Short day plants flower when the nights are longer than a critical length, in other words when they receive a long, uninterrupted dark period. In the UK they usually flower in late summer, autumn or winter. Examples include chrysanthemums, poinsettias, soya beans and the cocklebur (Xanthium).How phytochrome controls floweringIn short day plants, Pfr inhibits flowering.- In autumn and winter the nights are long, so there is time for most Pfr to convert back to Pr. The Pfr level falls low enough that flowering is no longer inhibited, florigen is produced and the plant flowers.
- In summer the nights are short, so Pfr stays high and flowering is inhibited.Night-break experimentsA short day plant given long nights flowers. If the long night is interrupted by a brief flash of red light, Pr is converted to Pfr, the night is effectively split into two short nights and the plant does not flower. If the red flash is immediately followed by far-red light, the Pfr is converted back to Pr and the plant flowers.
The cocklebur is so sensitive that a single long night can be enough to trigger flowering.Commercial useGrowers use this to control flowering. Chrysanthemums can be made to flower at any time of year by covering them with blackout screens to give long nights, or kept from flowering by lighting them for a short time in the middle of the night. Poinsettias are given long nights in autumn so that their coloured leaves are ready for Christmas.
- In summer the nights are short, so Pfr stays high and flowering is inhibited.Night-break experimentsA short day plant given long nights flowers. If the long night is interrupted by a brief flash of red light, Pr is converted to Pfr, the night is effectively split into two short nights and the plant does not flower. If the red flash is immediately followed by far-red light, the Pfr is converted back to Pr and the plant flowers.
The cocklebur is so sensitive that a single long night can be enough to trigger flowering.Commercial useGrowers use this to control flowering. Chrysanthemums can be made to flower at any time of year by covering them with blackout screens to give long nights, or kept from flowering by lighting them for a short time in the middle of the night. Poinsettias are given long nights in autumn so that their coloured leaves are ready for Christmas.