DOMINANT ROLE OF SEAWATER OSMOTIC-PRESSURE ON GERMINATION IN CYMODOCEA-NODOSA

On eastern parts of the French Mediterranean coasts, seeds of Cymodocea nodosa (Ucria) Aschers. are abundant all the year round, but they rarely germinate. Previous experiments showed that germination, which can be initiated at any time, increases in rapidity both with increasing temperature (from 17 to 40-degrees-C) and with decreasing salinity (from 27 to 0 parts per thousand). The present work shows that salinity is the determining factor and that removal of the seed coats was without effect at 38 parts per thousand, but was stimulating at 20 parts per thousand. Comparison of germination times in mannitol solutions with those in artificial seawater of the same osmotic pressures shows that the effect of salinity is due to osmotic pressure. The water content of seeds is high, 35% of wet weight, and germination is accompanied by supplementary water uptake. The amount of supplementary uptake shows an inverse relationship with external osmotic pressure, amounting to 3% and 12% at an osmotic pressure of, respectively, 0.6 osm l-1 and 0.15 osm l-1. Seeds of C nodosa thus show secondary dormancy induced by the normal environmental salinity of 38 parts per thousand. The possibility of genetic variation in that dormancy is discussed for C. nodosa and for the genus Zostera.