Plants Prefer Ammonium as a N Source: Implications for Space Crop Production

Long-duration space exploration missions will require a sustainable supply of food to support human crews. Due to high CO2 levels in space habitats occupied by astronauts, it is crucial to understand plant responses to elevated CO2 (eCO2). Nitrogen (N) is crucial for plant survival, and it is possible that eCO2 affects plant N uptake. Shoot nitrate reduction can be inhibited at eCO2, due to lower reductant availability for nitrate reduction through increased carbon fixation and decreased photorespiration, suggesting eCO2 may increase plant ‘preference’ for ammonium as a N source. However, changes in N preference in response to eCO2, to our knowledge, are poorly understood. Here, we show using differential stable isotope labelling that several varieties of hydroponically grown lettuce prefer ammonium over nitrate as a N source, at both ambient CO2 (410 ppm) and ISS-level eCO2 (3000 ppm). Moreover, two varieties, ‘Outredgeous’ and ‘Waldmann’s Dark Green’, displayed exacerbated ammonium preference at eCO2. Significant differences in ammonium preference across the four lettuce varieties tested were linked to plant size and total N content. Growth and yield of a subset of lettuce varieties was increased when grown in solutions containing 70 % of total N as ammonium, compared to control solutions containing 100 % of total N as nitrate. Preliminary results suggest this increase is more substantial in plants grown at eCO2. These results shed light on plant nitrogen uptake responses to eCO2 and suggest that future nutrient formulations should favor ammonium as a major N source for space crop production. Moreover, this work will inform waste recycling approaches as a step towards plant-based bioregenerative life support systems. This work was funded by NASA Space Biology.