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Abstract
Four-hour dark period interruptions of incandescent, red and far-red radiation each night during natural cooling hastened flowering by 22, 15 and 8 days, respectively, over natural daylengths. No additional acceleration occurred when night breaks were applied during natural cooling and forcing as compared to lighting during natural cooling only. The accelerating effect of night breaks was found to be the response to light alone and not total radiant energy or temperature. Bloom date acceleration by night breaks of incandescent light was due to a proportionate acceleration in flower bud initiation. For every week of acceleration there was an average decrease of 2 flower buds and 7 leaves per stem. Bulb potting depth also influenced bloom date. The depth effects on flowering time were independent of lighting treatments. A 4-day acceleration was obtained by setting the bulb nose at the soil line as compared to a 2 inch planting depth.
Plant height was independently influenced by photoperiod-light quality and by potting depth. Height was only slightly affected by supplemental lighting during natural cooling. After forcing temperatures were reached, red and incandescent lighting caused slight height increases while far-red caused pronounced stem stretching. Exposure of the bulb above the soil at potting reduced plant height at maturity.