Effects of simulated microgravity on the performance of the duckweeds Lemna aequinoctialis and Wolffia globosa

Duckweed is considered to be one of the most attractive higher plants to provide the life support requirements of the crew in long-duration space missions. Two species of duckweed, Lemna aequinoctialis and Wolffia globosa, were comparatively studied with focus on the effects of simulated microgravity conditions. A significant enhancement of plant relative growth rate (RGR) was observed under simulated microgravity conditions. The mean RGR of L. aequinoctialis was 32% increased, and W. globosa was 12% increased compared to control. Light and electron microscope analyses showed that the mesophyll cells of duckweed fronds were loosely arranged and many intercellular air spaces enclosed by mesophyll cells were observed in reaction to simulated microgravity, and few starch grains were detected in root tip cells in L. aequinoctialis under simulated microgravity conditions. For L. aequinoctialis, the starch content was significantly increased under simulated microgravity conditions. Thus, the starch content of W. globosa under simulated microgravity conditions was kept stable during all growth periods. This pattern might be associated with the different expression change of the rate-limiting gene APL in regulating starch levels. Taking into consideration of the duckweed growth, ultra-structure and starch content changes, we considered that simulated microgravity conditions do not have a significant adversely affect on the duckweed growth.