Comparative analysis of the photosynthetic ecophysiology of Myriophyllum aquaticum and congeneric milfoils and invasion potential into the Midwestern United States

Abstract

Parrotfeather (Myriophyllum aquaticum (Vell.) Verdc.) is a heterophyllous, perennial aquatic invasive plant. Since parrotfeather’s arrival in the U.S. in the late 1800s, there is yet to be wide-spread establishment in the Midwestern United States (Midwest). The current study utilized pulse amplitude modulated chlorophyll fluorescence and gas exchange analyses to determine if there is a physiological mechanism that limits parrotfeather colonization in the Midwest. Fluorescence parameters were taken at water temperatures between 0 and 45°C, and gas exchange measurements were examined at 4, 25 and 35°C. The photosynthetic ecophysiology of parrotfeather was compared to Eurasian watermilfoil (Myriophyllum spicatum L.) and a hybrid strain of watermilfoil (Myriophyllum spicatum x Myriophyllum sibiricum) as the latter two plants are widespread throughout the Midwest. The ratio of variable to maximal fluorescence (F_v/F_m) in emergent and submerged parrotfeather averaged 0.80 and 0.65, respectively, at temperatures between 0 and 35°C and declined at > 40°C. In addition, light-adapted quantum yield of photosystem II (ϕ_PSII) was lowest at temperatures between 0 and 10°C and 40–45°C in submersed leaves. Net photosynthetic rates were highest at 25°C for emergent and submerged parrotfeather and averaged 8.83 μmol m⁻² s⁻¹ and 0.00107 μmol g⁻¹ s⁻¹ FW, respectively. Carbon assimilation rates in submersed parrotfeather were significantly lower than those observed in the Eurasian and hybrid milfoils. Lastly, the light compensation points were consistently lower in submersed parrotfeather than the other milfoils with values ≤ 55 μmol m⁻² s⁻¹ at 4°C and 25°C. Based on current photosynthetic ecophysiology data, these species of Myriophyllum maintain positive photosynthetic rates at temperatures near 4°C.