Photoperiodic control of flowering

Eventhough the majority of orchids are day-neutral , there are a few species which are ' controllable' . In these plants, the hormone, phytochrome, is produced only when the day and night are of a particular duration . So by controlling the day-length , we are able to control the time of flowering in such species . This facet of orchid culture has been developed to the point of perfection in the ' labiata' group of Cattleyas. According to their light requirements , there are long-day and short-day plants. Arditti (1966) is of opinion that long night is more important in bringing about flowering than the short-day plants . In the genus Cattleya itself, there are both short-day (long night ) and long-day (short night) plants. eg. the Warscewiczii group (C. warsce-wiczii, C. gaskelliana , C. skinneri ,C. downiana etc. ) are long day plants while the Mossiae group (C.mossiae, C. labiata, C. trianne etc) are short-day plants.

The correct ratio between the light and dark periods and in some cases the correct temperature also initiate the production of phytochrome , which is a protein acting as an enzyme. It is a blue pigment and occurs in two reversible forms-one absorbing light in the red and the other in the far red region . Absorption of red rays stimulates growth whereas , absorption in the far region nullifies it (Arditti, 1966).

Dodson reports (1962) that filtered sunlight stimulates formation of the male flower in Catasetum while in direct sunlight only female flowers are produced.