Growth and photosynthesis of Hydrocotyle ranunculoides in Central Europe
Hydrocotyle ranunculoides (Apiaceae, EPPO A1 List) is present in the EPPO region in Azerbaijan, Belgium, France, Germany, Israel, Italy, the Netherlands, Portugal, and Spain. To characterize the probable further spread of the species, in particular in Central Europe, some of its ecological and ecophysiological characteristics were investigated in Nordrhein-Westfalen, Germany.
Life cycle of the plant in Nordrhein-Westfalen
H. ranunculoides showed rapid growth with a maximal growth rate in June and July. Starting from small plants or fragments after melt of ice, plants grew slowly in spring and formed small, up to 10 cm² wide leaves, most of which floated on the water surface. With increasing temperatures, length of photoperiod and light intensities, leaves grew larger and reached a height of up to 40 cm above water level. The plants flowered and fruited between May and October and the stands got denser. With a decrease in temperatures and light availability in autumn, plants developed smaller new leaves and most of the leaves died after the first night of frost. At this time, plants formed new floating and submerged leaves. Floating leaves died when enclosed by ice, but submerged leaves were able to survive the winter months. From these small submerged plants and leafless overwintering, stolons, new plants grew out in spring.
Growth and photosynthesis
H. ranunculoides has a high regeneration capacity and can form new shoots even from small stem fragments (1 cm in length with one node, and with or without leaves). In the experiment, single leaves and internode fragments of the shoots did not form new shoots. Development of new shoots was almost completed after 1 week regenerating from cuttings that were made up by a node with open leaf, and after 2 weeks if regeneration occurred from a node without attached leaves (90% achieved after the first week). Results showed increasing growth rates under increasing nutrient availability. H. ranunculoides net CO2 exchange peaked at 25-35°C.
From the experiments, it is clear that H. ranunculoides has a high capacity to become a vigorous invasive alien plant in Central Europe. Its high regeneration capacities showed that mechanical control will be difficult to apply without dispersing the plant via small stem fragments. Results of experiments suggested that this species is well adapted to the predicted climatic conditions which will prevail in Central Europe during the next decade (the species prefers high CO2 gas exchange and high temperatures).
Hussner A, Lösch R (2007) Growth and photosynthesis of Hydrocotyle ranunculoides in Central Europe. Flora 202, 653-660.