Toward understanding the ecological role of mucilage in seed germination of a desert shrub Henophyton deserti: interactive effects of temperature, salinity and osmotic stress

Seeds of Henophyton deserti (Brassicaceae), an endemic saharan shrub in south Tunisia, produce a pectinaceous mucilage layer that can imbibe a large amount of water when wetted. The aim of this study was to explore the role of mucilage in seed germination of this shrub under heterogeneous stressful environments. Germination of both intact and demucilaged seeds was tested over wide ranges of temperature, and in iso-osmotic solutions of NaCl and PEG. Recovery of germination after NaCl and Polyethylene Glycol (PEG)-6000 treatment was also tested. The effect of mucilage on water uptake was measured and the structure of the seed investigated. A considerable proportion of seed mass (30 %) is made up of mucilage, which is extremely hydrophilic and able to increase seed mass by 550 % over dry seeds. Mucilage water uptake appears to be unaffected by salt concentration, while higher concentrations of PEG inhibit mucilage hydration. Mucilage decreases germination specifically at 10 A degrees C and this effect can be interpreted in relation to oxygen uptake. High concentrations of NaCl and PEG decrease both germination percentage and rate, with some greater tolerance at 15 A degrees C and 20 A degrees C versus 25 A degrees C. Recovery was higher from higher concentrations of NaCl and PEG and lower temperatures, with a clear inhibitory effect of mucilage. The study has shown that the mucilage of H. deserti may act as a physical barrier for regulating diffusion of water and oxygen to the inner tissue of the seed and thereby prevent germination under unsuitable conditions.