Seed germination responses to low-level narrow-band light spectra for 14 ornamental plant genotypes

To investigate whether low-level lighting is necessary and which narrow-band light spectra are effective in seed germination, seeds of 14 genotypes from begonia (Begonia x tuberosa), echinacea (Echinacea spp.), gerbera (Gerbera jamesonii), petunia (Petunia x hybrida), and vinca (Catharanthus roseus) were germinated under ultraviolet-B (UVB), blue (B), green (G), red (R), far-red (FR) light, or darkness. Light-emitting diode (LED) fixtures provided all spectrum treatments except for UVB, which was provided by a narrow-band fluorescent light. The photon flux density at seed level was approximate to 18 mu mol.m(-2).s(-1) for B, G, R, and FR, and 0.4 mu mol.m(-2).s(-1) for UVB. Based on daily germination observations, final germination percentage, germination onset time, germination time spread, and germination speed were compared among different spectrum treatments for each of the plant genotypes. There were no promotive effects on final germination percentage, germination onset time, and germination time spread under the narrowband lights compared with darkness. For all plant genotypes, B had a similar effect as darkness on seed germination. FR inhibited seed germination relative to darkness by reducing final germination percentage by 31%-88%, delaying germination onset time by 30%-40%, and decreasing germination speed by 11%-48% in some genotypes. Under R, G, and UVB compared with darkness, germination speed was promoted for begonia 'Apricot Shade', a light-requiring genotype, and inhibited for vinca 'Burgundy', a light-inhibited genotype. Therefore, lighting at low levels used in our study is unnecessary for seed germination of the tested species except light-requiring genotypes, where R, G, and UVB are the most promotive among the tested narrow-band lights.