phytochrome to detect changes in light and dark periods. Phytochrome exists in two interconvertible forms: Pr (red-light absorbing) and Pfr (far-red light absorbing).

• Pr form: Absorbs red light (660nm) and converts to Pfr. This is the inactive form.
• Pfr form: Absorbs far-red light (730nm) and converts back to Pr. This is the active form, initiating physiological responses.

During daylight, red light predominates, converting Pr to Pfr. At night, Pfr slowly reverts to Pr, or is degraded. The ratio of Pfr to Pr at dawn is the critical factor determining the plant’s photoperiodic response. A high Pfr/Pr ratio indicates a long day, while a low ratio indicates a short day. This ratio acts as a signal triggering flowering or other developmental changes.

Categories of Plants Based on Photoperiodic Responses

Plants are categorized into three main groups based on their flowering responses to day length:

• Short-Day Plants (SDP): These plants flower when the day length is less than a critical photoperiod (i.e., the dark period is longer than a critical length). Examples include chrysanthemum, rice, soybean, and strawberry. They require an uninterrupted dark period for flowering.
• Long-Day Plants (LDP): These plants flower when the day length is greater than a critical photoperiod (i.e., the dark period is shorter than a critical length). Examples include spinach, lettuce, wheat, and barley.
• Day-Neutral Plants (DNP): These plants flower irrespective of day length. Their flowering is often influenced by factors like temperature, maturity, or nutrient availability. Examples include tomatoes, cucumbers, and sunflowers.

Plant Category	Critical Day Length	Flowering Trigger	Examples
Short-Day	Less than critical	Long dark period	Chrysanthemum, Rice
Long-Day	More than critical	Short dark period	Spinach, Wheat
Day-Neutral	Independent	Maturity, Temperature	Tomato, Sunflower

The Effect of Night Break

Interrupting the dark period with even a brief flash of light, known as a night break, can dramatically affect flowering in short-day plants.

• In SDPs: A night break converts Pfr back to Pr, effectively resetting the photoperiodic clock. This prevents the accumulation of sufficient Pfr to trigger flowering, even if the overall dark period remains long enough. Therefore, a night break can inhibit flowering in SDPs.
• In LDPs: A night break can promote flowering. The brief flash of light converts Pr to Pfr, increasing the Pfr level and satisfying the requirement for a short dark period.
• In DNPs: Night breaks generally have little to no effect on flowering.

The critical dark period is the uninterrupted period of darkness required for flowering. The phytochrome system is sensitive to this uninterrupted darkness, and any interruption resets the system. This sensitivity is exploited in horticulture to control flowering times in commercially important plants.