Environmental requirements for germination of dispersion units of Welwitschia mirabilis Hook. fil

Welwitschia mirabilis is gymnosperm endemic to the Namib Desert. Welwitschia seeds are dispersed enveloped by the winged perianth which contains, according to some authors, a germination inhibitor. In this work, after testing for the presence of an inhibitor, we determined the effects of environmental parameters (light, temperature, water limiting conditions and salinity) on the germination of dispersion units of Welwitschia. We found that the winged perianth does not contain a germination inhibitor, seeds can germinate in both light and dark, and germination occurs in a range of temperatures between 15 degrees C and 40 degrees C, with an optimum temperature of 25-30 degrees C. Further, germination time decreased progressively and significantly along with the increase in temperature. Dispersion units subjected to water limiting conditions in a water vapor state were able to germinate up to a threshold limit (-1.5 MPa) below which germination decreases to nil. Welwitschia may be regarded as an halophyte species because, when the seeds are in contact with the salt solution in the Petri dish technique, additional salt toxicity is not shown, and reduced germination should therefore be attributed exclusively to water stress rather than ionic effect of salt. (C) 2015 Elsevier Ltd. All rights reserved.