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Baby greens are becoming increasingly popular in the consumer market because of their desired flavor and leaf size. The short life cycles and fast response times to environmental stimuli make baby greens ideal for testing environmental conditions for space crop production. Additionally, far-red (FR) light has been used for microgreen and baby green research to enhance stem elongation, leaf expansion, and biomass; however, how it interacts with nutrient solution nitrogen (N) concentrations remains unclear. During this ground-based study, we characterized how FR light and N concentrations influenced the growth and morphology of Chinese cabbage (Brassica rapa var. chinensis cv. Tokyo Bekana) and kale (Brassica oleracea var. sabellica cv. Red Russian) baby greens under similar superelevated CO₂ and low relative humidity to levels observed in spaceflight. Plants were subject to combinations of four sole-source light spectra and three N concentrations (75, 125, and 175 mg⋅L⁻¹). At the same total photon flux density (PFD) of 200 μmol⋅m⁻²⋅s⁻¹, we maintained the same blue and green PFDs at 25 μmol⋅m⁻²⋅s⁻¹ each; the remaining 150 μmol⋅m⁻²⋅s⁻¹ comprised four red (R) and FR PFD combinations (FR: 0, 25, 50, and 75 μmol⋅m⁻²⋅s⁻¹). Increasing the FR PFD enhanced the typical shade-avoidance morphology of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’, exhibiting leaf length increases of 20% to 26% and 31% to 61%, respectively. Edible biomass did not increase with increasing FR PFDs for either species, regardless of the N concentration. Increasing the N concentration increased the Chinese cabbage ‘Tokyo Bekana’ fresh mass and dry mass by 32% to 59% and 37% to 74%, respectively, except under 25 μmol⋅m⁻²⋅s⁻¹ of FR light, with which shoot fresh mass increased by 55% with an increasing N concentration from 75 to 125 mg⋅L⁻¹; however, the shoot dry mass was unaffected. Increasing the N concentration did not affect kale ‘Red Russian’ growth under various FR PFDs. We conclude that partially substituting incremental FR light for R light elicits the shade-avoidance response, with little influence on the growth, of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’ baby greens under superelevated CO₂ and continuous light, and that the former, but not the latter, crop can benefit from increased N fertilization.

Compared with the ambient Earth carbon dioxide concentration (≈415 μmol⋅mol^–1), the International Space Station has superelevated carbon dioxide (≈2800 μmol⋅mol^–1), which can be a stressor to certain crops. Far-red light can drive plant photosynthesis and increase extension growth and biomass. However, the effects of far-red light under superelevated carbon dioxide are unclear. We grew hydroponic mustard (Brassica carinata) ‘Amara’ seedlings in four growth chambers using a randomized complete block design with two carbon dioxide concentrations (415 and 2800 μmol⋅mol^–1), two lighting treatments, and two blocks at temperature and relative humidity set points of 22 °C and 40%, respectively. Each growth chamber had two lighting treatments at the same total photon flux density of 200 μmol⋅m^–2⋅s^–1. Under the same blue and green light at 50 μmol⋅m^–2⋅s^–1 each, plants received either red light at 100 μmol⋅m^–2⋅s^–1 or red + far-red light at 50 μmol⋅m^–2⋅s^–1 each. At day 15 after planting, far-red light did not influence shoot fresh or dry mass at 415 μmol⋅mol^–1 carbon dioxide, but decreased both parameters by 22% to 23% at 2800 μmol⋅mol^–1 carbon dioxide. Increasing the carbon dioxide concentration increased shoot fresh and dry mass 27% to 49%, regardless of the lighting treatment. Far-red light decreased leaf area by 16% at 2800 μmol⋅mol^–1 carbon dioxide, but had no effect at 415 μmol⋅mol^–1 carbon dioxide. Increasing the carbon dioxide concentration increased leaf area by 21% to 33%, regardless of far-red light. Regardless of the carbon dioxide concentration, far-red light promoted stem elongation and decreased chlorophyll concentrations by 39% to 42%. These responses indicate far-red light elicited a crop-specific shade avoidance response in mustard ‘Amara’, increasing extension growth but decreasing leaf area, thereby reducing light interception and biomass. In addition, carbon dioxide enrichment up to 2800 μmol⋅mol^–1 increased the biomass of mustard ‘Amara’ but decreased the biomass of other crops, indicating crop-specific tolerance to superelevated carbon dioxide. In conclusion, mustard ‘Amara’ seedlings benefit from superelevated carbon dioxide, but exhibit growth reduction under far-red light under superelevated carbon dioxide.