Developing carbon assimilation methods in duckweed for insights into photosynthesis and growth mechanisms

Infrared gas analysis (IRGA) is the primary technique for quantifying CO2 uptake in plants, but technical obstacles have limited its application to aquatic species, inhibiting exploration of their photosynthetic mechanisms. Using an IRGA with a customised aquatic chamber, we developed a methodology for measuring CO2 responses in duckweed, a group of fast-growing, floating angiosperms. We compared three morphologically contrasting species which have commercial or environmental significance: Wolffia australiana, Spirodela polyrhiza, and Lemna minuta. Light response curves showed species variation in light saturation intensity and light saturated rates of photosynthesis, with W. australiana generally showing the highest rates of exchange per unit area under CO2 limited and CO2 saturated conditions. We estimated biochemical parameters (Vcmax, Jmax) from CO2 response curves, though direct measurement of conductance and intercellular CO2 (Ci) was not possible. Using estimated Ci, predicted Vcmax and Jmax values fell within ranges consistent with other angiosperms, and again W. australiana had the highest values. Temperature response curves suggested an optimum of 35°C. Combining gas exchange and chlorophyll fluorescence further revealed a higher photosynthetic performance in W. australiana grown at lower plant densities. The suite of methodologies presented here will enable future mechanistic studies of duckweed carbon assimilation, growth dynamics and environmental responses, advancing understanding to levels comparable with that of many other terrestrial plant species.