Information on the recommended way to quantify duckweed growth can be found in the 8th issue of Duckweed forum.
In addition, the 14th issue of Duckweed forum contains further suggestions to standardize the growth rate quantification.
When duckweed growth rates are presented in a scientific way, then the growth conditions should ideally be comparable between different studies by various researchers. This would allow the field to leverage the data resources produced by the community as a whole and minimize redundant work:
When there are no specific reasons for using a particular set of growth parameters, the following conditions are suggested if possible: 25°C, continuous light (24 hours per day), light intensity 100 µmol photons m-2 s-1, and a well-defined inorganic nutrient medium such as the commercially available Schenk & Hildebrandt (SH) salts (see Growth Media). When photoperiods shorter than 24 h per day were used, the periodic response of the measured parameter (e.g. starch content) must be tested. Only when there are no periodic changes light/darkness, parameters can be measures as in continuous light. “Lux” is not suitable as dimension of light intensity as this unit is adapted from human vision, not to responses to plants. In plants, the normal form of measurements should be “Photosynthetic Active Radiation” (PAR) which has to be given as µmol photons m-2 s-1. Only in very special experiments, radiation energy can be given as W m-2 together with the emission spectrum of the light source.
Duckweed growth must be given as exponential “relative growth rates” (RGR or just GR) with the dimension d-1 or h-1. Data as percentage growth supposes a linear growth of duckweed, which is in reality exponential. To make the results easier to understand, doubling time or specific yield (Ziegler et al., 2015) can be calculated from the same data set.