Q.What do you understand by photoperiodism and vernalisation?

Photoperiodism is the response of plants to the duration of light, so the answer is (B).

Photoperiodism is the effect of the relative lengths of light and dark periods (day length) on flowering and other responses in plants. Based on their response, plants are classed as:

• Short-day plants — flower when the day length is below a critical value.
• Long-day plants — flower when day length is above a critical value. …

Pea, barley, and oats are long day plants — they flower when exposed to day lengths longer than their critical photoperiod, typically in spring/early summer.

Based on their photoperiodic response, flowering plants are grouped as:

• Long Day Plants (LDPs): flower only when day length exceeds a critical minimum (i.e., when the dark period is shorter than a critical maximum) — commonly flowering in spring/summer as days lengthen. Examples: wheat, barley, oats, pea, spinach, radish, henbane.
• Short Day Plants (SDPs): flower only when day length is less than a critical maximum (long uninterrupted dark period needed) — typically flowering as days shorten in autumn. Examples: rice, tobacco, soybean, cocklebur (Xanthium), chrysanthemum.
• Day Neutral Plants (DNPs): flowering is not controlled by photoperiod. Examples: tomato, cotton, sunflower, cucumber. …

Photoperiodism is a plant's flowering (and other developmental) response to the relative durations of light and dark it experiences daily.

Many plants flower only when they are exposed to a light duration (photoperiod) either above or below a critical length, a phenomenon discovered by W.W. Garner and H.A. Allard. Based on this response, plants are classified as: short-day plants (SDPs), which flower when day length is shorter than a critical value (e.g., chrysanthemum, tobacco); long-day plants (LDPs), which flower when day length exceeds a critical value, generally in summer (e.g., spinach, radish); and day-neutral plants, whose flowering is unaffected by photoperiod (e.g., tomato, cotton). The site of perception of the light/dark stimulus …