Plant Growth And Development

Photoperiodism, the response of plants to the relative lengths of light and dark periods, is explained in NCERT Class 11, Chapter 15 (Plant Growth and Development). Short-day plants flower when the period of uninterrupted darkness exceeds a critical duration, which means the dark period is actually the decisive cue. The pigment that perceives these light and dark signals is phytochrome, located in the leaves; it exists in two interconvertible forms that respond to red and far-red light and thereby measure the photoperiod. Once the appropriate signal is perceived, a flowering stimulus moves to the shoot apex to induce flower formation. Chlorophyll a is the principal photosynthetic pigment that captures light energy for carbon fixation, not for measuring day length. Carotenoids are accessory photosynthetic pigments that broaden the absorption spectrum but do not sense photoperiod. Anthocyanins are water-soluble pigments contributing to flower and fruit colour, with no role in timing flowering. A consistency check supports the answer: because a brief flash of light during the dark period can prevent short-day plants from flowering, a specialised light-sensing pigment, phytochrome, must be detecting the photoperiodic signal.