Interactions between light and CO2 enhance the growth of Riccia fluitans

We grew the amphibious liverwort, Riccia fluitans, at saturating nitrogen and phosphorus concentrations in a cross-factorial design under three levels of light and three levels of CO2 making a matrix of nine treatments. Under these conditions, the relative growth rate (RGR) ranged from 0.011 d(-1) at low light and CO2 availability to 0.138 d(-1) at high light and CO2 with a significant positive interaction between light and CO2 on the RGR. After the growth experiments, a range of photosynthetic parameters were measured and in particular the maximum net photosynthesis (NPmax) showed a strong acclimation to light and CO2 availability. NPmax decreased significantly with increasing light intensities but increased with increasing CO2 concentration. Surprisingly, no positive correlation between the dark respiration (R) and the RGR was found. Rather, a strong positive correlation between NPmax and R was present and thus a positive correlation between R and RGR cannot be obtained since NPmax and RGR did not develop in parallel with increasing light and CO2 availability. The CO2 compensation point for photosynthesis was also strongly affected by the availability of light and CO2. The CO2 compensation point was very low a high light and low CO2 and increased at low light and high CO2 and there were significant interactions between light and CO2 on the CO2 compensation point throughout the entire experimental matrix. The observed responses to changes in light and CO2 availability and the interactions between the two will alleviate CO2 limitations in dense buoyant mats where the light is typically high. On the other hand, these interactions will also allow penetration to greater depths where light is scarce because the higher CO2 near the bottom can increase the light use efficiency.