Mustard ‘Amara’ Benefits from Superelevated CO2 While Adapting to Far-red Light Over Time

Authors:
Emily J. Kennebeck Department of Plant and Soil Sciences, University of Delaware, 531 South College Avenue, Newark, DE 19716, USA

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Qingwu Meng Department of Plant and Soil Sciences, University of Delaware, 531 South College Avenue, Newark, DE 19716, USA

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Abstract

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.

Space-grown food crops are imperative for advancing space research and exploration by supporting astronauts’ dietary intake directly. Currently, the astronaut diet is comprised primarily of prepackaged and freeze-dried meals that degrade in nutritional quality after 1 to 3 years in storage (Cooper et al. 2017). Space-grown crops supplement essential minerals and nutrients lost through degradation during storage. Flavorful variety from fresh-grown produce can also alleviate the menu fatigue experienced from underconsumption resulting from the cyclic menu onboard the International Space Station (ISS) (Tibbetts 2019). However, the unique environment of the ISS imposes a range of environmental stressors, such as microgravity, low relative humidity levels, and highly elevated CO2 concentrations (i.e., “superelevated”), which can affect crop growth negatively, and thus the efficient and consistent fresh food supplementation in the astronaut diet. Therefore, recent space crop production research has focused on understanding crop responses to these stressors, and identifying environmental and cultural parameters (e.g., light quality and intensity, growing media, nutrient delivery) suitable for space-grown crops.

Elevated CO2 concentrations can, depending on the level, vary in their impacts on crop development. Raising the CO2 concentration (700–1200 μmol⋅mol–1) above the ambient-Earth concentration can increase the net photosynthetic rate and crop growth (Bugbee et al. 1994; Gutiérrez et al. 2014; Xu 2015). However, CO2 elevation above this level can decrease plant biomass and delay seed set in spring and winter wheat (Triticum aestivum), and decrease ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) concentration and activity in kidney bean (Phaseolus vulgaris cv. Linden), potato (Solanum tuberosum), lambsquarters (Chenopodium album), cabbage (Brassica oleracea), and eggplant (Solanum melongena) (Bugbee et al. 1994; Gutiérrez et al. 2014; Sage et al. 1989; Xu 2015). The decreased RuBisCo activity impacts the Calvin cycle directly, thus limiting the photosynthetic capacity of plants (Sharkey et al. 2007). When grown at elevated CO2 concentrations (900–1350 μmol⋅mol–1), Chinese cabbage (Brassica rapa cv. Tokyo Bekana) exhibited a decrease in leaf area and biomass accumulation, and significant chlorosis and necrosis of leaf tissues (Burgner et al. 2019, 2020). Furthermore, the ambient-ISS CO2 concentration (≈2800 μmol⋅mol–1) is almost seven times that of the ambient CO2 concentration on Earth’s surface (Massa et al. 2017). The superelevated CO2 concentrations, such as those seen on the ISS, have elicited stress responses in some, but not all, leafy greens (Burgner et al. 2020; Johnson et al. 2021). These findings emphasize the need for a focus on cultivar-specific stress mitigation testing in space crop production research to increase edible biomass production and ensure adequate supplementation of the astronaut diet.

In addition to the CO2 concentration, light quality plays a significant role in regulating plant growth and development. Photosynthetically active radiation (PAR, 400–700 nm) affects plant growth through plant photosystems (Horton and Ruban 2005; McCree 1981). Far-red (FR) light (700–750 nm) is outside the conventionally defined PAR range, but can increase photosynthetic efficiency synergistically with shorter wavelength photons, including red light (600–700 nm), by equally exciting photosystems I and II (PSI, PSII) (Zhen and van Iersel 2017). In addition, the ratio of red light to far-red light mediates shade avoidance responses through phytochromes. Under red light, phytochromes primarily exist in their active form, translocate into the nucleus, and subsequently suppress phytochrome-interacting factors (PIFs) 4 and 5, which inhibit extension growth (Franklin 2008). In contrast, under FR light, phytochromes partially convert to their inactive form, which alleviates the suppression of PIFs and promotes extension growth (Sharrock 2008).

The effects of superelevated CO2 concentration and FR light on leafy greens have been mostly studied separately. Their combined effects and potential interactions are less understood in space crop production. The objective of this ground-based study was to determine how partial substitution of FR light for red light influenced growth, morphology, and pigmentation of mustard green (Brassica juncea) ‘Amara’ under the ambient-Earth CO2 concentration and the typical ISS superelevated CO2 concentration. We postulated that FR light would increase plant growth but decrease chlorophyll concentration and pigmentation under the superelevated CO2 concentration.

Materials and Methods

Plant material and treatments.

We grew mustard green ‘Amara’ in four reach-in plant growth chambers (E41L2; Percival Scientific, Perry, IA, USA) in a randomized complete block design with two simultaneous replications from 7 Apr to 5 May 2022. Each set of two chambers was designated an experimental replication (block). The two vertical shelves in each chamber shared similar environmental conditions and thus were not used as a block. We rinsed and soaked 240 rockwool plugs (2.5 cm long × 2.5 cm wide × 3.5 cm tall; AO 25/40, Grodan, Milton, ON, Canada) with reverse-osmosis water for 10 to 15 min until saturation before draining the excess water. Subsequently, we sowed two seeds (Johnny’s Selected Seeds, Winslow, ME, USA) per rockwool plug and placed 30 rockwool plugs in each of eight plastic trays (52.0 cm long × 25.7 cm wide × 6.1 cm tall). We added reverse-osmosis water up to half of the plug height and covered the trays with transparent humidity domes.

We placed the eight trays centrally on top of lidded bus tubs (50.8 cm long × 38.1 cm wide × 12.7 cm tall; Choice Foodservice Equipment Co., Layton, UT, USA) centered on two shelves in each of the four growth chambers, which delivered four treatments per replication with two CO2 concentrations (415 and 2800 μmol⋅mol–1) and, at each CO2 concentration, two photon spectra (blue + green + red + FR at 50 μmol⋅m–2⋅s–1 each and blue at 50 μmol⋅m–2⋅s–1 + green at 50 μmol⋅m–2⋅s–1 + red at 100 μmol⋅m–2⋅s–1) (Fig. 1). The light-emitting diode array with four independent color channels on each shelf provided the same total photon flux density of 200 μmol⋅m–2⋅s–1 (400–750 nm) and the same 18-h photoperiod. These light parameters were selected to resemble previously tested set points of 180 to 200 μmol⋅m–2⋅s–1 at a photoperiod of 16 to 18 h in space crop research (Massa et al. 2017; Mickens et al. 2018). In a previous study (Spencer et al. 2020) that also simulated the space growing environment, adding FR light at 16 μmol⋅m–2⋅s–1 to white light at a photosynthetic photon flux density of 320 μmol⋅m–2⋅s–1 increased the shoot dry mass, but not fresh mass, of mustard green ‘Amara’. Because the advanced plant habitat onboard the ISS can deliver FR light of up to 50 μmol⋅m–2⋅s–1 (Monje et al. 2020; Spencer et al. 2020), we selected 50 μmol⋅m–2⋅s–1 as the target FR photon flux density to determine its maximal effects. Because FR light also contributes to photosynthesis (Zhen and van Iersel 2017), we maintained the total photon flux density while substituting FR light partially for red light. The adjustment of each lighting treatment was based on 12 spectral measurements at plant height across the treatment area with a spectroradiometer (SS-110; Apogee Instruments, Logan, UT, USA). All chambers maintained the same air temperature of 22.0 ± 0.4 °C and relative humidity of 40.0% ± 3.9% throughout the experiment. Chamber environments were recorded by internal chamber sensors and additional third-party temperature and relative humidity sensors (H5072001; Govee, Shenzhen, China) and CO2 sensors (YEM-40L; GZAIR, Guangzhou, China) at plant height.

Fig. 1.
Fig. 1.

Comparison of light spectral graphs for two experimental lighting treatments with and without far-red light with the same total photon flux density. Color names are abbreviated: blue (B), green (G), red (R), and far-red (FR). Light intensity, measured in μmol·m−2·s−1 is shown in subscript after each waveband abbreviation.

Citation: HortScience 59, 2; 10.21273/HORTSCI17522-23

On day 4, we removed the humidity domes and subirrigated seedlings by maintaining the nutrient solution level at one-fourth the plug height. We prepared the nutrient solution by dissolving a premixed base fertilizer (12-4-16 RO FeED; JR Peters, Inc., Allentown, PA, USA) and MgSO4 (JR Peters, Inc.) in reverse-osmosis water [electrical conductivity (EC) = 1.30 ± 0.05 dS⋅m–1, pH = 5.80 ± 0.01), which supplied the following nutrients initially: N (124.82 mg⋅L–1), P (18.31 mg⋅L–1), K (138.13 mg⋅L–1), Ca (72.81 mg⋅L–1), Mg (48.47 mg⋅L–1), S (39.02 mg⋅L–1), B (0.12 mg⋅L–1), Cu (0.47 mg⋅L–1), Fe (1.77 mg⋅L–1), Mn (0.52 mg⋅L–1), Mo (0.13 mg⋅L–1), and Zn (0.56 mg⋅L–1). We thinned seedlings with tweezers to one plant per plug on day 7 and transplanted six randomly selected seedlings into a mini hydroponic setup under each treatment on day 15. We grew six plants in two rows of three net pots (6.35-cm diameter × 5.08-cm height; Cz Garden Supply, West Bloomfield, MI, USA) spaced 18 cm apart and embedded in each bus tub containing nutrient solution. Between transplant and the final harvest, we maintained the nutrient solution level at one-half the plug height by adding nutrient solution daily. We recorded the nutrient solution pH and EC in each bus tub daily and maintained the nutrient solution pH at 5.8 ± 0.3 (Table 1). We adjusted the nutrient solution pH manually with diluted solutions of food-grade H3PO4 (pH Down; General Hydroponics, Santa Rosa, CA, USA), and KOH and K2CO3 (pH Up, General Hydroponics). We did not maintain the nutrient solution EC at a set point with daily replenishment of the original nutrient solution.

Table 1.

Environmental parameters of four treatments across two replications.

Table 1.

Data collection and analysis.

On the day of transplant (day 15), we quantified growth (shoot fresh and dry mass), morphology (length and width of the longest true leaf, total leaf area, and stem width), and pigmentation (chlorophyll concentration and leaf color indices) of 10 randomly selected seedlings from each treatment. We cut each plant at the stem base in contact with the rockwool plug and measured its shoot fresh mass (and, subsequently, shoot dry mass) with a precision balance (GF-1003A; A&D Weighing, Ann Arbor, MI, USA). We used a digital caliper (model 41101; Wiha Tools, Monticello, MN, USA) to measure stem width, and used a leaf area meter (CI-202; CID Bio-Science, Inc., Camas, WA, USA) to measure the total leaf area. In addition, we used a chlorophyll meter (MC-100; Apogee Instruments, Inc.) to measure the relative chlorophyll concentration of the longest true leaf and used a color reader (CR-10; Konica Minolta Sensing, Ramsey, NJ, USA) to measure leaf color indices in the L*a*b* color space [L* (0–100): 0 = black and 100 = white; a* (–150 to 150): lower = greener and higher = redder; b* (–150 to 150): lower = bluer and higher = yellower]. Subsequently, we dried seedlings for 5 d in a drying oven at 65 °C before weighing shoot dry mass. We harvested mature plants from all treatments on day 29 and measured the same parameters as for seedlings, along with root length and root dry mass. Because wet roots took different amounts of time to achieve the same dryness in ambient air depending on the treatment, we did not measure root fresh mass, and determined that root dry mass was a more accurate indicator of root growth.

We photographed one representative plant per treatment per replication on day 15 and day 29 for visual comparisons of treatment effects (Fig. 2). We collected environmental parameter data in all growth chambers weekly to ensure they were on target throughout the experiment (Table 1). We conducted pairwise comparisons using analysis of variance and Tukey’s honestly significant difference test (α = 0.05) in JMP Pro (ver. 16.0.0; SAS Institute, Inc., Cary, NC, USA). We pooled data from two replications when there was no treatment × replication interaction or when data trends among treatments were consistent across replications.

Fig. 2.
Fig. 2.

Images of representative plants from two experimental replications. The mustard ‘Amara’ seedlings and mature plants were subjected to lighting treatments with (+) and without (–) far-red light under two CO2 concentrations (415 and 2800 μmol⋅mol–1).

Citation: HortScience 59, 2; 10.21273/HORTSCI17522-23

Results

Shoot and root growth.

All numerical differences described hereafter are statistically significant. Under the same lighting treatment at both growth stages, mustard ‘Amara’ grown at the superelevated CO2 concentration of 2800 μmol⋅mol–1 was larger than plants grown at the CO2 concentration of 415 μmol⋅mol–1. At the seedling stage (days 0–15), increasing the CO2 concentration from 415 to 2800 μmol⋅mol–1 increased seedling shoot fresh and dry mass by 27% to 30% with FR light and 49% without FR light (Fig. 3, Table 2). However, CO2 concentration did not affect shoot moisture content or shoot height of seedlings. At the mature stage (days 16–29), increasing the CO2 concentration from 415 to 2800 μmol⋅mol–1 increased shoot fresh and dry mass by 51% to 71% with FR light and 32% to 59% without FR light. At 2800 μmol⋅mol–1 CO2, shoot moisture content decreased by 1% with FR light and 2% without FR light, when compared with 415 μmol⋅mol–1 CO2. The superelevated CO2 concentration with FR light increased shoot height by 20% but did not affect it without FR light.

Fig. 3.
Fig. 3.

Shoot fresh and dry mass, and leaf area of mustard ‘Amara’ at the seedling growth stage (day 15) and the mature growth stage (day 29). ‘Amara’ was grown at two CO2 concentrations (415 and 2800 μmol⋅mol–1), and under two lighting treatments without far-red light (–FR) and with far-red light (+FR). Different letters within each column are significantly different by Tukey’s honestly significant difference test at P ≤ 0.05. Error bars represent standard error.

Citation: HortScience 59, 2; 10.21273/HORTSCI17522-23

Table 2.

Destructive measurements of the biomass and morphology of mustard ‘Amara’ at the seedling (day 15) and mature (day 29) growth stages.

Table 2.

Plants were taller when grown with FR light than without. At the seedling stage, the addition of FR light decreased shoot fresh and dry mass by 22% and 23%, respectively, at 2800 μmol⋅mol–1 CO2, but had no effect at 415 μmol⋅mol–1 CO2 (Fig. 3, Table 2). Shoot moisture content increased by 0.4% when FR light was added at 415 μmol⋅mol–1 CO2. In addition, FR light increased seedling shoot height by 40% to 41% regardless of CO2 concentration. At the mature stage, FR light did not affect shoot fresh or dry mass at either CO2 concentration, but decreased shoot moisture content by 1% at 415 μmol⋅mol–1 CO2. FR light increased shoot height of mature plants by 539% and 615% at 415 and 2800 μmol⋅mol–1 CO2, respectively. A decrease in the effect of FR light on mustard ‘Amara’ between the seedling and mature stages suggests an environmental adaptation to FR light over time.

Increasing CO2 concentration from 415 to 2800 μmol⋅mol–1 decreased root length, root dry mass, and the ratio of root to shoot mass by 45%, 35%, and 62%, respectively, without FR light, but had no effect with FR light (Table 3). FR light increased root length by 50% at 2800 μmol⋅mol–1 CO2, but had no effect at 415 μmol⋅mol–1 CO2. FR light decreased root dry mass and the ratio of root to shoot mass by 44% and 45%, respectively, at 415 μmol⋅mol–1 CO2, but had no effect at 2800 μmol⋅mol–1 CO2.

Table 3.

Destructive measurements of root morphology at the mature growth stage (day 29).

Table 3.

Leaf morphology and coloration.

Increasing the CO2 concentration from 415 to 2800 μmol⋅mol–1 increased seedling leaf length and leaf area by 8% and 21%, respectively, with FR light, and by 22% and 33%, respectively, without FR light (Fig. 3, Table 2). The CO2 concentration increase did not affect mature leaf length, but increased leaf area by 30% with FR light. At the seedling stage, FR light increased leaf length by 21% to 37% regardless of CO2 concentration. Although FR light did not affect leaf area at 415 μmol⋅mol–1 CO2, it decreased leaf area by 16% at 2800 μmol⋅mol–1 CO2. At the mature stage, FR light increased leaf length by 12% at 415 μmol⋅mol–1 CO2 and 13% at 2800 μmol⋅mol–1 CO2, but did not affect leaf area. These results again suggest that mustard ‘Amara’ adapts to the effects of FR light as the cultivar matures. Carbon dioxide concentration did not affect leaf relative chlorophyll concentration (Table 4). At the seedling and mature stages, FR light decreased relative chlorophyll concentration by 39% to 42% and 50% to 51%, respectively. FR light increased foliage brightness (L*) by 15% to 20% in seedlings and 25% to 34% in mature plants. FR light decreased a* by 42% in seedlings and 27% to 48% in mature plants. FR light also increased foliage yellowness (b*) by 81% to 89% in seedlings and 88% to 141% in mature plants. These results are consistent with the typical shade avoidance response caused by FR light (Demotes-Mainard et al. 2016).

Table 4.

Chlorophyll concentration and coloration indices of mustard ‘Amara’ leaves at the seedling (day 15) and mature (day 29) growth stages.

Table 4.

Discussion

The superelevated CO2 concentrations on the ISS can act as a stressor on plant growth, as was seen when Chinese cabbage ‘Tokyo Bekana’ had stunted growth and severe chlorosis at superelevated CO2 concentrations (Burgner et al. 2020). Elevated CO2 concentrations (e.g., 900–1000 μmol⋅mol–1) decreased RuBisCO concentration and activity in five C3 species (Sage et al. 1989). A decrease in RuBisCO activity affects N use efficiency negatively, ultimately decreasing the photosynthetic rate and plant growth (Sage et al. 1989). In our study, increasing the CO2 concentration from 415 to 2800 μmol⋅mol–1 increased shoot fresh and dry mass, leaf length, and leaf area of mustard ‘Amara’ seedlings, regardless of FR light. For mature mustard ‘Amara’ plants, this CO2 concentration increase also increased shoot fresh and dry mass by 32% to 71% and leaf area by 22% to 30%, regardless of FR light. In studies on wheat, increasing the CO2 concentration initially (800–1200 μmol⋅mol–1) increased growth responses through the CO2 fertilization effect. Elevating the CO2 concentration past this point either did not increase growth further or decreased fresh mass and seed set (Bugbee et al. 1994; Gutiérrez et al. 2014; Xu 2015). However, the maximum threshold of the CO2 fertilization effect may differ depending on the cultivar studied. For example, compared with the ambient-Earth CO2 concentration (400 μmol⋅mol–1), superelevated CO2 concentrations (3000–6000 μmol⋅mol–1) can increase fresh mass of lettuce (Lactuca sativa cvs. Dragoon and Outredgeous), mustard greens ‘Amara’, pak choi (Brassica rapa cv. Extra Dwarf), shungiku (Glebionis coronaria), kale (Brassica napus cv. Red Russian and Brassica oleracea cv. Toscano), and chard (Beta vulgaris cv. Barese Swiss) (Wheeler et al. 2021).

Increasing CO2 concentration from 415 to 2800 μmol⋅mol–1 increased shoot height of mature mustard ‘Amara’ with FR light. In some herbaceous species [e.g., dwarf bean (Phaseolus vulgaris cv. ‘Tender Green’), salad burnet (Sanguisorba minor), birds-foot trefoil (Lotus corniculatus), hoary plantain (Plantago media), and kidney vetch (Anthyllis vulneraria)], a slightly elevated CO2 concentration (590 μmol⋅mol–1) promoted leaf epidermal cell expansion compared with the ambient CO2 concentration (350 μmol⋅mol–1) (Ferris and Taylor 1994; Ranasinghe and Taylor 1995). However, for edible gynura (Gynura bicolor), wheat, and radish (Raphanus sativus), superelevated CO2 concentrations decreased or did not influence shoot height (Jiang et al. 2017; Stryjewski and Eraso 2002; Wang et al. 2015). These results support this hypothesis and our findings of increased growth for mustard ‘Amara’ under superelevated, ambient-ISS CO2 concentrations.

In our study, superelevated CO2 concentrations decreased root length, root dry mass, and the ratio of root-to-shoot mass of mustard ‘Amara’. In edible gynura, superelevated CO2 concentrations (3000 μmol⋅mol–1) decreased root length but did not influence root dry mass compared with the ambient CO2 concentration (Wang et al. 2015). In contrast, superelevated CO2 concentrations increased root mass and lateral root formation in winter wheat and orchid plantlets (Arachnis hookeriana × Ascocenda cv. Mokara Yellow and Cymbidium cv. Music Hour Maria) compared with the ambient CO2 concentration (Gouk et al. 1997; Jiang et al. 2017; Norikane et al. 2010). High CO2 concentrations increased the ratio of root-to-shoot mass through assimilate partitioning in a variety of herbaceous species [e.g., velvetleaf (Abutilon theophrasti), Napa cabbage (Brassica pekinesis), Bigelow’s sedge (Carex bigelowii), soybean (Glycine max), Marsh Labrador Tea (Ledum palustre), and cherry tomato (Lycopersicon esculentum)] (Farrar and Williams 1991). Taken together, CO2 concentration has no consistent effect on root and shoot biomass allocation; effects can depend on the plant species, developmental stage, environment, and experimental procedures (Rogers et al. 1995).

We postulated that the effects of FR light would alleviate the growth inhibition from superelevated CO2 concentrations observed in previous research. In our study, FR light either did not influence or decreased the shoot fresh and dry mass, and leaf area of mustard ‘Amara’ seedlings, and did not affect these growth parameters of mature mustard ‘Amara’, regardless of CO2 concentration. Similarly, 23 μmol⋅m–2⋅s–1 of supplemental FR light did not increase shoot fresh mass of mature mustard ‘Amara’ (Spencer et al. 2020). It is unclear why responses of mustard ‘Amara’ to FR light differ from typical shade avoidance responses induced by FR light (e.g., shoot elongation, leaf extension, and leaf area expansion) (Franklin 2008). When grown under supplemental FR light, leaf lettuce exhibited typical shade avoidance response patterns, including increased shoot fresh mass and leaf area (Meng et al. 2019). In comparison, mustard ‘Amara’ is a less volume-efficient crop with a different growth habit; it grows vertically with leaves extending sideways individually instead of the layered-leaf structure in lettuce. These differences in morphology could explain this unusual FR light response. When comparing lettuce cultivars grown with and without supplemental FR light, the head lettuce cultivars Cherokee and Little Gem responded with the typical growth responses seen under the shade avoidance response. However, the loose-leaf lettuce cultivar Green Saladbowl did not exhibit increases in dry weight under FR light, unlike what was seen in the head-lettuce cultivars (Liu and van Iersel 2022). This again suggests that morphology plays an integral role in the response of leafy greens to supplemental FR light. FR light could be supplemented for crops that exhibit the shade avoidance response to increase yield such as head lettuce (Spencer et al. 2020), whereas FR light may not be necessary for other less volume-efficient crops to increase yield.

In our study, FR light decreased root dry mass and the ratio of root-to-shoot mass of mature mustard ‘Amara’. In arabidopsis (Arabidopsis thaliana), FR light affects lateral root formation and density negatively through the ELONGATED HYPOCOTYL5 (HY5) transcription factor (van Gelderen et al. 2018). Supplemental FR light promoted expression of HY5 in lateral root primordia, which downregulated PIN-FORMED3 and LIKE-AUX1 3 that control the auxin flow from the shoot toward the root (van Gelderen et al. 2018). This could explain the increases in root length seen under some treatments with FR light to ensure adequate nutrient uptake when lateral root formation may be suppressed through this mechanism.

In mustard ‘Amara’, FR light had a more pronounced influence on the growth of seedlings than mature plants. This suggests that mustard ‘Amara’ can acclimate to FR light over time. In arabidopsis, exposure to FR light for long periods of time resulted in overstimulation and subsequent partial degradation of PSI, and an increase in PSII antenna size to equalize the electron transport rates (ETRs) of both photosystems (Hu et al. 2021). After being placed under actinic lighting, strong PSII photoinhibition was observed from the PSII ETR bottleneck caused by the PSI reduction (Hu et al. 2021). Increasing the antenna size makes PSII even more susceptible to damage from high light-intensity stress than its high baseline sensitivity, furthering photoinhibition and the ETR bottleneck (Shi et al. 2022). PSII photoinhibition decreases the photosynthetic rate, which could explain the negative or lack of influence of FR light on fresh and dry mass and leaf area in mustard ‘Amara’ seedlings. However, this does not explain the difference in the effect of FR light on the same measured parameters at the mature stage. The PSII photorepair system for high light-intensity-damaged PSII structures may explain this difference (Shi et al. 2022). Repairing a damaged (or in this case, altered) PSII structure mainly involves the disassembly of the PSII complex, replacement of the D-1 reaction center subunit, and reassembly of the whole structure. Studies in vivo have found that the D-1 subunit is the main component that becomes damaged during photoinhibition of PSII. The filamentous temperature-sensitive H (FtsH) and degradation of periplasmic proteins (Deg) protease families are mainly responsible for the degradation of the damaged subunit. The entire PSII structure is then disassembled, and a replacement subunit is selected for reassembly into the newly repaired PSII (Nixon et al. 2010). The replacement of the altered PSII structure through this photorepair system would alleviate the ETR bottleneck with an increase in functional photosystems. This step might not have occurred at the seedling phase for mustard ‘Amara’, but as the plants matured further into their growth cycle, this repair mechanism could have been implemented.

In summary, we observed unique responses of mustard ‘Amara’ to FR light and CO2 concentration in space-relevant conditions (e.g., temperature and relative humidity level). The typical shade avoidance response elicited by FR light influences plant growth patterns toward elongated shoots, expanded leaf area, and overall biomass accumulation (Franklin 2008). However, mustard ‘Amara’ grown under FR light does not follow these growth trends. Specific cultivar morphology appears to play a role in the growth response to FR light. Leafy green varieties with a stacked canopy, such as in head lettuce, do follow the typical shade avoidance response patterns. Loose-leaf lettuces and other less volume-efficient cultivars, such as mustard ‘Amara’, do not exhibit this same response, possibly because of decreased light penetration into the plant canopy (Liu and van Iersel 2022). As such, we suggest conducting ground testing with cultivars under FR light before approval for use in flight to determine the necessity of including the additional light waveband in space crop production systems. In earlier studies, superelevated CO2 concentrations found in ambient-ISS conditions affected leafy green growth negatively by decreasing RuBisCO activity and concentration directly (Sage et al. 1989). We have shown that the most common CO2 fertilization effect curve (with maximum positive responses observed ≈800–1200 μmol⋅mol–1) is not representative of mustard ‘Amara’ (Bugbee et al. 1994; Gutiérrez et al. 2014; Xu 2015). Superelevated CO2 concentrations increased mustard ‘Amara’ growth regardless of the lighting treatment. In other ground-testing studies, mustard ‘Amara’ also exhibited increased growth at greater CO2 concentrations than the level used in our study (Wheeler et al. 2021). Based on our results, we advise the implementation of ground testing with FR light and ISS-simulated environmental parameters to observe cultivar-specific responses and the nutritional quality of the crops. This continued research will ensure adequate mineral and nutrient supplementation of the astronaut diet through space crop production.

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  • Farrar JF, Williams ML. 1991. The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration. Plant Cell Environ. 14(8):819830. https://doi.org/10.1111/j.1365-3040.1991.tb01445.x.

    • Search Google Scholar
    • Export Citation
  • Ferris R, Taylor G. 1994. Elevated CO2, water relations and biophysics of leaf extension in four chalk grassland herbs. New Phytol. 127(2):297307. https://doi.org/10.1111/j.1469-8137.1994.tb04280.x.

    • Search Google Scholar
    • Export Citation
  • Franklin KA. 2008. Shade avoidance. New Phytol. 179(4):930944. https://doi.org/10.1111/j.1469-8137.2008.02507.x.

  • Gouk SS, Yong JWH, Hew CS. 1997. Effects of super-elevated CO2 on the growth and carboxylating enzymes in an epiphytic CAM orchid plantlet. J Plant Physiol. 151(2):129136. https://doi.org/10.1016/S0176-1617(97)80144-7.

    • Search Google Scholar
    • Export Citation
  • Gutiérrez D, Morcuende R, Del Pozo A, Martínez-Carrasco R, Pérez P. 2014. Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat. J Plant Physiol. 170(15):13371343. https://doi.org/10.1016/j.jplph.2013.05.006.

    • Search Google Scholar
    • Export Citation
  • Horton P, Ruban A. 2005. Molecular design of the photosystem II light-harvesting antenna: Photosynthesis and photoprotection. J Expt Bot. 56(411):365373. https://doi.org/10.1093/jxb/eri023.

    • Search Google Scholar
    • Export Citation
  • Hu C, Nawrocki WJ, Croce R. 2021. Long-term adaptation of Arabidopsis thaliana to far-red light. Plant Cell Environ. 44(9):30023014. https://doi.org/10.1111/pce.14032.

    • Search Google Scholar
    • Export Citation
  • Jiang F, Shen Y, Ma C, Zhang X, Cao W, Rui Y. 2017. Effects of TiO2 nanoparticles on wheat (Triticum aestivum L.) seedlings cultivated under super-elevated and normal CO2 conditions. PLoS One. 12(5):e0178088. https://doi.org/10.1371/journal.pone.0178088.

    • Search Google Scholar
    • Export Citation
  • Johnson CM, Boles HO, Spencer LE, Poulet L, Romeyn M, Bunchek JM, Fritsche R, Massa GD, O’Rourke A, Wheeler RM. 2021. Supplemental food production with plants: A review of NASA research. Front Astron Space Sci. 8:734343. https://doi.org/10.3389/fspas.2021.734343.

    • Search Google Scholar
    • Export Citation
  • Liu J, van Iersel MW. 2022. Far-red light effects on lettuce growth and morphology in indoor production are cultivar specific. Plants. 11(20):2714. https://doi.org/10.3390/plants11202714.

    • Search Google Scholar
    • Export Citation
  • Massa GD, Dufour NF, Carver JA, Hummerick ME, Wheeler RM, Morrow RC, Smith TM. 2017. VEG-01: Veggie hardware validation testing on the International Space Station. Open Agric. 2(1):3341. https://doi.org/10.1515/opag-2017-0003.

    • Search Google Scholar
    • Export Citation
  • McCree KJ. 1981. Photosynthetically active radiation, p 41–55. In: Lange OL, Nobel PS, Osmond CB, Ziegler H (eds). Physiological plant ecology I: Responses to the physical environment. Springer, New York, NY, USA. https://doi.org/10.1007/978-3-642-68090-8_3.

  • Meng Q, Kelly N, Runkle ES. 2019. Substituting green or far-red radiation for blue radiation induces shade avoidance and promotes growth in lettuce and kale. Environ Exp Bot. 162:383391. https://doi.org/10.1016/j.envexpbot.2019.03.016.

    • Search Google Scholar
    • Export Citation
  • Mickens MA, Skoog EJ, Reese LE, Barnwell PL, Spencer LE, Massa GD, Wheeler RM. 2018. A strategic approach for investigating light recipes for ‘Outredgeous’ red romaine lettuce using white and monochromatic LEDs. Life Sci Space Res. 19:5362. https://doi.org/10.1016/j.lssr.2018.09.003.

    • Search Google Scholar
    • Export Citation
  • Monje O, Richards JT, Carver JA, Dimapilis DI, Levine HG, Dufour NF, Onate BG. 2020. Hardware validation of the advanced plant habitat on ISS: Canopy photosynthesis in reduced gravity. Front Plant Sci. 11:673. https://doi.org/10.3389/fpls.2020.00673.

    • Search Google Scholar
    • Export Citation
  • Nixon PJ, Michoux F, Yu J, Boehm M, Komenda J. 2010. Recent advances in understanding the assembly and repair of photosystem II. Ann Bot. 106(1):116. https://doi.org/10.1093/aob/mcq059.

    • Search Google Scholar
    • Export Citation
  • Norikane A, Takamura T, Morokuma M, Tanaka M. 2010. In vitro growth and single-leaf photosynthetic response of Cymbidium plantlets to super-elevated CO2 under cold cathode fluorescent lamps. Plant Cell Rep. 29(3):273283. https://doi.org/10.1007/s00299-010-0820-1.

    • Search Google Scholar
    • Export Citation
  • Ranasinghe S, Taylor G. 1995. Mechanism for increased leaf growth in elevated CO2. J Expt Bot. 47(3):349358. https://doi.org/10.1093/jxb/47.3.349.

    • Search Google Scholar
    • Export Citation
  • Rogers HH, Prior SA, Runion GB, Mitchell RJ. 1995. Root to shoot ratio of crops as influenced by CO2. Plant Soil. 187(2):229248. https://doi.org/10.1007/BF00017090.

    • Search Google Scholar
    • Export Citation
  • Sage RF, Sharkey TD, Seemann JR. 1989. Acclimation of photosynthesis to elevated CO2 in five C3 species. Plant Physiol. 89(2):590596. https://doi.org/10.1104/pp.89.2.590.

    • Search Google Scholar
    • Export Citation
  • Sharkey TD, Bernacchi CJ, Farquhar GD, Singsaas EL. 2007. Fitting photosynthetic carbon dioxide response curves for C3 leaves. Plant Cell Environ. 30(9):10351040. https://doi.org/10.1111/j.1365-3040.2007.01710.x.

    • Search Google Scholar
    • Export Citation
  • Sharrock RA. 2008. The phytochrome red/far-red photoreceptor superfamily. Genome Biol. 9(8):17. https://doi.org/10.1186/gb-2008-9-8-230.

    • Search Google Scholar
    • Export Citation
  • Shi Y, Ke X, Yang X, Liu Y, Hou X. 2022. Plants response to light stress. J Genet Genomics. 49(8):735747. https://doi.org/10.1016/j.jgg.2022.04.017.

    • Search Google Scholar
    • Export Citation
  • Spencer L, Wheeler R, Romeyn M, Massa G, Mickens M. 2020. Effects of supplemental far-red light on leafy green crops for space. https://hdl.handle.net/2346/86368. [accessed 23 Nov 2023].

  • Stryjewski E, Eraso I. 2002. Paper session III-B: Characterization of potential ISS/space shuttle environmental conditions on growth and development of R. sativus: Ground studies for the Rasta space flight experiment. https://commons.erau.edu/space-congress-proceedings/proceedings-2002-39th/may-2-2002/12/. [accessed 23 Nov 2023].

  • Tibbetts J. 2019. Gardening of the future: From outer to urban space: Moving from freeze-dried ice cream to fresh-picked salad greens. Bioscience. 69(12):962968. https://doi.org/10.1093/biosci/biz115.

    • Search Google Scholar
    • Export Citation
  • van Gelderen K, Kang C, Paalman R, Keuskamp D, Hayes S, Pierik R. 2018. Far-red light detection in the shoot regulates lateral root development through the HY5 transcription factor. Plant Cell. 30(1):101116. https://doi.org/10.1105/tpc.17.00771.

    • Search Google Scholar
    • Export Citation
  • Wang M, Dong C, Fu Y, Liu H. 2015. Growth, morphological and photosynthetic characteristics, antioxidant capacity, biomass yield and water use efficiency of Gynura bicolor DC exposed to super-elevated CO2. Acta Astronaut. 144:138146. https://doi.org/10.1016/j.actaastro.2015.05.010.

    • Search Google Scholar
    • Export Citation
  • Wheeler R, Spencer L, Romeyn M, Massa G, Bunchek J. 2021. Effects of carbon dioxide concentrations on salad crops for space food production (abstr). 43rd COSPAR Scientific Assembly. F4.5-0010-21. The SAO/NASA Astrophysics Data System, Cambridge, MA, USA.

  • Xu M. 2015. The optimal atmospheric CO2 concentration for the growth of winter wheat (Triticum aestivum). J Plant Physiol. 184:8997. https://doi.org/10.1016/j.jplph.2015.07.003.

    • Search Google Scholar
    • Export Citation
  • Zhen S, van Iersel MW. 2017. Far-red light is needed for efficient photochemistry and photosynthesis. J Plant Physiol. 209:115122. https://doi.org/10.1016/j.jplph.2016.12.004.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    Comparison of light spectral graphs for two experimental lighting treatments with and without far-red light with the same total photon flux density. Color names are abbreviated: blue (B), green (G), red (R), and far-red (FR). Light intensity, measured in μmol·m−2·s−1 is shown in subscript after each waveband abbreviation.

  • Fig. 2.

    Images of representative plants from two experimental replications. The mustard ‘Amara’ seedlings and mature plants were subjected to lighting treatments with (+) and without (–) far-red light under two CO2 concentrations (415 and 2800 μmol⋅mol–1).

  • Fig. 3.

    Shoot fresh and dry mass, and leaf area of mustard ‘Amara’ at the seedling growth stage (day 15) and the mature growth stage (day 29). ‘Amara’ was grown at two CO2 concentrations (415 and 2800 μmol⋅mol–1), and under two lighting treatments without far-red light (–FR) and with far-red light (+FR). Different letters within each column are significantly different by Tukey’s honestly significant difference test at P ≤ 0.05. Error bars represent standard error.

  • Bugbee B, Spanarkel B, Johnson S, Monje O, Koerner G. 1994. CO2 crop growth enhancement and toxicity in wheat and rice. Adv Space Res. 14(11):257267. https://doi.org/10.1016/0273-1177(94)90306-9.

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  • Burgner S, Morrow R, Massa G, Wheeler R, Romeyn M, Mitchell C. 2019. Troubleshooting performance failures of Chinese cabbage for veggie on the ISS. https://hdl.handle.net/2346/84553. [accessed 23 Nov 2023].

  • Burgner SE, Nemali K, Massa GD, Wheeler RM, Morrow RC, Mitchell CA. 2020. 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. Life Sci Space Res. 27:8388. https://doi.org/10.1016/j.lssr.2020.07.007.

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  • Cooper M, Perchonok M, Douglas GL. 2017. Initial assessment of the nutritional quality of the space food system over three years of ambient storage. npj Microgravity. 3(1):14. https://doi.org/10.1038/s41526-017-0022-z.

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  • Demotes-Mainard S, Péron T, Corot A, Bertheloot J, Le Gourrierec J, Pelleschi-Travier S, Crespel L, Morel P, Huché-Thélier L, Boumaza R, Vian A. 2016. Plant responses to red and far-red lights, applications in horticulture. Environ Exp Bot. 121:421. https://doi.org/10.1016/j.envexpbot.2015.05.010.

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  • Farrar JF, Williams ML. 1991. The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration. Plant Cell Environ. 14(8):819830. https://doi.org/10.1111/j.1365-3040.1991.tb01445.x.

    • Search Google Scholar
    • Export Citation
  • Ferris R, Taylor G. 1994. Elevated CO2, water relations and biophysics of leaf extension in four chalk grassland herbs. New Phytol. 127(2):297307. https://doi.org/10.1111/j.1469-8137.1994.tb04280.x.

    • Search Google Scholar
    • Export Citation
  • Franklin KA. 2008. Shade avoidance. New Phytol. 179(4):930944. https://doi.org/10.1111/j.1469-8137.2008.02507.x.

  • Gouk SS, Yong JWH, Hew CS. 1997. Effects of super-elevated CO2 on the growth and carboxylating enzymes in an epiphytic CAM orchid plantlet. J Plant Physiol. 151(2):129136. https://doi.org/10.1016/S0176-1617(97)80144-7.

    • Search Google Scholar
    • Export Citation
  • Gutiérrez D, Morcuende R, Del Pozo A, Martínez-Carrasco R, Pérez P. 2014. Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat. J Plant Physiol. 170(15):13371343. https://doi.org/10.1016/j.jplph.2013.05.006.

    • Search Google Scholar
    • Export Citation
  • Horton P, Ruban A. 2005. Molecular design of the photosystem II light-harvesting antenna: Photosynthesis and photoprotection. J Expt Bot. 56(411):365373. https://doi.org/10.1093/jxb/eri023.

    • Search Google Scholar
    • Export Citation
  • Hu C, Nawrocki WJ, Croce R. 2021. Long-term adaptation of Arabidopsis thaliana to far-red light. Plant Cell Environ. 44(9):30023014. https://doi.org/10.1111/pce.14032.

    • Search Google Scholar
    • Export Citation
  • Jiang F, Shen Y, Ma C, Zhang X, Cao W, Rui Y. 2017. Effects of TiO2 nanoparticles on wheat (Triticum aestivum L.) seedlings cultivated under super-elevated and normal CO2 conditions. PLoS One. 12(5):e0178088. https://doi.org/10.1371/journal.pone.0178088.

    • Search Google Scholar
    • Export Citation
  • Johnson CM, Boles HO, Spencer LE, Poulet L, Romeyn M, Bunchek JM, Fritsche R, Massa GD, O’Rourke A, Wheeler RM. 2021. Supplemental food production with plants: A review of NASA research. Front Astron Space Sci. 8:734343. https://doi.org/10.3389/fspas.2021.734343.

    • Search Google Scholar
    • Export Citation
  • Liu J, van Iersel MW. 2022. Far-red light effects on lettuce growth and morphology in indoor production are cultivar specific. Plants. 11(20):2714. https://doi.org/10.3390/plants11202714.

    • Search Google Scholar
    • Export Citation
  • Massa GD, Dufour NF, Carver JA, Hummerick ME, Wheeler RM, Morrow RC, Smith TM. 2017. VEG-01: Veggie hardware validation testing on the International Space Station. Open Agric. 2(1):3341. https://doi.org/10.1515/opag-2017-0003.

    • Search Google Scholar
    • Export Citation
  • McCree KJ. 1981. Photosynthetically active radiation, p 41–55. In: Lange OL, Nobel PS, Osmond CB, Ziegler H (eds). Physiological plant ecology I: Responses to the physical environment. Springer, New York, NY, USA. https://doi.org/10.1007/978-3-642-68090-8_3.

  • Meng Q, Kelly N, Runkle ES. 2019. Substituting green or far-red radiation for blue radiation induces shade avoidance and promotes growth in lettuce and kale. Environ Exp Bot. 162:383391. https://doi.org/10.1016/j.envexpbot.2019.03.016.

    • Search Google Scholar
    • Export Citation
  • Mickens MA, Skoog EJ, Reese LE, Barnwell PL, Spencer LE, Massa GD, Wheeler RM. 2018. A strategic approach for investigating light recipes for ‘Outredgeous’ red romaine lettuce using white and monochromatic LEDs. Life Sci Space Res. 19:5362. https://doi.org/10.1016/j.lssr.2018.09.003.

    • Search Google Scholar
    • Export Citation
  • Monje O, Richards JT, Carver JA, Dimapilis DI, Levine HG, Dufour NF, Onate BG. 2020. Hardware validation of the advanced plant habitat on ISS: Canopy photosynthesis in reduced gravity. Front Plant Sci. 11:673. https://doi.org/10.3389/fpls.2020.00673.

    • Search Google Scholar
    • Export Citation
  • Nixon PJ, Michoux F, Yu J, Boehm M, Komenda J. 2010. Recent advances in understanding the assembly and repair of photosystem II. Ann Bot. 106(1):116. https://doi.org/10.1093/aob/mcq059.

    • Search Google Scholar
    • Export Citation
  • Norikane A, Takamura T, Morokuma M, Tanaka M. 2010. In vitro growth and single-leaf photosynthetic response of Cymbidium plantlets to super-elevated CO2 under cold cathode fluorescent lamps. Plant Cell Rep. 29(3):273283. https://doi.org/10.1007/s00299-010-0820-1.

    • Search Google Scholar
    • Export Citation
  • Ranasinghe S, Taylor G. 1995. Mechanism for increased leaf growth in elevated CO2. J Expt Bot. 47(3):349358. https://doi.org/10.1093/jxb/47.3.349.

    • Search Google Scholar
    • Export Citation
  • Rogers HH, Prior SA, Runion GB, Mitchell RJ. 1995. Root to shoot ratio of crops as influenced by CO2. Plant Soil. 187(2):229248. https://doi.org/10.1007/BF00017090.

    • Search Google Scholar
    • Export Citation
  • Sage RF, Sharkey TD, Seemann JR. 1989. Acclimation of photosynthesis to elevated CO2 in five C3 species. Plant Physiol. 89(2):590596. https://doi.org/10.1104/pp.89.2.590.

    • Search Google Scholar
    • Export Citation
  • Sharkey TD, Bernacchi CJ, Farquhar GD, Singsaas EL. 2007. Fitting photosynthetic carbon dioxide response curves for C3 leaves. Plant Cell Environ. 30(9):10351040. https://doi.org/10.1111/j.1365-3040.2007.01710.x.

    • Search Google Scholar
    • Export Citation
  • Sharrock RA. 2008. The phytochrome red/far-red photoreceptor superfamily. Genome Biol. 9(8):17. https://doi.org/10.1186/gb-2008-9-8-230.

    • Search Google Scholar
    • Export Citation
  • Shi Y, Ke X, Yang X, Liu Y, Hou X. 2022. Plants response to light stress. J Genet Genomics. 49(8):735747. https://doi.org/10.1016/j.jgg.2022.04.017.

    • Search Google Scholar
    • Export Citation
  • Spencer L, Wheeler R, Romeyn M, Massa G, Mickens M. 2020. Effects of supplemental far-red light on leafy green crops for space. https://hdl.handle.net/2346/86368. [accessed 23 Nov 2023].

  • Stryjewski E, Eraso I. 2002. Paper session III-B: Characterization of potential ISS/space shuttle environmental conditions on growth and development of R. sativus: Ground studies for the Rasta space flight experiment. https://commons.erau.edu/space-congress-proceedings/proceedings-2002-39th/may-2-2002/12/. [accessed 23 Nov 2023].

  • Tibbetts J. 2019. Gardening of the future: From outer to urban space: Moving from freeze-dried ice cream to fresh-picked salad greens. Bioscience. 69(12):962968. https://doi.org/10.1093/biosci/biz115.

    • Search Google Scholar
    • Export Citation
  • van Gelderen K, Kang C, Paalman R, Keuskamp D, Hayes S, Pierik R. 2018. Far-red light detection in the shoot regulates lateral root development through the HY5 transcription factor. Plant Cell. 30(1):101116. https://doi.org/10.1105/tpc.17.00771.

    • Search Google Scholar
    • Export Citation
  • Wang M, Dong C, Fu Y, Liu H. 2015. Growth, morphological and photosynthetic characteristics, antioxidant capacity, biomass yield and water use efficiency of Gynura bicolor DC exposed to super-elevated CO2. Acta Astronaut. 144:138146. https://doi.org/10.1016/j.actaastro.2015.05.010.

    • Search Google Scholar
    • Export Citation
  • Wheeler R, Spencer L, Romeyn M, Massa G, Bunchek J. 2021. Effects of carbon dioxide concentrations on salad crops for space food production (abstr). 43rd COSPAR Scientific Assembly. F4.5-0010-21. The SAO/NASA Astrophysics Data System, Cambridge, MA, USA.

  • Xu M. 2015. The optimal atmospheric CO2 concentration for the growth of winter wheat (Triticum aestivum). J Plant Physiol. 184:8997. https://doi.org/10.1016/j.jplph.2015.07.003.

    • Search Google Scholar
    • Export Citation
  • Zhen S, van Iersel MW. 2017. Far-red light is needed for efficient photochemistry and photosynthesis. J Plant Physiol. 209:115122. https://doi.org/10.1016/j.jplph.2016.12.004.

    • Search Google Scholar
    • Export Citation
Emily J. Kennebeck Department of Plant and Soil Sciences, University of Delaware, 531 South College Avenue, Newark, DE 19716, USA

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Qingwu Meng Department of Plant and Soil Sciences, University of Delaware, 531 South College Avenue, Newark, DE 19716, USA

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Contributor Notes

This work was supported by a grant from the National Aeronautics and Space Administration Established Program to Stimulate Competitive Research Rapid Response Research (Grant No. 80NSSC21M0142).

We thank Eva Birtell, Evyn Appel, and Ian Kelly for experimental assistance; and Gioia Massa for supporting this research.

Q.M. is the corresponding author. E-mail: qwmeng@udel.edu.

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  • Fig. 1.

    Comparison of light spectral graphs for two experimental lighting treatments with and without far-red light with the same total photon flux density. Color names are abbreviated: blue (B), green (G), red (R), and far-red (FR). Light intensity, measured in μmol·m−2·s−1 is shown in subscript after each waveband abbreviation.

  • Fig. 2.

    Images of representative plants from two experimental replications. The mustard ‘Amara’ seedlings and mature plants were subjected to lighting treatments with (+) and without (–) far-red light under two CO2 concentrations (415 and 2800 μmol⋅mol–1).

  • Fig. 3.

    Shoot fresh and dry mass, and leaf area of mustard ‘Amara’ at the seedling growth stage (day 15) and the mature growth stage (day 29). ‘Amara’ was grown at two CO2 concentrations (415 and 2800 μmol⋅mol–1), and under two lighting treatments without far-red light (–FR) and with far-red light (+FR). Different letters within each column are significantly different by Tukey’s honestly significant difference test at P ≤ 0.05. Error bars represent standard error.

 

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