Can a high red: Far red ratio replace temperature-induced inflorescence development in Phalaenopsis?

Phalaenopsis is one of the most economically important ornamental crops in the Netherlands, but due to its growth requirements during its various developmental stages, it is a costly crop to grow in terms of energy. Warming the greenhouse in the winter during the vegetative phase, and cooling the greenhouse to 19 °C in the summer to induce flowering costs a great deal of energy. Because flower induction is primarily controlled by altering the balance of plant hormones, it might well be possible to steer flower induction by changing the light spectrum instead of changing the temperature. The objective of the present study was to investigate the role of the light spectrum on bud break and inflorescence elongation in Phalaenopsis. To this end an experiment was designed with two Phalaenopsis cultivars, Quincy and Red Stones in a matrix of temperature and spectral treatments. Light treatments were used with a relatively high red to far-red ratio (R:FR) and therefore a high phytochrome photostationary state (PSS) value of 0.85, and with a relatively lower R:FR ratio resulting in a PSS value of 0.71, similar to that of natural daylight. Our hypothesis was that low temperature treatments can, at least partially, be substituted by a light spectrum with a high PSS value in order to suppress apical dominance of the primary flowering inflorescence and thus realize multiple inflorescences. A cooling period (19 °C) of 8 weeks always resulted in a high percentage of multiple inflorescences, both from Red Stones as well as Quincy. This implies that a flower induction period of 8 weeks with cooling is effective, regardless of the light spectrum. An induction period of 8 weeks with “red light” always resulted in a high percentage of multiple inflorescences, regardless of the temperature (19 or 22 °C). A cooling period (19 °C) of 4 weeks followed by 4 weeks 22 °C only resulted in a higher percentage of multiple inflorescences under the light spectrum with a high PSS value during the second 4 week-period. This suggests that the plant hormones responsible for bud break and inflorescence elongation, can be stimulated by light with a high PSS value, as well as by a cool phase. In order to stimulate multiple flowering inflorescences, the temperature and/or light spectrum during the second 4-week-period appears to be more important than during the first 4-week-period of the induction phase. Presumably this is due to suppression of the apical dominance of the first emerging inflorescence by low temperature and/or relatively high R:FR ratio during this phase. This also implies that the energy costly cool-induction period during the summer can be replaced, at least in part, by red light or a relatively high R:FR