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.

在最初考虑让宇航员从国际空间站 (ISS) 的 Veggie 植物生长装置中采摘和食用的候选叶类蔬菜中，大白菜 ( Brassica rapa L. cv. Tokyo Bekana) 在地面筛选研究中排名靠前。然而，随后在严格的类似 ISS 的地面生长环境中优化生长的尝试因叶片萎黄、坏死和生长不均匀的发展而受挫。在 VEG-03B 和 C 飞行期间在国际空间站上生长的“东京 Bekana”（“TB”）出现了类似的压力症状。经过长时间的故障排除工作，以确定在高度受控的环境中低于标准增长的原因，超高的 CO ₂ISS 上的植物持续暴露的浓度（平均 2,800 µmol/mol）成为导致观察到的植物胁迫症状的候选环境条件。随后的地面研究发现，与近地正常 CO ₂对照相比，在素食环境和文化条件下持续暴露于 ISS 水平的 CO ₂可显着抑制“TB”的生长。本研究调查了“TB”对 sub-ISS 的生长和气体交换反应，但与 450 μmol/mol CO 相比，CO ₂水平仍然升高（900 或 1,350 µmol/mol）以及与 ISS/Veggie 相关的其他潜在压力因素₂控制。在 450 µmol•mol ^-1 CO _{2 下}生长的植物的茎干质量28 天比在 900 µmol•mol ^-1 CO ₂和 1,350 µmol•mol ^-1 CO _{2 下}生长的植物分别高 96% 和 80% 。对照的叶数和叶面积显着高于在 1,350 µmol•mol ^-1 CO _{2 下}生长的植物。对于在 900 µmol•mol ^-1 CO ₂下生长的植物，使用叶试管测量的光合速率显着低于对照。对于生长在 450 µmol•mol ^{-1 的}植物，叶片内部 CO ₂浓度 (C _i ) 与比色皿环境 CO ₂浓度 (C _a ) 的比率显着较低CO ₂比在高 CO _{2 下}生长的植物。因此，持续升高的 CO ₂与素食栽培系统相结合会降低大白菜 'Tokyo Bekana' 的生长、叶面积和光合效率。这项研究的结果表明，“Tokyo Bekana”对这种生长环境中持续升高的 CO ₂非常敏感，并表明需要改进对 CO _{2 的}环境控制以及在国际空间站太空飞行环境中成功作物生产的可能根区因素。其他沙拉作物对 ISS/Veggie 生长环境的不同敏感性也是可能的，因此在地面筛选期间尽可能精确地模拟可控的类似 ISS 的环境条件非常重要。