Contrasting responses in two populations of Zostera noltii Hornem. to experimental photoperiod manipulation at two salinities

Zostera noltii plants from two contrasting habitats (the intertidal Zandkre ek in the SW Netherlands and a quiet cove Cala Jonquet on the Mediterranean coast of Catalonia, NE Spain) were grown for 28 weeks in a common garden e xperiment. In this experiment the plants were exposed to two salinities (15 and 35 parts per thousand S) and two seasonal daylength regimes (one repre senting 22 degrees Northern Latitude or the southern limit of this species in Mauretania and the other of 55 degrees NL or northern Denmark, close to the northern limit). Plants from both populations suffered considerable mortality at the high sa linity (25% survival for the Dutch and 60% for the Catalonian population) b ut survival was good at the low salinity. Dutch plants realised similar sho ot production as in the field only in the treatment combination of 15 parts per thousand S-55 degrees NL. Surviving Catalonian shoots produced compara ble numbers of shoots as in the field in all four treatments and hence were less sensitive to daylength seasonality. Although the differences between populations had decreased during the exper iment, observed variation in leaf width and rhizome diameter at the end of the experiment was explained largely by the factor source population (contr ibuted 73 and 83%, respectively, to total variation in a threeway ANOVA com paring effects of source population, daylength regime and salinity). For to tal biomass produced, shoot length and weight, root length as well as rhizo me internode length, the experimental salinities and daylength regimes ofte n also explained comparable proportions of variance. It is concluded that: (a) the observed quantitative differences between the two populations of Z. noltii must have a considerable genetic basis, (b) p lants from both populations were not well equipped to survive at worldwide average seawater salinity of 35 parts per thousand S, and (c) the Dutch pop ulation responded more vigorously to the more pronounced daylength seasonal ity of northern latitudes, which is coupled to its higher photosynthetic li ght requirements. (C) 2000 Elsevier Science B.V. All rights reserved.