Fennel pondweed: a world citizen : geographic variation in life-cycle characteristics of the clonal water plant Potamogeton pectinatus L.

 

 

The performance of broadly distributed plants is potentially constrained by geographic variation in climatic factors. Several patterns of response have been proposed that can be considered of adaptive value if variation in abiotic conditions is pronounced. Firstly, species may possess a high capacity for phenotypic plasticity that results in increased tolerance to variation in abiotic factors. Secondly, locally adapted genotypes may evolve that show phenotypic characteristics that are only suited for a restricted set of environmental conditions. Thirdly, populations may avoid unfavourable conditions by showing changes in phenology that result in a compression of the life cycle.

As compared to terrestrial species, many aquatic plants are widespread and have the ability to thrive in different climatic regions. To evaluate to what extent phenotypic plasticity, local adaptation and differences in phenology might contribute to the globally wide distribution of the aquatic macrophyte fennel pondweed ( Potamogeton pectinatus L.), a series of experiments were performed that focused on geographic variation in life-history traits. For this purpose we used up to 15 clones obtained from a gradient in latitude (24-68°N) and studied their performance in dependence of variation in abiotic factors that relate to climate. We thereby focussed at various stages of the life cycle, such as tuber sprouting, vegetative growth and asexual reproduction.

At northern localities, where the length of the growth season is restricted, genetically fixed changes in phenology result for P. pectinatus in a compression of the life cycle. To prevent young plants to be damaged from low spring temperatures, tubers of higher-latitude clones possess a higher thermal threshold for sprouting. In addition, northern clones show early reproduction, which constrains the size of the tubers. Furthermore, adaptive phenotypic plasticity allows P. pectinatus to grow at contrasting environmental conditions ) . Although thermal acclimation in gas-exchange is constrained, plastic changes in morphology are of special importance to attain a comparable biomass between 15/20 and 30°C. P. pectinatus can also cope with considerable differences in the light climate. Increased light capture through canopy formation and acclimative changes in photosynthesis result in a relatively high biomass yield at low irradiance (i.e. 2.5% of full sunlight in temperate regions). Similarly, photoperiods varying between 13 and 22 h did result in plastic changes in morphology and physiology that limited the loss of biomass productivity at shorter days. Despite the fact that changes in phenology and high phenotypic plasticity allowed P. pectinatus to grow and reproduce in different climatic regions, the Mediterranean clones showed local adaptation resulting from increased perenniality and the absence of asexual reproduction.

In conclusion, this thesis has shown that not a single evolutionary mechanism is responsible for the ability of P. pectinatus to thrive in different climatic regions, but that at different stages of the plant's life cycle phenotypic plasticity, local adaptation and changes in phenology play an important role in maintaining the observed distributional pattern.