Final answer:
The statement that the first stage of how plants prepare for subzero temperatures involves the photoperiod is true.
Step-by-step explanation:
The first stage of how plants prepare for subzero temperatures indeed involves the photoperiod, making the statement true. It is a critical factor in how plants detect seasonal changes and prepare for them. Plants utilize the phytochrome system, a light-sensitive chemical, to sense day length, which is a more stable indicator of season than temperature or light intensity which can vary yearly.
As the days grow shorter in the fall, the photoperiod triggers physiological responses in plants, such as dormancy and changes in leaf coloration, to survive impending cold conditions. This process ensures that flowering in plants like beet, which requires prior exposure to cool temperatures, will only occur at the optimal time. Therefore, the length of daylight sensed by plants dictates vital survival strategies and reproductive processes like flowering, where long-day plants and short-day plants respond to specific daylight lengths to initiate blooming.