Abstract
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.
Baby greens, although not a legally defined term, have become increasingly popular among consumers and, consequently, their production for the produce market has increased (Saini et al. 2016; Treadwell et al. 2020). These greens are young and full of flavor, and they contain high levels of health-promoting compounds compared with those of their mature counterparts (Martínez-Sánchez et al. 2012; Saini et al. 2016; Waterland et al. 2017). Baby greens are generally defined as young plants between the microgreens and mature stages with at least two pairs of true leaves (Saini et al. 2016; Waterland et al. 2017). Because of their increased flavor and beneficial compounds, baby greens are ideal candidates for use in spaceflight, during which the diet of astronauts primarily comprises prepackaged, freeze-dried food with nutritional value that degrades after 1 to 3 years in storage (Tibbetts 2019). Foods with increased flavor are also beneficial for use during space travel because astronauts can lose some of their ability to taste and smell as a side effect of fluid redistribution as the human body enters microgravity (Herridge and National Aeronautics and Space Administration 2021; Taylor et al. 2020). However, the environment onboard the International Space Station (ISS) provides some unique parameters that can hinder desired crop growth. For example, the low relative humidity levels and superelevated CO2 concentrations at ambient conditions onboard the ISS can decrease crop yield and produce chlorosis and necrosis in some leafy greens, such as tomato (Solanum lycopersicum cv. DiFen), lettuce (Lactuca sativa var. capitata), and Chinese cabbage (Brassica rapa var. chinensis cv. Tokyo Bekana) (Amitrano et al. 2019, 2021; Burgner et al. 2019, 2020; Massa et al. 2017). Baby greens also exhibit responses to environmental stimuli at their early developmental stage, reducing experimental durations and expediting data collection. However, these shorter harvest periods require additional seed and growth media to adequately supplement the astronauts’ diet, ultimately increasing the expense because of the heavier payload.
The lighting environment also influences plant growth and development. Far-red (FR) light (700–800 nm) exists adjacent to the traditional photosynthetically active radiation (PAR) spectra (400–700 nm). However, recent research has shown that FR light is effective at driving photosynthesis (Zhen and van Iersel 2017). This light waveband, in tandem with red (R) light, forms the R-to-FR ratio, which elicits various physiological responses from plants through plant phytochrome photoreceptors (Demotes-Mainard et al. 2016; Franklin 2008). When the R photon is detected, a phytochrome undergoes a conformational change toward its active form (Pfr) (Sharrock 2008). This form actively suppresses genetic expressions for stem elongation, leaf area expansion, and overall biomass accumulation, which are typical morphology changes seen with the shade-avoidance response (Demotes-Mainard et al. 2016; Franklin 2008; Zou et al. 2019). When the FR photon is detected, the phytochrome undergoes a subsequent conformational change to the inactive form (Pr), thereby alleviating the suppression of the aforementioned genetic expressions (Sharrock 2008). By increasing edible biomass, FR light could potentially increase crop yield when grown under supraoptimal or suboptimal conditions (e.g., superelevated CO2 and low relative humidity), like those on the ISS.
The availability of nitrogen (N) is critical to amino acid synthesis and subsequent structural and enzymatic protein synthesis in plants (Kirkby 1981; Lam et al. 1996; Leghari et al. 2016). Metabolically active cells, like those involved in meristematic activity and photosynthesis, contain a larger amount of N per dry weight compared with cells involved in storage and transport (Lawlor et al. 1988, 2001). The light-dependent and independent reactions of photosynthesis are influenced by N as a main component of the involved proteins, thus making crop growth N-dependent (Evans and Seemann 1989; Greenwood et al. 1991; Lawlor et al. 2001). The electrical conductivity (EC) of the nutrient solution used in a hydroponic system can also affect crop growth. For example, increasing EC from 0.5 to 0.9 or 1.2 dS⋅m−1 increased the fresh mass of lettuce ‘Batavia-Caipira’ and basil (Ocimum basilicum cv. Emily), respectively, whereas further increases to 2 dS⋅m−1 decreased fresh mass (Hosseini et al. 2021). Furthermore, high EC levels (up to 10 dS⋅m−1) increased the leaf nitrate content of most tested lettuce cultivars (Kiber, Attirai, Rouxai, Cencibel, and Sintia) and decreased the fresh mass of all tested cultivars compared with lettuce grown at low EC levels (0.5–1.3 dS⋅m−1) (Kappel et al. 2021; Samarakoon et al. 2006).
Both FR light and the N concentration of the nutrient solution influence plant growth; however, whether they have interactions under superelevated CO2 and continuous light is unclear. The objective of this ground-based experiment was to characterize the growth and morphology of baby greens treated with varying FR photon flux densities (PFDs) and N concentrations under superelevated CO2 and continuous light. We postulated that increasing the FR PFD and/or the N concentration would increase biomass and extension growth of baby greens.
Materials and Methods
We grew baby greens of Chinese cabbage ‘Tokyo Bekana’ and kale (Brassica oleracea var. sabellica cv. Red Russian) (Johnny’s Selected Seeds, Winslow, ME, USA) using a randomized complete block design with two simultaneous replications. We conducted this experiment at the Delaware Indoor Agricultural Laboratory at the University of Delaware (Newark, DE, USA) from 9 to 31 Jan 2023. We sowed two seeds in each of 336 rockwool plugs (2.5 cm long × 2.5 cm wide × 3.5 cm tall; Grodan, Roermond, the Netherlands) per cultivar. Before seeding, we rinsed and soaked all rockwool plugs in reverse-osmosis water to remove any lingering surfactants from the manufacturing process. After seeding, we placed 14 rockwool plugs into each plastic container and covered them with transparent lids (22 cm long × 15 cm wide × 4.5 cm tall). Each experimental replication was conducted in two reach-in plant growth chambers (E41L2; Percival Scientific, Perry, IA, USA). Plants were grown under four light spectra (blue25 + green25 + red150; blue25 + green25 + red125 + far red25; blue25 + green25 + red100 + far red50; or blue25 + green25 + red75 + far red75) and three N concentrations (75, 125, and 175 mg⋅L−1) (Fig. 1). The subscript following each waveband is its PFD in μmol⋅m−2⋅s−1. The temperature, relative humidity, and CO2 concentration were held constant at ambient ISS conditions at 22 °C, 50%, and 2800 μmol⋅mol−1, respectively, under a 24-h photoperiod (Table 1). We set up lighting treatments by obtaining 12 PFD measurements per treatment with an extended PAR quantum sensor (PQ-612; Apogee Instruments, Logan, UT, USA) and spectral distributions with a spectroradiometer (SS-110; Apogee Instruments). Environmental parameters were monitored by third-party sensors as well as internal chamber sensors. We placed a temperature and relative humidity sensor (MX1101; Onset, Bourne, MA, USA) on a wooden block under a radiation shield in the middle of each treatment shelf to log data every 15 min. Additionally, we placed a CO2 sensor (YEM-40L; GZAIR, Guangzhou, China) on the top shelf of each chamber to log data every 10 min. We downloaded environmental data once per week to ensure the targeted environmental parameters.
Spectral distribution curves of four sole-source lighting treatments with fixed blue and green photon flux densities and varying red and far-red photon flux densities. The number following each waveband denotes its photon flux density in μmol⋅m−2⋅s−1.
Citation: J. Amer. Soc. Hort. Sci. 149, 2; 10.21273/JASHS05352-23
Temperature, relative humidity, and CO2 concentration (mean ± SD) throughout the experiment on reach-in plant growth chamber shelves that delivered light spectra with four far-red (FR) photon flux densities.
On day 4, we removed the transparent lids to expose the seedlings to the ambient chamber environment. On day 5, we began to manually thin the seedlings to one plant per plug as they germinated. From day 5 until the end of the experiment, we fertigated the seedlings with their respective nutrient solutions with varying N concentrations. Each nutrient solution was made with a base fertilizer [5N–5.2P–21.6K (5–12–26 Part A); JR Peters, Inc., Allentown, PA, USA] and calcium nitrate [15N–0P–0K (15–0–0 Part B); JR Peters, Inc.] dissolved in reverse-osmosis water with an adjusted pH of 5.80 ± 0.02. All treatments received the same amount of base fertilizer (1000 mg⋅L−1) and varying amounts of calcium nitrate (167, 500, and 833 mg⋅L−1) to deliver the three N concentrations of 75, 125, and 175 mg⋅L−1, respectively. The level of nutrient solution was maintained at one-fourth of the rockwool plug height through sub-irrigation as needed.
We conducted a destructive harvest for six randomly selected baby greens per treatment per cultivar per replication at canopy closure (on day 18 for Chinese cabbage ‘Tokyo Bekana’ and day 21 for kale ‘Red Russian’). We collected measurements of biomass (shoot fresh and dry mass), morphology (shoot height, longest leaf length, and total leaf area), and the relative chlorophyll concentration using a chlorophyll meter (MC-100; Apogee Instruments, Inc.). Plant material was dried at 65 °C for 7 d before dry mass measurements. We photographed a representative plant per treatment per cultivar per replication for visual treatment comparisons (Fig. 2). Additionally, we downloaded environmental data from the internal chamber sensors and third-party sensors throughout the experiment (Table 1). We analyzed plant data with mean separations in pairwise comparisons using an analysis of variance and Tukey’s honestly significant difference test (α = 0.05) in JMP Pro (version 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.
Representative images of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’ baby greens grown at four far-red photon flux densities (0, 25, 50, and 75 μmol⋅m−2⋅s−1) and three nutrient solution nitrogen concentrations (75, 125, and 175 mg⋅L−1).
Citation: J. Amer. Soc. Hort. Sci. 149, 2; 10.21273/JASHS05352-23
Results
FR light.
For Chinese cabbage ‘Tokyo Bekana’, increasing the FR PFD generally did not influence the shoot fresh mass and dry mass and total leaf area regardless of the N concentration, although increasing it from 0 to 25 μmol⋅m−2⋅s−1 notably decreased them at the N concentration of 175 mg⋅L−1 (Fig. 3). Increasing the FR PFD generally increased leaf length and decreased relative chlorophyll concentration (Fig. 4). As the FR PFD increased from 0 to 75 μmol⋅m−2⋅s−1, leaf length increased by 20% to 26%, whereas the relative chlorophyll concentrations decreased by 44% to 47%. For kale ‘Red Russian’, including FR light decreased shoot fresh mass by 30% at the N concentration of 125 mg⋅L−1 (Fig. 3). The total leaf area decreased by 35% as the FR PFD increased from 0 to 25 μmol⋅m−2⋅s−1 at the N concentration of 125 mg⋅L−1. Increasing the FR PFD generally increased the shoot height and leaf length and decreased the relative chlorophyll concentration (Fig. 4). As the FR PFD increased, the shoot height increased by 27% to 32%, whereas the leaf length increased by 31% to 61%, except at the N concentration of 125 mg⋅L−1. The relative chlorophyll concentration decreased by 52% to 58% with increasing FR PFDs.
Shoot fresh mass, shoot dry mass, and total leaf area of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’ baby greens grown under superelevated CO2 and continuous light. Plants were subject to treatments comprising light spectra with four far-red photon flux densities (0, 25, 50, and 75 μmol⋅m−2⋅s−1) and three nutrient solution nitrogen concentrations (75, 125, and 175 mg⋅L−1). Data were pooled from two replications. Values (mean ± SE) within each species and parameter followed by different letters are statistically different (P ≤ 0.05).
Citation: J. Amer. Soc. Hort. Sci. 149, 2; 10.21273/JASHS05352-23
Shoot height, length of the longest leaf, and relative chlorophyll concentration of Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’ baby greens grown under superelevated CO2 and continuous light. Plants were subject to treatments comprising light spectra with four far-red photon flux densities (0, 25, 50, and 75 μmol⋅m−2⋅s−1) and three nutrient solution nitrogen concentrations (75, 125, and 175 mg⋅L−1). Data were pooled from two replications. Values (mean ± SE) within each species and parameter followed by different letters are statistically different (P ≤ 0.05).
Citation: J. Amer. Soc. Hort. Sci. 149, 2; 10.21273/JASHS05352-23
N concentration.
The N concentration affected most of the measured growth parameters of Chinese cabbage ‘Tokyo Bekana’, but it generally did not affect those of kale ‘Red Russian’. For Chinese cabbage ‘Tokyo Bekana’, the N concentration generally increased the shoot fresh mass and dry mass and increased the leaf length and total leaf area in a few treatments (Figs. 3 and 4). As the N concentration increased from 75 to 175 mg⋅L−1, Chinese cabbage ‘Tokyo Bekana’ shoot fresh mass increased by 32% to 59%, except under 25 μmol⋅m−2⋅s−1 of FR light, at which the shoot fresh mass increased by 55% with an increasing N concentration from 75 to 125 mg⋅L−1; however, it did not change with an increasing N concentration from 125 to 150 mg·L−1 (Fig. 3). The shoot dry mass increased by 37% to 74% as the N concentration increased, except under 25 μmol⋅m−2⋅s−1 of FR light (Fig. 3). The leaf length increased by 15% with an increasing N concentration from 75 to 175 mg⋅L−1 under 75 μmol⋅m−2⋅s−1 of FR light (Fig. 4). The leaf length also increased by 16% as the N concentration increased from 75 to 125 mg⋅L−1 under 25 μmol⋅m−2⋅s−1 of FR light. The total leaf area increased with an increasing N concentration by 40% and 35% under 0 and 75 μmol⋅m−2⋅s−1 of FR light, respectively (Fig. 3). Under 25 μmol⋅m−2⋅s−1 of FR light, the total leaf area increased by 37% when the N concentration increased from 75 to 125 mg⋅L−1.
Discussion
Both Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’ exhibited typical morphologies akin to the shade-avoidance response under increasing FR PFDs. The increased leaf length and stem elongation and decreased relative chlorophyll concentration are typical responses to increasing FR light in a variety of plants (Demotes-Mainard et al. 2016; Franklin 2008; Legendre and van Iersel 2021). However, under continuous light in our study, partially substituting FR light for R light under fixed blue and green light did not have an overall effect on the edible biomass (shoot fresh and dry mass) or total leaf area of either crop, except in a few treatments for kale ‘Red Russian’. Similarly, partially substituting 32% FR light for white light at the same total PFD of 100 followed by 190 μmol⋅m−2⋅s−1 did not affect the biomass of basil (Ocimum basilicum cv. Dark Opal), cabbage (Brassica oleraceae var. capitata cv. Red Cabbage), or kale ‘Red Russian’ microgreens grown under a 12-h photoperiod followed by a 18-h photoperiod (Hooks et al. 2022). In contrast, adding 160 μmol⋅m−2⋅s−1 of FR light to 300 μmol⋅m−2⋅s−1 of white light increased the shoot fresh mass and dry mass of lettuce ‘Red Cross’ baby greens grown under a 16-h photoperiod (Li and Kubota 2009). Similarly, adding 30 to 75 μmol⋅m−2⋅s−1 of incremental FR light to 180 or 360 μmol⋅m−2⋅s−1 of blue+R light (1:1) generally increased the shoot dry mass of lettuce ‘Rex’ and ‘Rouxai’ seedlings grown under continuous light (Meng and Runkle 2019). This indicates that although continuous light may be a stress source, it cannot readily explain the lack of FR light effects in our study. However, FR light additions did not influence the shoot fresh mass of lettuce ‘Rex’ and ‘Rouxai’ seedlings under 360 μmol⋅m−2⋅s−1 of blue+R light (Meng and Runkle 2019). In addition, FR-light-induced growth enhancement diminished as the blue PFD decreased under continuous light (Meng and Runkle 2019). Taken together, the effects of FR light on biomass accumulation depend on crop species, cultivars, how FR light is included (i.e., substitution of or addition to existing light), and the existing light environment (e.g., the total PFD and the blue PFD).
The N concentration in the nutrient solution had varied effects on biomass accumulation between Chinese cabbage ‘Tokyo Bekana’ and kale ‘Red Russian’. In Chinese cabbage ‘Tokyo Bekana’, as the N concentration increased, plant edible biomass and most morphological parameters increased. Similarly, increasing the nutrient solution N concentration from 0 to 200 mg⋅L−1 quadratically increased the shoot fresh mass of three substrate-grown microgreen species, arugula (Eruca sativa), mizuna (Brassica rapa var. japonica), and mustard (Brassica juncea cv. Garnet Giant) (Allred 2017). Because N plays a significant role in protein synthesis, enzyme synthesis, and photosynthesis, increasing the available N can increase photosynthetic activity and, thus, growth (Lawlor et al. 2001). However, the N concentration generally did not affect the biomass and morphology of kale ‘Red Russian’. The N availability is positively correlated with the growth of crops [e.g., winter wheat (Triticum aestivum)] until it becomes N-saturated (Bock and Hergert 1991; Lawlor et al. 2001). Therefore, the lack of N responses in kale ‘Red Russian’ may be attributed to its lower N saturation thresholds than those for Chinese cabbage ‘Tokyo Bekana’. Additionally, elevated CO2 concentrations (70 Pa) increased the photosynthetic N-use efficiency and biomass of sunflower (Helianthus annuus cv. Teddy Bear) (Zerihun et al. 2000). Elevated CO2 concentrations (from 400 to 800 μmol⋅mol−1), together with increased N concentrations (from 0.06 to 0.24 g N⋅kg−1 soil), increased the cucumber (Cucumis sativus cv. Jinmei No. 3) yield, whole-plant productivity, and carbon accumulation in fruits (Dong et al. 2018). However, superelevated CO2 concentrations (from 800 to 1200 μmol⋅mol−1) decreased cucumber yield, likely through the inhibition of N metabolism (Dong et al. 2018). The limitation of superelevated CO2 on N metabolism in a potential crop-specific manner may also explain the unresponsiveness of kale ‘Red Russian’ to the N concentration under superelevated CO2 in our study.
Increasing the FR PFD can inhibit N assimilation and affect the synthesis of key enzymes including nitrate reductase and nitrite reductase (Demotes-Mainard et al. 2016), which can explain the reduced chlorophyll concentration with increasing FR PFDs in our study because N is an integral component of chlorophylls. Nitrate reductase activity in squash (Cucurbita maxima) exhibited phytochrome-mediated decreases when exposed to FR light pulses or R followed by FR light pulses, compared with solely R light (Rajasekhar et al. 1988). Catalyzation from nitrate reductase is the first step in assimilating inorganic N for plant use; therefore, it is essential for growth (Campbell 1999). A decrease in nitrate reductase activity caused by FR light may explain the diminishing influence of the N concentration on the growth of kale ‘Red Russian’ in the presence of FR light in our study.
Chlorosis was present in both crops under all treatments, especially at the lowest N concentration of 75 mg⋅L−1. This can be attributed to a deficiency in N, which is critical in chlorophyll development (Shimshi 1967). In addition, Chinese cabbage ‘Tokyo Bekana’ showed necrosis under all treatments, which is consistent with its stunted, chlorotic, and necrotic growth observed under superelevated CO2 in arcillite growing media (Burgner et al. 2019, 2020). Another possible cause is stress induced by continuous light. Tomato grown under continuous light had increased chlorosis and decreased photosynthetic rates, which could be attributed to increased starch and hexose accumulation in leaves, although continuous light either did not affect or increased lettuce growth compared with a shorter photoperiod (Sysoeva et al. 2010). This indicates that plant responses to continuous light are species-specific. In lettuce ‘Grand Rapids’, increasing the photosynthetic PFD from 250 to 450 μmol⋅m−2⋅s−1 did not influence biomass but exacerbated chlorosis in bottom leaves, which was attributed to excess sugar accumulation under continuous light (Oda et al. 1989). Therefore, it is possible that the continuous light in our study limited the responsiveness of kale ‘Red Russian’ to increasing N concentrations by overloading the electron transport chain and photo-oxidating chlorophylls (Sysoeva et al. 2010). In this case, the N concentration may not be a limiting factor in plant growth under stress posed by continuous light.
In conclusion, the two crops in this study exhibited varying responses to both FR light and N concentration under superelevated CO2 and continuous light. Chinese cabbage ‘Tokyo Bekana’ was more responsive than kale ‘Red Russian’ to growth influences from either variable. Chinese cabbage ‘Tokyo Bekana’ appears to require optimal growing conditions for the desired yield and quality, whereas kale ‘Red Russian’ is more tolerant of conditions that are otherwise supraoptimal or suboptimal for other leafy greens. To grow at least these two crops as baby greens under spaceflight conditions, partially substituting FR light for R light is unnecessary, whereas a nutrient solution N concentration of 175 or 75 mg⋅L−1 is sufficient for Chinese cabbage ‘Tokyo Bekana’ or kale ‘Red Russian’, respectively.
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