Flowering and Day-Length

Plants are sensitive to the absence of light (photoperiodism). Many plant responses are affected by photoperiodism including breaking of dormancy and flowering.Phytochrome exists in two interconvertible forms P_r(P₆₆₀) sensitive to red light and P_fr (P₇₃₀) the bioactive form which is sensitive to far-red light.In short-day plants, P_fr inhibits flowering. During long periods of darkness P_fr levels fall allowing flowering to take place.In long-day plants, P_fr stimulates flowering. During short nights very little P_fr is converted to P_r allowing flowering to take place.