For many long-day plants (LDP), adding far red light (FR, 700 to 800 nm) to red light (R, 600 to 700 nm) to extend the day or interrupt the night promotes extension growth and flowering. Blue light (B, 400 to 500 nm) independently inhibits extension growth, but its effect on flowering is not well described. Here, we determined how R-, FR-, or B-deficient (R _d , FR _d , or B _d , respectively) photoperiods influenced stem extension and flowering in five LDP species: Campanula carpatica Jacq., Coreopsi × grandiflora Hogg ex Sweet, Lobelia × speciosa Sweet, Pisum sativum L., and Viola × wittrockiana Gams. Plants were exposed to R _d , FR _d , B _d , or normal (control) 16-hour photoperiods, each of which had a similar photosynthetic (400 to 700 nm) photon flux. Compared with that of the control, the R _d environment promoted extension growth in C. carpatica (by 65%), C. × grandiflora (by 26%), P. sativum (by 23%), and V. × wittrockiana (by 31%). The FR _d environment suppressed extension growth in C. × grandiflora (by 21%), P. sativum (by 17%), and V. × wittrockiana (by 14%). Independent of the R: FR ratio, the B _d environment promoted stem extension (by 10% to 100%) in all species, but there was little or no effect on flowering percentage and time to flower. Extension growth was generally linearly related to the incident wide band (100 nm) R: FR ratio or estimated phytochrome photoequilibrium except when B light was specifically reduced. A high R: FR ratio (i.e., under the FR _d filter) delayed flower initiation (but not development) in C. carpatica and C. × grandiflora and inhibited flower development (but not initiation) in V .× wittrockiana . Therefore, B light and the R: FR ratio independently regulate extension growth by varying magnitudes in LDP, and in some species, an FR _d environment can suppress flower initiation or development.

Phalaenopsis orchids are among the most valuable potted flowering crops commercially produced throughout the world because of their long flower life and ease of crop scheduling to meet specific market dates. During commercial production, Phalaenopsis are usually grown at an air temperature ≥28 °C to inhibit flower initiation, and a cooler night than day temperature regimen (e.g. 25/20 °C day/night) is used to induce flowering. However, the specific effect of day and night temperature on flower initiation has not been well described, and the reported requirement for a diurnal temperature fluctuation to elicit flowering is unclear. Two Phalaenopsis clones were grown in glass greenhouse compartments with constant temperature set points of 14, 17, 20, 23, 26, or 29 °C and fluctuating day/night (12 h/12 h) temperatures of 20/14, 23/17, 26/14, 26/20, 29/17, or 29/23 °C. The photoperiod was 12 h, and the maximum irradiance was controlled to ≤150 μmol m⁻² s⁻¹. After 20 weeks, ≥80% of plants of both clones had a visible inflorescence when grown at constant 14, 17, 20, or 23 °C and at fluctuating day/night temperatures of 20/14 °C or 23/17 °C. None of the plants were reproductive within 20 weeks when grown at a constant 29 °C or at 29/17 °C or 29/23 °C day/night temperature regimens. The number of inflorescences per plant and the number of flower buds on the first inflorescence were greatest when the average daily temperature was 14 °C or 17 °C. These results indicate that a day/night fluctuation in temperature is not required for inflorescence initiation in these two Phalaenopsis clones. Furthermore, the inhibition of flowering when the day temperature was 29 °C and the night temperature was 17 °C or 23 °C suggests that a warm day temperature inhibits flower initiation in Phalaenopsis.