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ISHS Acta Horticulturae 1134: VIII International Symposium on Light in Horticulture

Investigating reciprocity of intensity and duration of photoperiodic lighting to regulate flowering of long-day plants

Authors:   M.G. Blanchard, E.S. Runkle
Keywords:   boom lighting, Campanula carpatica, cyclic lighting, night interruption, Petunia ×hybrida, photoperiod
DOI:   10.17660/ActaHortic.2016.1134.6
When the natural photoperiod is short, commercial growers typically create long days (LDs) to stimulate flowering of LD crops using fixed lamps that deliver 1 to 
2 µmol m-2 s-1. As an alternative to fixed lamps, growers have experimented with “boom lighting”, which is a form of intermittent (cyclic) lighting in which lamps are attached to irrigation booms and the booms operate during the night. We simulated boom lighting in a research greenhouse to determine whether there was a reciprocal relationship between irradiance and lighting duration and whether regulation of flowering simply required delivering a specific quantity of photons. Two LD crops were grown under a 9-h photoperiod and night-interruption (NI) lighting was delivered by high-pressure sodium (HPS) or incandescent lamps that operated continuously or cyclically (e.g., 2 min every 15 min) for 4 h. Cyclic and 4-h continuous NI lighting treatments delivered an NI light integral (NILint) of 3,600 or 14,400 µmol 
m-2, which included the warm-up time of the HPS lamps. In the qualitative LD plant Campanula carpatica, HPS lamps that operated cyclically for 2 min every 15 or 30 min for 4 h and an NILint of 14,400 µmol m-2 promoted flowering similarly to HPS lamps that operated continuously for 4 h with the same NILint. However, a 2.4- or 24-min single continuous NI treatment did not promote complete flowering even with an NILint of 14,400 µmol m-2. In contrast, an NI for at least 2.4 min delivered continuously and with an NILint of 14,400 µmol m-2 promoted flowering of the quantitative LD plant Petunia ×hybrida. The cyclic lighting treatment with the longest interval (2 min every 45 min) and an NILint of 3,600 µmol m-2 also promoted flowering of Petunia similar to the 4-h continuous NI at the same NILint. We conclude that the law of reciprocity held true for Petunia under both NILint studied, but did not for Campanula.

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