Effects of fragment size and water depth on performance of stem fragments of the invasive, amphibious, clonal plant Ipomoea aquatica

Abstract

Many aquatic, clonal plants spread by vegetative fragments consisting of one to several ramets. Fragment size and physiological integration between ramets are likely to affect the establishment and growth of these fragments, and water depth is likely to affect the success of fragments in aquatic species that root in the sediment, but these effects have been little studied. To test the hypotheses that larger fragments perform better, that fragments perform better in shallower water, and that integration produces differences in the growth of ramets within fragments, we grew stem fragments of the cultivated, invasive, amphibious, floating, rooting plant Ipomaea aquatica with 1, 2, 3, or 4 nodes, which function as ramets, in water depths of 5, 20, or 40cm for 8 weeks in the greenhouse. Final mean mass and leaf area per node were greater in larger fragments (e.g., 0.46g and 49.3cm2 in 4-node versus 0.16g and 18.5cm2 in 2-node fragments). Fragments accumulated about 3 times more mean biomass and leaf area in 5cm (1.78g and 196.7cm2) than in 20cm (0. 49g and 49.9cm2) or 40cm (0.36g and 34.9cm2) deep water. Within larger fragments, most of the mean final mass was contained in the youngest ramet (66% in 3-node and 67% in 4-node fragments). Results supported all three hypotheses, and suggest that only stem fragments with 2 or more nodes are likely to promote vegetative reproduction in I. aquatica, and that physiological integration may concentrate growth in younger ramets.