Main content area

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

Blanchard, M. G., Runkle, E. S.
Acta horticulturae 2016 no.1134 pp. 41-48
Campanula carpatica, Petunia hybrida, crops, flowering, greenhouses, growers, incandescent lamps, irrigation, light intensity, photons, photoperiod, sodium
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.