Abstract. Geographic variation can lead to the evolution of different local varieties within a given species, therefore influencing its distribution and genetic structure. We investigated the contribution of plasticity and local adaptation to the performance of a common aquatic plant (Potamogeton pectinatus) in contrasting climates, using reciprocal transplants at three experimental sites across a latitudinal cline in Europe. Plants from 54 genets, originally collected from 14 populations situated within four climatic regions (subarctic, cold temperate, mild temperate, and mediterranean) were grown in three different localities within three of these regions (cold temperate, Norway; mild temperate, The Netherlands; mediterranean, Spain). Tuber production was highest for the mild-temperate genets, irrespective of locality where the genets were grown. Selection coefficients indicated that populations at the European center of the species distribution perform better than all other populations, at all sites. However, marginal populations showed changes in life-history traits, such as compressed life cycles in the north and true perenniality in the south, that may allow them to perform better locally, at the limits of their distribution range. Our results thus suggest that local adaptation may overlap spatially with center-periphery gradients in performance caused by genetic factors (such as genetic drift and inbreeding in range-marginal populations).

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Summary1 Broadly distributed plants have to cope with dramatic differences across latitude in the prevailing environmental temperature. We investigated the effect of water temperature on plant morphology, biomass accumulation and oxygen-exchange for five clones of the submerged aquatic macrophyte Potamogeton pectinatus L., originating from 42 to 68 ° N. 2 We tested whether P. pectinatus clones show local adaptation to the prevailing environmental temperatures (in which case high-latitude clones would perform better at lower temperatures and vice versa). Alternatively, pronounced phenotypic plasticity in the response to temperature could enable individual clones to perform well over a wide range of environmental temperatures (i.e. high degree of thermal tolerance). 3 The overall pattern of thermal response was similar for all clones. In addition, we detected acclimative phenotypic plasticity in both physiological and morphological plant parameters. The optimum temperatures for gross or for net photosynthesis did not vary with growth temperature, but morphological acclimation partly compensated for the loss of photosynthetic capacity at higher temperatures, enabling comparable rates of ambient gross photosynthesis. Respiratory reactions also showed some degree of thermal acclimation to higher temperatures. 4 As a result of the combined effects of changes in morphology and physiology, all clones produced similar amounts of plant biomass over a relatively wide range of water temperatures. We therefore conclude that P. pectinatus is thermally tolerant and not locally adapted.

Summary1 Widely distributed plants are exposed to contrasting gradients in irradiance and photoperiod across latitude. We investigated the relative contribution of local specialization and phenotypic plasticity to variation in plant growth for three clones of the aquatic angiosperm Potamogeton pectinatus L., originating from 42.5 to 68 ° N. Plants were grown at a factorial combination of two irradiances (50 and 350 µ mol m − 2 s − 1 ) and three photoperiods (13, 16 and 22 h) and morphology, gas-exchange rate and biomass accumulation were recorded. 2 The overall response to variation in irradiance and photoperiod was similar for all three clones. 3 Differences in irradiance resulted in strong acclimative changes in morphological and physiological characteristics. At low irradiance, pronounced vertical shoot extension compensated for the limited plasticity in leaf area production, while photosynthetic capacity, apparent quantum yield and total chlorophyll concentration increased. As a result, biomass yield at the end of the experimental period was similar in both treatments. 4 A decrease in photoperiod also resulted in plastic changes in morphology (increase of leaf biomass per unit plant biomass) and physiology (increase of photosynthetic capacity). However, these acclimative responses did not fully compensate for differences in photoperiod, since biomass was significantly lower under 13 and 16 h photoperiods than at 22 h. 5 P. pectinatus is therefore phenotypically plastic, rather than locally specialized to differences in irradiance and photoperiod.