Lettuce growth response to increased sodium chloride and reduced potassium concentrations in the nutrient solution at the Plant Characterization Unit

Geremia Pellegri^1*Antonio Pannico²Marco Gatti³Øyvind Mejdell Jakobsen⁴Norbert Kirchgessner¹Stefania De Pascale²Emmanuel Frossard¹

• ¹ETH Zürich, Zurich, Switzerland
• ²University of Naples Federico II, Naples, Italy
• ³EnginSoft S.p.A, Bergamo, Italy
• ⁴CIRIS, Trondheim, Norway

Bioregenerative Life Support Systems (BLSS) will be essential in long-term Space missions to reduce the requirement for supply from Earth. Most BLSSs will include crops to generate oxygen (O2), water, and food needed by astronauts, while capturing carbon dioxide (CO2) from the atmosphere. While previous research has shown that plants deliver these services when grown under optimal mineral nutrition, it is also important to study the impact of suboptimal plant nutrition, as any decrease in O2, water, and food production or CO2 capture could have long-term effects in the BLSS. To this end, we conducted four crop tests in the Plant Characterization Unit (PCU) of the European Space Agency, examining the impact of adding sodium chloride (NaCl) and reducing potassium (K) in nutrient solutions on lettuce (Lactuca sativa L.). The control treatment with standard nutrient solution composition was run twice (Control_1 and Control_2) to investigate the repeatability of the PCU crop tests. Plant growth during the experiment was monitored by measuring O2 and water production, projected leaf area, and canopy temperature throughout the crop tests. At harvest, photosynthesis-related parameters (Fv/Fm, SPAD, stomatal conductance, and transpiration rate) were collected, and leaf, stem, and roots were weighed and analyzed for total elemental composition. Assessing the impact of suboptimal mineral nutrition on plants was challenged by high plant-to-plant variability within each test. Furthermore, considerable differences were observed in plant growth in the two control treatments. Comparisons with previous and similar experiments suggested that Control_2 provided a more realistic representation of the control treatment. Compared to Control_2, the addition of 27 mM NaCl in the nutrient solution did not cause any significant decrease in biomass production or photosynthesis-related parameters at harvest. Net water and O2 production were also comparable to Control_2. However, projected leaf area showed retardation in plant development compared to Control_2. The reduction of K in the nutrient solution caused a retardation in plant development, reflected in projected leaf area, and a 20% decrease in shoot dry biomass. This, in turn, led to decreased O2 and water production of the PCU relative to Control_2.