Define photoperiodism.

• A. A plant's growth and developmental responses to the length of day or night
• B. A plant's response to soil pH
• C. A plant's response to nutrient availability
• D. A plant's response to temperature changes

Answer: (A)

Photoperiodism is the plant’s growth and developmental response to the length of the light period, meaning the duration of day and night. This concept explains why some plants flower only when days are long enough or when nights reach a certain length, while others flower under shorter days. The plant uses internal timing mechanisms and light receptors to measure night length, translating that information into timing of developmental steps like flowering or dormancy. For example, long-day plants flower when days are long (nights short), while short-day plants flower when nights are long (days short). Other environmental factors such as soil pH, nutrient availability, or temperature influence different physiological responses, but they are not about measuring day length.