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 (FRd) 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 FRd environment to facilitate flowering with minimal extension growth. In one experiment, plants were grown under a 16-hour FRd 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 FRd 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 FRd 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.
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