The site of perception of light in plants during photoperiodism is :

Photoperiodism is the physiological response of plants to the relative lengths of day and night. For this response to occur, the plant must first detect the presence and duration of light.

The primary site where light is perceived for photoperiodic signaling is the leaf. Leaves contain specialized photoreceptor proteins, such as phytochromes and cryptochromes, that absorb red/far‑red and blue light respectively. These photoreceptors undergo conformational changes when illuminated, initiating a cascade of molecular signals.

Experimental evidence supports the leaf as the perception organ:

• When leaves are removed or covered, plants fail to exhibit normal flowering or other photoperiod‑dependent responses, even though the shoot apex and stem remain intact.
• Conversely, exposing only the leaves to appropriate light periods while shielding other parts restores the normal photoperiodic behavior.

After light detection in the leaf, the signal is transmitted through the plant’s vascular system to the shoot apex, where the actual developmental switch (e.g., flowering) is executed. Thus, the leaf functions as the “sensor” while the shoot apex acts as the “response” site.

In summary, the site of perception of light during photoperiodism in plants is the leaf.