2017 ASHS Annual Conference

Lighting can extend short days to promote flowering of long-day plants and inhibit flowering of short-day plants. Red/far-red and blue light control flowering primarily through phytochromes and cryptochromes, respectively. Green light acts antagonistically to blue light in cryptochrome-mediated physiological responses such as stomatal opening; however, the role of green light in photoperiodic regulation of flowering is not well understood. We grew four long-day plants [two petunia (Petunia ×hybrida) cultivars, ageratum (Ageratum houstonianum), and snapdragon (Antirrhinum majus)] and four short-day plants [three chrysanthemum (Chrysanthemum × morifolium) cultivars and marigold (Tagetes erecta)] at a set point of 20 °C in a research greenhouse with supplemental high-pressure sodium lighting. Plants received 9-hour natural short days with or without 7-hour day-extension lighting from green light (peak = 522 nm) at 2, 13, or 25 µmol∙m^–2∙s^–1 or red+white+far-red light at 2 µmol∙m^–2∙s^–1. We observed a dose-response relationship between green light intensity and flowering time. As green light intensified from 0 to 25 µmol∙m^–2∙s^–1, petunia flowered earlier, and chrysanthemum flowered later. Ageratum and marigold perceived green light at 2 µmol∙m^–2∙s^–1 as a saturating long-day signal. However, petunia flowered most rapidly when green light was ≥13 µmol∙m^–2∙s^–1. In contrast, green light at 25 µmol∙m^–2∙s^–1 promoted flowering of snapdragon marginally. Therefore, depending on the intensity, green light can effectively control flowering of some, but not all, photoperiodic floriculture crops.