Growth and photosynthetic responses of Chinese cabbage (Brassica rapa L. cv. Tokyo Bekana) to continuously elevated carbon dioxide in a simulated Space Station "Veggie" crop-production environment

Abstract

Among candidate leafy vegetable species initially considered for astronauts to pick and eat from the Veggie plant-growth unit on the International Space Station (ISS), Chinese cabbage (Brassica rapa L. cv. Tokyo Bekana) ranked high in ground-based screening studies. However, subsequent attempts to optimize growth within rigorous ISS-like growth environments on the ground were frustrated by development of leaf chlorosis, necrosis, and uneven growth. 'Tokyo Bekana' ('TB') grown on ISS during the VEG-03B and C flights developed similar stress symptoms. After lengthy troubleshooting efforts to identify causes of sub-par growth in highly controlled environments, the super-elevated CO₂ concentrations that plants on ISS are exposed to continuously (average of 2,800 µmol/mol) emerged as a candidate environmental condition responsible for the observed plant-stress symptoms. Subsequent ground-based studies found continuous exposure to ISS levels of CO₂ under Veggie environmental and cultural conditions to significantly inhibit growth of 'TB' compared to near-Earth-normal CO₂ controls. The present study investigated growth and gas-exchange responses of 'TB' to sub-ISS but still elevated CO₂ levels (900 or 1,350 µmol/mol) in combination with other potential stressors related to ISS/Veggie compared to 450 µmol/mol CO₂ controls. Shoot dry mass of plants grown at 450 µmol•mol^-1 CO₂ for 28 days was 96% and 80% higher than that of plants grown at 900 µmol•mol^-1 CO₂ and 1,350 µmol•mol^-1 CO_2, respectively. Leaf number and leaf area of controls were significantly higher than those of plants grown at 1,350 µmol•mol^-1 CO₂. Photosynthetic rate measured using a leaf cuvette was significantly lower for plants grown at 900 µmol•mol^-1 CO₂ than for controls. The ratio of leaf internal CO₂ concentration (C_i) to cuvette ambient CO₂ concentration (C_a) was significantly lower for plants grown at 450 µmol•mol^-1 CO₂ than for plants grown at elevated CO₂. Thus, continuously elevated CO₂ in combination with a Veggie cultivation system decreased growth, leaf area, and photosynthetic efficiency of Chinese cabbage 'Tokyo Bekana'. The results of this study suggest that 'Tokyo Bekana' is very sensitive to continuously elevated CO₂ in such a growth environment, and indicate the need for improved environmental control of CO₂ and possibly root-zone factors for successful crop production in the ISS spaceflight environment. Differential sensitivity of other salad crops to an ISS/Veggie growth environment also is possible, so it is important to mimic controllable ISS-like environmental conditions as precisely as possible during ground-based screening.

Keywords: Brassica; CO(2); Chlorosis; ISS; Photosynthesis; Veggie.