Improved Tetrazolium Viability Testing In Orchid Seeds With A Thick Carapace (Dactylorhiza Fuchsii) Or Dark Seed Coat (Vanda Curvifolia)

The Orchidaceae are characterised by their very small seeds of apparently simple structure. Assessment of seed quality is often by germination, which can be a slow process and require precise conditions in vitro, whilst the alternative approach to determining quality using vital staining, e.g. tri-phenyl tetrazolium chloride (Tz), can be impeded by the presence of dark or impermeable seed coat structures. We developed modified Tz protocols for seeds of two orchid species: tropical Vanda curvifolia (dark testa) and temperate Dactylorhiza fuchsii (distinct carapace). Bleaching of the dark testa of previously Tz-stained seeds of Vanda curvifolia for five minutes with NaOCl (0.5%) produced clearly observable stained embryos, with values comparable with seed germination. Higher concentrations and longer disinfection times reduced apparent viability. Scarification with NaOCl (0.5%) for an optimum time of two-and-a-half minutes prior to Tz staining also resulted in a close correlation between viability and germination of Dactylorhiza fuchsii seeds which have a distinct carapace around the embryo. Overcoming morphological impediments to orchid seed viability assessment will enable greater quality assurance prior to seed storage.