Density-dependent root morphology and root distribution in the submerged plant Vallisneria natans

Root morphology, root distribution and biomass allocation in relation to plant nutrient concentration were investigated in the submerged macrophyte Vallisneria natans, growing on two types of sediment (clay and a mixture of sandy loam and clay) with three kinds of initial density (290, 650 and 1300 plants m−2). Both initial density and sediment type had significant impacts on biomass accumulation, root morphology, root distribution, and plant N and P concentrations, whereas biomass allocation was affected by sediment type alone, rather than plant density. For the same sediment type, biomass of the individual was highest in 290 plants m−2, intermediate in 650 plants m−2 and lowest in 1300 plants m−2. When initial density increased from 290 to 1300 plants m−2, root diameter decreased from 0.36 to 0.31 mm in the mixed sediment and from 0.43 to 0.35 mm in clay, but specific root length increased from 3.3 to 5.9 m g−1 in the mixed sediment and from 3.0 to 4.6 m g−1 in clay. For the plants grown in the mixed sediment, root distribution ratio in 4–8 cm depth increased from 3.3 to 30.8% when initial density increased from 290 to 1300 plants m−2. Increase of density led to decreased plant N and P concentrations. It is concluded that plasticity in root morphology and root distribution exists in V. natans as a response to density, and can help to adapt to competitive environments by increasing efficiency of nutrient acquisition.