Differential photosynthetic yield across a wide range of temperatures for Northern and invasive Watermilfoils

Abstract

Eurasian watermilfoil (Myriophyllum spicatum) is one of the most challenging invasive aquatic plant species in Minnesota. Eurasian watermilfoil has hybridized with the native Northern watermilfoil (Myriophyllum sibiricum), creating a more complex issue when it comes to invasions. The hybridization of watermilfoil has increased difficulty for management and an underlying biological mechanism explaining this difficulty has yet to be determined. We compared the photosynthetic temperature response of the parental Northern and Eurasian parental genotypes to five hybrid watermilfoils using pulse amplitude modulated (PAM) chlorophyll fluorometry to determine if there are intrinsic physiological differences in photosynthesis between genotypes. We examined both the ratio of variable to maximal fluorescence (Fv/Fm) and light-adapted quantum yield of photosystem II (PSII) electron transfer (ΦPSII) every 5 °C from 0 to 45 °C representing sub- to supraoptimal water temperatures. On average, hybrids had 6% and 8% higher Fv/Fm and ΦPSII values respectively, than parental genotypes across all temperatures. In both parental and hybrid genotypes Fv/Fm values were consistent from 0 to 35 °C and declined 36% as water temperature approached 45 °C. Optimal water temperature for ΦPSII was between 25-35 °C and hybrids had 17% higher values for suboptimal temperatures between 5-15 °C. Our findings indicate that although the many strains of watermilfoil are morphologically similar, they differ in their photosynthetic capacity and may indicate heterosis. Innate differences between parental strains and hybrids could impact overall primary productivity, life history characteristics, and overall invasiveness. These factors could also be contributing to the documented management failures seen in field populations.