Light stimulation and darkness requirement for the symbiotic germination of Dactylorhiza majalis (Orchidaceae) in vitro

Germination percentage of Dactylorhiza majalis (Rchb.f.) Hunt & Summerh. (Orchidaceae) was increased by illuminating surface‐sterilized, rinsed and incubated seeds with white light in 16 h photoperiods. Optimal exposures (10–14 days) raised germination from 40 to 75%. Longer light treatments resulted in reduced germination percentage and smaller seedlings. Only about 2% of the seeds could actually germinate in photoperiods; the germination of the rest was delayed by initial light and required about 14 days in constant darkness. Interruption of this darkness period with two consecutive photoperiods increased the germination percentage when the interruption occurred before day 8, but not when occurring later than that.