Question

The inhibitory effect of red light on flowering during the critical dark period in short day plants can be overcome by
A. Blue light
B. Far-red light
C. Infra-red rays
D. Ultraviolet rays

Answer 1

Physiological reaction of organisms to the length of the night or a dark period is called photoperiodism. In addition, infra-red light is responsible for the conversion of phytochrome in its inactive form.

Complete answer:
Photoperiodism occurs in all plants and animals. It can be defined as the response of developing plants to the relative length of light and dark period. According to their photoperiod plants are classified as short-day plants, long-day plants, and day-neutral plants. -Photochrome or cryptochrome is a photoreceptor protein that senses the seasonal changes in night length or photoperiod. During the day period red light is present which is responsible for the conversion of phytochrome ${{P}_{fr}}$ and ${{P}_{r}}$ which is the active form and triggers the growth in plants. -On the contrary, the active form ${{P}_{fr}}$ is converted into inactive form ${{P}_{r}}$ in presence of infra-red rays which is present in dark or shade regions. This will inhibit the growth of plants and hence flowering does not take place. Flowering in short-day plants occurs when the night length exceeds their critical photoperiod. Interruption of the dark period by exposure of red light will inhibit the growth in short-day plants and flowering will not occur. This can be overcome by exposing infra-red rays as it converts ${{P}_{fr}}$ into ${{P}_{r}}$ and initiates flowering.
So, option C: infra-red rays are the correct answers as it can overcome the inhibitory effect of red light on short-day plants.

Note: The plants sense whether it is day or night depending on the conversions of phytochromes ${{P}_{fr}}$ and ${{P}_{r}}$. Depending upon the seasonal changes and daylength the biological and behavioral changes occur in plants as well as in animals.