Influences of light intensity variations on growth characteristics of Myriophyllum aquaticum

Myriophyllum aquaticum is a heterophyllous plant that is native to South America and has been introduced to the United States. The reallocation of resources to emergent or submersed growth forms likely allows M. aquaticum to overcome changes in light availability and to invade different habitats. Our objective was to determine the effects of light availability on plant length, biomass allocation, and relative growth rate through replicated mesocosm experiments. Myriophyllum aquaticum was grown in full sunlight, 30%, 50%, and 70% shade in replicated treatments. Total plant length, emergent shoot length, submersed shoot length, and the total of number of emergent and submersed shoots were recorded. Plants were harvested and sorted into emergent shoots, submersed shoots, roots, and stolons, then dried and weighed to assess biomass allocation. After 12 weeks, biomass was different among shade treatments. Differences in plant mass were a result of greater plant growth in the 30% shade treatment. Total plant length was greatest in the 50% shade treatment with a reduction in plant length observed in full sunlight. Emergent shoot length was reduced in full sunlight, while an increase in submersed shoot length occurred in 70% shade. Our data suggest that intermediate light availability is optimal for M. aquaticum growth and that the growth of two leaf forms is a physiological response to changes in environmental conditions.