Final answer:
Phytochrome is a plant pigment protein that mediates photoperiodism, controlling plant lifecycle events like flowering. It acts as a biological light switch, responding to changes in day length, which plants use as an indicator of seasonal change, crucial for their survival.
Step-by-step explanation:
The phytochrome is a plant pigment protein that mediates photoperiodism. This process involves the plant's response to the timing and duration of day and night, and is critical for the adjustment of the plant's lifecycle events, such as flowering, setting of winter buds, and overall vegetative growth. Phytochrome plays a crucial role in sensing changes in day length, which is a reliable indicator of seasonal variation essential for plant survival.
When irradiated by red light, the photoreceptor is converted to its far-red light-absorbing form (Pfr), which controls not only germination and flowering in response to day length but also activates photosynthesis in dormant plants. While the phytochrome system acts as a biological light switch, monitoring the level, intensity, duration, and color of environmental light, cryptochromes, and phototropins—other plant photoreceptors—are responsible for processes such as phototropism.
It is important to note, the effect of red light on phytochrome can be reversed by far-red light, and this interconversion regulates the presence of the active form of phytochrome (Pfr), which in turn can activate or repress specific gene expression. This is a fine example of how the phytochrome system interfaces with the plant's circadian rhythms to produce photoperiodic responses and modulate plant growth and development.