PLASTIC RESPONSES MORPHOLOGY OF POTAMOGETON PECTINATUS L TO SEDIMENT AND ABOVE-SEDIMENT CONDITIONS AT 2 SITES IN THE NORTHERN BALTIC PROPER

The morphology of Potamogeton pectinatus L. varies markedly between tw o sites, with different degrees of wave exposure, in the northern Balt ic proper. To study the morphological variation attributable to phenot ypic plasticity, tubers and sediment were reciprocally transplanted be tween the two sites. Several plant characters respond plastically to s ediment type. The average values in muddy compared with sandy sediment was: for plant dry weight 1.0 g and 0.4 g; for total rhizome length 1 .3 m and 0.6 m; for root:shoot ratio 0.08 and 0.21 respectively. Plant s collected at the sheltered site were significantly more phenotypical ly plastic in total growth and rhizome length, than plants originating from the exposed site. The size of the transplanted tubers had a sign ificant impact on growth and survival of the plants. Shoot length and branching were greatly affected by the above-sediment conditions, i.e. mainly wave exposure. The shoots were on average 21 cm tall at the sh eltered site and only 10 cm at the wave exposed site. This was mainly due to a plastic response in internode length and to a minor extent to dislodgement of shoot parts. Many of the observed plastic responses i n morphology of P. pectinatus result from phenotypic plasticity in gro wth. The plasticity in the root:shoot ratio to the nutrient content of the sediment and the plasticity in growth pattern to the degree of wa ve exposure are however suitable adjustments in resource allocation to the environmental conditions. All measured characters showed signific ant phenotypic plasticity in the experiment, nevertheless large morpho logical differences remained between plants from the two sites, when g rown under the same environmental conditions.