Photoperiodism and Vernalization Questions in English

(C) Statement $i$ is incorrect because phytochrome is present in almost all parts of the plant,not just leaves.
Statement $ii$ is correct; it exists as $P_r$ and $P_{fr}$.
Statement $iii$ is correct; $P_r$ and $P_{fr}$ are interconvertible by red and far-red light.
Statement $iv$ is incorrect because phytochromes are located in the cytoplasm or cell membrane of green cells,not specifically in the chloroplast.
Statement $v$ is correct; $P_r$ form is inactive and does not promote flowering in $LDP$ (Long Day Plants) during the dark period.
Therefore,statements $i$ and $iv$ are incorrect.

(C) Photoperiodism is the physiological reaction of organisms to the length of night or a dark period.
Short-day plants $(SDP)$ require a long,uninterrupted dark period to flower. If this dark period is interrupted by a flash of light,they fail to flower.
Long-day plants $(LDP)$ require a short dark period. $A$ flash of light during the dark period effectively shortens the night,which promotes flowering in $LDP$.
Since plant '$A$' produced flowers after a light flash,it must be a long-day plant $(LDP)$.
Since plant '$B$' did not produce flowers after a light flash,it must be a short-day plant $(SDP)$ because the light flash interrupted the critical dark period required for its flowering.
Therefore,'$A$' is a long-day plant and '$B$' is a short-day plant.

(D) Vernalization is the process of inducing flowering by exposure to prolonged cold temperatures.
Apart from promoting flowering,vernalization is known to increase the resistance of plants against fungal diseases.
This is because the cold treatment can trigger physiological and biochemical changes in the plant,such as the production of specific proteins or secondary metabolites,which enhance the plant's defense mechanism against pathogens,particularly fungi.