Seed dormancy concepts in orchids: Dendrobium cruentum as a model species.

Generally, orchids produce dust-like seeds in which endosperm reduction and embryo undifferentiation represent a derived state shared with species in about 11 other plant families. Orchid seeds are proposed to have a special kind of morphological or morphophysiological dormancy. We test this proposition, overcoming several design limitations of earlier studies, specifically that the germination method for orchid seeds uses pro-oxidants for disinfection and incorporates nitrate in the medium; both ‘treatments’ might contribute to dormancy breaking, potentially confounding judgement on the depth and nature of the dormant state. Seeds of the tropical orchid Rchb. f., were sown both , on a nutrient medium, and , on plain agar omitting prior disinfection with sodium hypochlorite. Seeds previously stored and fresh seeds were incubated under combinations of vitro conditions, light treatments, constant or alternating temperatures and nitrate concentration. Seeds of are very small but have a large embryo that occupies most of the seed. Over a range of constant temperature seeds germinated to the spherical protocorm stage just as well as . Neither light nor nitrate were prerequisites for germination. The ability of seed to germinate in the absence of environmental or chemical stimuli suggests that mature seed can be non-dormant. Our results support the proposition that neither all DUST seed fit a dormancy class nor all orchids produce morphological or morphophysiological seeds. Finally, embryo/seed volume determinations in orchids may prove as valuable in studies on the evolution and ecology of germination and dormancy as embryo:seed ratios in other angiosperm species.