Impact of Different Substrate Moisture Levels on Lettuce Plants during Ground Based Experiment in SVET-2 Space Greenhouse

Plant experiments carried out in space has proved that microgravity alters conditions in the plant growth facilities especially in the root modules and thus affects plant growth and development. Microgravity changes behavior of fluids and gases in the porous media used as plant growth substrates which causes problems with the control of water supply systems and this often leads to excess water input (overmoistening) and oxygen deficiency. The pattern of fluid and gas distribution in substrate medium in microgravity could not be repeated on Earth but some processes could be imitated. Overmoistening of the substrate medium and the subsequent oxygen deficiency could be replaced with the waterlogging on Earth. Ground experiment was carried out in the laboratory prototype of SVET-2 Space Greenhouse (SVET-2 SG) to study the effect of different root-zone moisture conditions and waterlogging on growth, photosynthesis and chlorophyll content of lettuce plants (Lactuca sativa L. cv. Lolo Rossa). The increase in height and biomass was suppressed while leaf dry matter increased during the waterlogging treatment suggesting assimilates accumulation in the leaves and slow translocation to the roots. Waterlogging caused a rapid decline in net photosynthetic rate (Pn). The reduction of Pn could not be attributed only to diffusion limitations resulting from stomatal closure but also to metabolic inhibition due to accumulation of assimilates in the leaves. The chlorophyll content decreased during the waterlogging and slowly recovered after termination of the waterlogging treatment. Lettuce plants showed decline in Pn and overall growth during waterlogging and demonstrated fast recovery after waterlogging removal. The results suggested that lettuce plants were waterlogging-resistant to a certain degree but yield is greatly affected.