Mechanical strength of stems in aquatic macrophytes

In populations of submerged macrophytes, individuals are selected in terms of resistance to the effect of hydrodynamic forces. The aim of this study was to check whether individuals growing in river water are more tensile and bending resistant than plants occurring in places not exposed to flow stress. We determined the ultimate tensile strength of stems in four macrophyte species, Potamogeton natans, P. pectinatus, Batrachium fluitans and Chara fragilis, which occur in two environmental variants: in running (current velocity of 0.1-0.6 m/s-1) and stagnant water. In addition, flexure of P. natans stems from both environmental variants was examined. What is more, the type and arrangement of strengthening structures in stems of the plants under study were determined. Potamogeton natans stems are the most resistant to stretching (15.6±4.7 N), while stems of P. pectinatus (3.3±1.0 N) and Batrachium fluitans (2.6±0.8 N) are less resistant. Chara fragilis (0.6±0.3 N) has the least resistant stems. Batrachium, Chara and P. pectinatus are more resistant to stretching if they occur in a river current, whereas P. natans, in stagnant lake water. Ultimate bending moment of P. natans stems from lakes is also much greater than of stems from a river (9.75•10-3 – 4.25•10-3 Nm as compared to 2.12•10-3 – 1.00•10-3 Nm). The resistance of stems to breaking is directly proportional to the stem and thallus cross sectional areas. On the one hand, in all the studied Cormophyta species, the more resistant stems (in P. natans from stagnant water, in the others from running water) are thicker and characterised by a higher contribution of air spaces in the overall stem cross-section. On the other hand, the stems retain their species specific structure and have a similar proportion of strengthening elements.