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In protected cultivation of short-day (SD) plants, flowering can be inhibited by lighting from incandescent (INC) lamps during the night. INC lamps are being phased out of production and replaced by light-emitting diodes (LEDs), but an effective spectrum to control flowering has not been thoroughly examined. We quantified how the red [R (600 to 700 nm)] to far red [FR (700 to 800 nm)] ratio (R:FR) of photoperiodic lighting from LEDs influenced flowering and extension growth of SD plants. Chrysanthemum (Chrysanthemum ×morifolium), dahlia (Dahlia hortensis), and african marigold (Tagetes erecta) were grown at 20 °C under a 9-hour day with or without a 4-hour night interruption (NI) treatment by INC lamps or LEDs with seven different R:FR ranging from all R to all FR. Flowering in the most sensitive species, chrysanthemum, was not inhibited by an R:FR of 0.28 or lower, whereas an R:FR of 0.66 or above reduced flowering percentage. Flowering in dahlia was incomplete under the FR-only NI and under SDs, but time to flower was similar under the remaining NI treatments. The least sensitive species, african marigold, flowered under all treatments, but flowering was most rapid under the FR-only NI and under SDs. For all species, stem length increased quadratically as the R:FR of the NI increased, reaching a maximum at R:FR of ≈0.66. We conclude that in these SD plants, a moderate to high R:FR (0.66 or greater) is most effective at interrupting the long night, blue light is not needed to interrupt the night, and FR light alone does not regulate flowering.

Commercial greenhouse growers often produce bedding plants from midwinter to early summer, and thus crops are grown under a wide range of environmental conditions. Despite bedding plants' high economic value, the interactions of temperature and photosynthetic daily light integral (DLI) on growth and flowering have not been determined for many bedding plants. We grew celosia (Celosia argentea L. var. plumosa L.) and seed impatiens (Impatiens wallerana Hook.f.) in glass greenhouses in a range of temperature (15 to 27 °C) and DLI (8 to 26 mol·m^-2·d^-1) conditions to quantify effects on growth and flowering. Growth (e.g., plant dry mass at flowering) and flowering characteristics (e.g., time to flowering and flower bud number) were modeled in response to the average daily temperature and DLI by using multiple regression analysis. Rate of progress to flowering (1/days to flower) of celosia increased as temperature increased up to ≈25 °C and as the average DLI increased to 15 ·mol·m^-2·d^-1. Impatiens grown under a DLI <15 mol·m^-2·d^-1 flowered progressively earlier as temperature increased from 14 to 28 °C, whereas temperature had little effect on flowering time when plants were grown under the highest DLI treatments. Number of flowers and flower buds at first flowering increased in both species as temperature decreased or DLI increased. Shoot dry mass at first flowering followed a similar trend, except celosia dry mass decreased as temperature decreased. The models developed to predict flowering time and plant quality could be used by commercial growers to determine the impacts of changing growing temperature, growing plants at different times of the year, and providing supplemental lighting.

Plastics that selectively reduce the transmission of far-red light (FR, 700 to 800 nm) reduce extension growth of many floricultural crops. However, FR-deficient (FR_d) environments delay flowering in some long-day plants (LDPs), including `Crystal Bowl Yellow' pansy (Viola ×wittrockiana Gams). Our objective was to determine if FR light could be added to an otherwise FR_d environment to facilitate flowering with minimal extension growth. In one experiment, plants were grown under a 16-hour FR_d photoperiod, and FR-rich light was added during portions of the day or night. For comparison, plants were also grown with a 9-hour photoperiod [short-day (SD) control] or under a neutral (N) filter with a 16-hour photoperiod (long day control). Flowering was promoted most (i.e., percent of plants that flowered increased and time to flower decreased) when FR-rich light was added during the entire 16-hour photoperiod, during the last 4 hours of the photoperiod, or during the first or second 4 hours after the end of the photoperiod. In a separate experiment, pansy was grown under an FR_d or N filter with a 9-hour photoperiod plus 0, 0.5, 1, 2, or 4 hours of night interruption (NI) lighting that delivered a red (R, 600 to 700 nm) to FR ratio of 0.56 (low), 1.28 (moderate), or 7.29 (high). Under the N filter, the minimum NI duration that increased percent flowering was 2 hours with a moderate or low R:FR and 4 hours with a high R:FR. Under the FR_d filter, 2 or 4 hours of NI lighting with a moderate or low R:FR, respectively, was required to increase percent flowering, but a 4-hour NI with a high R:FR failed to promote flowering. Pansy appears to be day-neutral with respect to flower initiation and a quantitative LDP with respect to flower development. The promotion of reproductive development was related linearly to the promotion of extension growth. Therefore, it appears that in LDPs such as pansy, light duration and quality concomitantly promote extension growth and flowering, and cannot readily be separated with lighting strategies.

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.

Adding green [G (500–600 nm)] radiation to blue [B (400–500 nm)] and red [R (600–700 nm)] radiation creates white radiation and improves crop inspection at indoor farms. Although G radiation can drive photosynthesis and elicit the shade-avoidance response, its effects on plant growth and morphology have been inconsistent. We postulated G radiation would counter the suppression of crop growth and promotion of secondary metabolism by B radiation depending on the B photon flux density (PFD). Lettuce (Lactuca sativa ‘Rouxai’) was grown in a growth room under nine sole-source light-emitting diode (LED) treatments with a 20-hour photoperiod or in a greenhouse. At the same photosynthetic photon flux density [PPFD (400–700 nm)] of 180 μmol·m⁻²·s⁻¹, plants were grown under warm-white LEDs or increasing B PFDs at 0, 20, 60, and 100 μmol·m⁻²·s⁻¹ with or without substituting the remaining R radiation with 60 μmol·m⁻²·s⁻¹ of G radiation. Biomass and leaf expansion were negatively correlated with the B PFD with or without G radiation. For example, increasing the B PFD decreased fresh and dry mass by up to 63% and 54%, respectively. The inclusion of G radiation did not affect shoot dry mass at 0 or 20 μmol·m⁻²·s⁻¹ of B radiation, but it decreased it at 60 or 100 μmol·m⁻²·s⁻¹ of B radiation. Results suggest that the shade-avoidance response is strongly elicited by low B radiation and repressed by high B radiation, rendering G radiation ineffective at controlling morphology. Moreover, substituting R radiation with G radiation likely reduced the quantum yield. Otherwise, G radiation barely influenced morphology, foliage coloration, essential nutrients, or sensory attributes regardless of the B PFD. Increasing the B PFD increased red foliage coloration and the concentrations of several macronutrients (e.g., nitrogen and magnesium) and micronutrients (e.g., zinc and copper). Consumers preferred plants grown under sole-source lighting over those grown in the greenhouse, which were more bitter and less acceptable, flavorful, and sweet. We concluded that lettuce phenotypes are primarily controlled by B radiation and that G radiation maintains or suppresses lettuce growth depending on the B PFD.

A model was constructed to predict shoot-tip temperature of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) according to an energy-balance equation by using five greenhouse environmental factors: dry-bulb, wet-bulb, and sky (glazing or shade screen) temperature; transmitted shortwave radiation; and air velocity. An experiment was conducted to collect the five environmental variables that were used as model inputs, and shoot-tip temperature data were used to validate the predicted shoot-tip temperature in a commercial greenhouse. The standard deviation of the difference between predicted and measured shoot-tip temperature was 0.798 and was calculated by using 8547 data points, and >84% of the actual and predicted data points were within 1 °C. A sensitivity analysis performed with the model indicated that, among the three temperatures measured, plant shoot-tip temperature was primarily influenced by the dry-bulb temperature. For example, shoot-tip temperature increased an average of 0.74 °C for every 1 °C increase in dry-bulb temperature when dry-bulb temperature varied from 28 to 42 °C, wet-bulb temperature was 27.8 °C, sky temperature was 39.8 °C, shortwave radiation (285 to 2800 nm) was 760 W·m^-2, and air velocity was 0.44 m·s^-1. Under these conditions and a dry-bulb temperature of 32.6 °C, an increase in shortwave radiation of 500 W·m^-2 increased the shoot-tip temperature by an average of 3.3 °C. This developed model may be a useful tool to predict shoot-tip temperature and evaluate the effect of greenhouse environmental factors on shoot-tip temperature.

Chrysanthemum (Chrysanthemum ×morifolium) is a common ornamental crop with a qualitative short-day flowering response. Extending a short day with moderate blue [B (400–500 nm)] light inhibits flowering in greenhouse conditions with sunlight but does not indoors (without sunlight) under B + red [R (600–700 nm)] light or white light. We postulated that the contrasting responses to B light as a day extension depended on far-red [FR (700–800 nm)] light during the day, which is plentiful under sunlight but lacking indoors under B+R or white light-emitting diodes. To study this response in three chrysanthemum cultivars, we delivered indoor lighting treatments at two locations with an 11-hour main photoperiod of B, green [G (500–600 nm)], R, and FR light, where subscript values indicate the photon flux density (in µmol·m⁻²·s⁻¹) of each waveband: B₆₀R₁₂₀, B₆₀G₆₀R₆₀, and B₆₀R₆₀FR₆₀. After each short main photoperiod, plants received 0 or 4 hours of day-extension lighting of 60 µmol·m⁻²·s⁻¹ of B light (B₆₀). Under all treatments except B₆₀R₆₀FR₆₀ with day-extension B₆₀, it took ‘Chelsey Pink’, ‘Gigi Gold’, and ‘Gigi Yellow’ 13 to 17 days to reach the first visible inflorescence and 42 to 51 days to the first open flower. In contrast, plants grown under B₆₀R₆₀FR₆₀ with day-extension B₆₀ took 41 to 67 days to reach the first visible inflorescence with few plants developing open flowers. Plants were tallest at the first open flower and after 9 weeks of treatments when grown under B₆₀R₆₀FR₆₀ with day-extension B₆₀. These results indicate that the inclusion of FR light, but not G light, in the main photoperiod is necessary for day-extension B light to inhibit flowering in chrysanthemum. On the basis of these results and those of other studies, we postulate that the spectral dependence of flowering in chrysanthemum depends on whether and how the phytochrome photoequilibrium changes during the day. In particular, a sufficiently high daytime phytochrome photoequilibrium (e.g., under B+R and B+G+R light) could establish a predominant mode of floral signaling that prevents perception of subsequent B light as a long day.