Abstract
Over half of global food waste occurs in retail and households, highlighting the importance of improving shelf life. Many factors affect shelf life, including storage temperature, harvest time, processing method, and packaging. Modified atmosphere packaging (MAP), such as nitrogen (N2), has been used to improve the shelf life of produce. This study aimed to determine the effect of nitrogen-enriched packaging on the shelf life of greenhouse- and garden-grown kale in raised beds outdoors. Five kale cultivars (Black Magic, Darkibor, Lacinato, Red Russian, and Westlander) were grown in the greenhouse, and four cultivars (Curly Roja, Dwarf Green Curled, Meadowlark, and Vates) outdoors in raised beds, harvested and stored in nitrogen-enriched or nonenriched bags for 6 days. Leaf water content, water activity, color, visual score, and texture were measured for each replicate. Nitrogen-enriched packaging did not affect the kale shelf life. Garden-grown kale exhibited significantly lower water activity (0.979 vs. 0.999) and noticeably lower water content (88.6 vs. 90.6%) than greenhouse-grown kale. The cultivars were significant for all factors tested except for texture on day 4 and change in light to dark (Δ L*) and blue to yellow (Δ b*) color ratios. At the end of the study, cultivars Darkibor and Curly Roja had the highest visual score (4.4) and texture (305 g), respectively. The high performance of ‘Darkibor’ and ‘Curly Roja’ in visual score and texture may be due to decreased (Δ L*) and (Δ b*) values and decreased water content and water activity values. Future breeding efforts focused on the shelf life of kale should use the top-performing cultivars identified in this study and a diverse genetic population tested in a range of environments to develop cultivars with a longer shelf life.
Global vegetable production totaled slightly more than 1 billion metric tons in 2018, with cabbages and other brassicas comprising 69 million metric tons (Food and Agriculture Organization of the United Nations and Agricultural Research Centre for International Development, 2021). In 2017, ∼4.3 million acres of vegetables were harvested in the United States, including more than 430,000 acres of leafy greens that include lettuce (Lactuca sativa; 345,965 acres), spinach (Spinacia oleracea; 69,969 acres), and kale (Brassica oleracea var. acephala; 15,325 acres) [U.S. Department of Agriculture, National Agricultural Statistics Service (USDA NASS), 2017]. Kale is a low-calorie, nutrient-rich leafy green that contains 10% or more of 17 essential nutrients, including prebiotic carbohydrates, minerals, and vitamins (Thavarajah et al., 2019). Kale’s popularity and production have significantly increased over the past decade in conventional and organic agriculture systems due to its perceived health benefits. In 2021, kale comprised 1.3% of total organic produce sales, valued at more than USD 17 billion in 2018 (Pullano, 2015; Šamec et al., 2019; USDA NASS, 2017). Unfortunately, not all produce harvested is used, resulting in food waste. Food waste is detrimental to the world economy, with the total social, economic, and environmental cost of global annual food waste estimated to be USD 2.6 trillion (FAOSTAT, 2014). An estimated 931 million tons of food waste was generated in 2019, which accounted for 17% of global food production (Forbes et al., 2021). Moreover, the detrimental effects of food waste have environmental implications because global carbon emissions associated with food waste may account for 8% to 10% of total greenhouse gas emissions (Mbow et al., 2019).
Shelf life is an essential consideration for food waste: if a food product has a more extended shelf life, it will last longer and is more likely to be used before it spoils. Indeed, 35% of vegetable waste in the United States occurs at retail or after consumer purchase (Gunders and Bloom, 2017). The need to extend shelf life is supported by the high proportion of global food waste generated at the household level; in 2021, 61% of global food waste was more than twice the 2011 estimate (Forbes et al., 2021). The shelf life of leafy greens depends on various factors, including time of harvest, method of processing, storage temperature, and packaging material (Albornoz and Cantwell, 2016; Cantwell et al., 2016; Casajús et al., 2021; Sripong et al., 2018). Postharvest treatment of leafy greens such as kale usually includes one or multiple rinses in chlorinated water, hydrocooling, and additional cooling and packing with ice (Boyette et al., 1992). Bagging harvested kale reduces moisture loss, maintains color, and reduces senescence (Albornoz and Cantwell 2016; Sidhu 2013). One way to increase the shelf life of vegetables is to use modified atmosphere packaging (MAP), which usually uses varying concentrations of nitrogen (N2), oxygen (O2), and carbon dioxide (CO2) mixtures (Sandhya, 2010). Nitrogen is an odorless, tasteless, colorless, and relatively nonreactive gas that inhibits the growth of aerobic bacteria and has been used to inhibit microbial growth in fresh-cut vegetables (Koseki and Itoh, 2002; Sandhya, 2010). The optimal mix of N2, O2, and CO2 varies with the product, although lower O2 and higher levels of CO2 generally inhibit respiration (Sandhya, 2010). However, increased levels of highly soluble CO2 can enter the product resulting in package collapse but can be avoided with N2 (Sandhya, 2010). Studies with fresh-cut lettuce and cabbage show decreased browning and microbial growth in N2 MAP compared with atmospheric air, but research on the effect of nitrogen-enriched MAP on kale is lacking (Koseki and Itoh, 2002). Therefore, this study is focused on elucidating the impact of nitrogen-enriched packaging on the shelf life of greenhouse- and garden-grown kale. Nitrogen-enriched packaging was expected to improve the shelf life of kale. Additionally, kale cultivars were expected to exhibit differential shelf life performance across water content, water activity, texture, color, and visual score.
Materials and Methods
Kale seeds
Nine kale cultivars were used in this study (Table 5). Commercial seeds were purchased from High Mowing Seeds (Wolcott, VT, USA) and Johnny’s Selected Seeds (Winslow, ME, USA). A list of market classifications of kale cultivars can be found in Reda et al. (2022).
Greenhouse design
This study was conducted from Aug through Nov 2021 at the Clemson University Greenhouses and Tiger Gardens, located in Clemson, SC (lat. 34.40274°N, long. 82.49565°W). The kale cultivars Black Magic, Darkibor, Lacinato, Westlander, and Red Russian were sown in a germination mix (Sungro Propagation Mix, Sungro Horticulture, Agawam, MA, USA) in 72 cell-count trays on 16 Aug 2021 in Clemson, SC, USA, at the Clemson University Greenhouses. Seven seedlings per cultivar (n = 35) were transplanted to 1-gallon pots of soilless media (Sungro Professional Growing Mix, Sungro Horticulture) on 30 Aug 2021. The plants were grown under natural irradiance supplemented by a 14-h photoperiod using Phantom Photobio•T PTB3330LS4X high-efficiency LED lights (Phantom Photobio, Petaluma, CA, USA) spaced 122 cm (4 ft) above the plant canopy. Greenhouse conditions averaged 24.8/22.4 ± 0.1.0/1 0.5 °C day/night and 88.8/97.7 ± 6.2/3.0% relative humidity day/night. Plants were watered using drip irrigation daily for 3 min using an Orbit Model 24600 irrigation timer (Orbit Irrigation Products, North Salt Lake, UT, USA) and received 250 mL of 150 ppm N twice per week with a 20N–20P–20K water-soluble fertilizer (Peter’s Professional 20–20–20 General Purpose Fertilizer, Everris Na Inc., Dublin, OH, USA) for a total of 75,000 μg N per week. Plants were harvested at physiological maturity (having greater than six true leaves) on 5 Oct 2021 (Waterland et al., 2017).
Raised beds
Cultivars Curly Roja, Dwarf Green Curled, Meadowlark, and Vates were sown in 72-count deep insert trays in the greenhouse in mid-September. Six seedlings per cultivar (n = 24) were transplanted to 1-gallon pots of soilless media (Sungro Professional Growing Mix, Sungro Horticulture) on 6 Oct 2021. Plants were moved outdoors on 14 Oct 2021 and hardened before being transplanted into raised beds on 19 Oct 2021. Plants were spaced 30 cm apart within and between rows. Raised beds were 1.22 m wide × 2.13 m long × 0.61 m tall, and beds were ∼0.91 m apart (Supplemental Fig. 1). Beds were filled with Sungro Professional Growing Mix (Sungro Horticulture). The average air temperature was 13.6 ± 6.1 °C with a high of 27 °C and a low of 1.6 °C; the average relative humidity was 76 ± 9.7%, within a range of 25% to 97% (wunderground.com). Plants were watered daily through drip irrigation for 3 min using a Mister Timer irrigation timer (Mister Landscaper Mister Timer, Mister Landscaper Drip Irrigation, and Micro Spray, Dundee, FL, USA). Each plant was hand-fertilized once a week with 250 mL of Peter’s Professional 20–20–20 General Purpose Fertilizer (Everris Na Inc., Dublin, OH, USA) at 150 ppm N (75,000 μg N per week). Plants were harvested at physiological maturity on 16 Nov 2021.
Harvest and postharvest storage
Leaves were cut at the base, double-rinsed in ice water, and spun dry in a 5-gallon ChefMaster centrifugal hand dryer (ChefMaster, Melville, NY, USA). Then leaves were cut into 3- to 4-cm-wide strips and placed into clear 1-gallon low-density polyethylene bags (Hefty Storage Slider bags, Reynolds Consumer Products, Lake Forest, IL, USA). MAP was included as the plastic bags were filled with analytical grade N2 gas by inserting a 1/8-inch (0.32-cm) tube connected to an N2 canister, sliding the bag closed except for the tube, pressing the air out, and flushing the bag with N2, quickly removing the tube, and then immediately closing the bag. The presence of the other gasses was not tested. Samples were stored at 10 °C and analyzed on days 0, 2, 4, and 6. Bags were refilled with N2 after each sampling. Greenhouse-grown cultivars had seven replications: five replications were stored in nitrogen-enriched bags, and two were stored without added N2. Cultivars grown outdoors in raised beds had six replications: three replications stored in nitrogen-enriched bags and three without added N2.
Water content.
For greenhouse-grown kale, 3 g of leaf tissue (± 0.03 g) from each sample was oven-dried at 105 °C for 2 h. Water content was measured using 0.5 g of leaf tissue (± 0.01 g) for garden-grown kale. Fresh weight was measured before oven-drying, and dry weight was measured following oven-drying. Dried samples were stored in a desiccator and weighed to determine water content. Water content is reported as a percentage of fresh weight.
Water activity.
Two to four leaf strips were removed from each plastic bag for greenhouse and garden-grown kale and placed into METER Aqualab disposable sample containers (Meter Group Inc., Pullman, WA, USA) until they covered the bottom of the plastic container. The cup was then placed in an AQUALAB 4TE chilled mirror dew point-sensing water activity meter (Meter Group Inc.), and the water activity (aw) was determined. The standard for water activity meter calibration was 13.41 mol/kg (0.250 aw) LiCl (Meter Group Inc.). The standard was run at the start of each run to ensure the water activity meter was functioning properly.
Color.
Leaf strips were removed from the plastic bag, placed in a rectangular plastic weigh boat, and analyzed by a spectrophotometer (Aeros Spectrophotometer, Hunter Associates Laboratory, Reston, VA. Values were reported as L*, a*, and b*, where L* describes how light or dark an object is [values range from 0 (black) to 100 (diffuse white)], and a* values describe the level of redness or greenness of an object (values are negative for green and positive for red). b* values describe whether an object is blue or yellow (values are negative for blue and positive for yellow). Data are reported as changes in respective color values, that is, Δ L*, Δ a*, and Δ b*, to account for cultivars of varying coloration; this allows for comparison between green- and red-colored cultivars without their inherent colors producing statistically significant differences. Values of Δ L*, Δ a*, and Δ b* were calculated using the following equations: Δ L* = L*Day X – L*Day 0; Δ a* = a*Day X − a*Day 0; and Δ b* = b*Day X − b*Day 0.
Texture.
Texture was measured by force required to puncture a leaf (shear texture), measured in grams using a CTX Texture Analyzer (Brookfield Ametek, Middleborough, MA, USA). The CTX measured shear texture using the Volodkevitch Bite Jaws in a compression test with the trigger set at 75 g, the deformation peak at 5.0 mm, and a speed of 10 mm per second. These settings always punctured the leaf, regardless of age or state of decomposition. Leaf samples were placed on the lower portion of the jaw and removed once punctured by the upper part of the jaw.
Visual score.
Visual score is an assessment of the overall quality of kale leaves over time intended to estimate suitability for use and sale. Samples were assigned a visual score of 1 to 9, adapted from Albornoz and Cantwell (2016), where 9 represented the best quality leaves, 5 represented the limit of marketability, 3 represented the limit of use, and 1 represented the worst-quality leaves (see Table 6 later in the article). All samples were assigned a score of 9 on day 0 (day of harvest). A single individual performed a visual score rating to avoid variability.
Statistical analysis.
Plants were arranged in a completely randomized design, with seven replicates for greenhouse-grown plants and six replicates for garden-grown plants. All cultivars were initially planned to be grown in the greenhouse; however, four cultivars were moved to the garden following insect pressure in the greenhouse. The greenhouse and garden-grown kale data were pooled to compare cultivars of different growing environments. Statistical models were developed for the dependent variables of water content, water activity, visual score, texture, and color, including the independent variables of cultivar, packaging type, and cultivar × packaging type. Separate models were used for days 0, 2, 4, and 6. Analysis of variance was used to determine if the independent variables had a significant effect on the different dependent variables; if there was an effect, then the means for the different levels of the independent variables were separated using Fisher’s protected least significant difference test. Principal component analysis (PCA) was performed on the dependent variables of water content, water activity, visual score, texture, color, and day to determine a combination of the variables that explained the separation among the cultivars and packaging. All statistical calculations were performed using JMP Pro 16 software (SAS Institute Inc., Gary, NC, USA), and P values less than 0.05 (in some cases, P ≤ 0.10) were considered statistically significant.
Results and Discussion
The results from this study demonstrate growing environment and nitrogen-enriched packaging have largely insignificant effects on shelf life (Table 1). The kale cultivar has a significant impact (P < 0.05) on water content, water activity, color, texture, and visual score (Table 1). These results are essential for kale breeding and germplasm improvement because genotype is responsible for shelf life. In this study, ‘Darkibor’ and ‘Curly Roja’ were among the top-performing genotypes for visual score and texture (Tables 2 and 3).
Analysis of variance for water content, water activity, color, visual score, and force for kale grown in this study.
Visual score and change in red:green color ratio (Δ a*) in greenhouse- and garden-grown kale.
Leaf texture [force (g) to puncture] of kale grown in the greenhouse- and garden-grown kale.
Water content was significantly affected by cultivar on all days (P < 0.05 for days 0, 2, and 6 for greenhouse and garden-grown kale; P < 0.01 for day 4) and packaging type and the interaction between the cultivar (cultivar × packaging type) on day 4 (P < 0.05) (Table 1). Similarly, a study reported that water loss over time was significantly affected by the genotype for kale shelf life (Dumičić et al., 2014). Mean water content was generally higher among greenhouse-grown kale (90.6 vs. 88.6%). Overall, the water content of kale grown in this study was higher than the 85% to 86% reported by Armesto et al. (2017) and Lisiecka et al. (2019). The standard error for the water content of kale grown in this study is too great to allow for clear segregation between growing environments, indicating that genotype alone may not be solely responsible for determining the water content of leaves (Table 4). Moreover, postharvest bagging of kale significantly affected water content in another study on kale shelf life, indicating factors other than genotype may affect water content (Dumičić et al., 2014). Growing environment, cultivar, postharvest storage, and growing environment × cultivar interaction may all affect water content over time (Table 1).
Water content (WC%) and water activity (aw) of greenhouse- and garden-grown kale.
Cultivars used in this study at physiological maturity.
Visual scoring reference for different market classes of kale: 9 represents the highest visual quality and best-looking leaves; 5 represents the limit of marketability; 3 represents the limit of usability; and 1 represents the lowest visual quality and worst looking leaves.
This study provides evidence of a significant effect (P < 0.0001) of kale cultivar on water activity (Table 1). The growing environment entirely separated these cultivars for mean water activity on each day of data collection, with cultivars grown in the greenhouse having significantly higher water activity than garden-grown cultivars (Table 2B; Figs. 1 and 2A). All cultivars demonstrated decreased water activity over time (Figs. 1 and 2A). Our study did not measure the microbial growth for different cultivars with different water activities.
Water activity of greenhouse- and garden-grown kale from this study. Numbers on the x-axis represent days after harvest. Days with different letters on the same day indicate a significant difference in water activity between growing environments (P < 0.01). Error bars are ± SD (n = 36).
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Water activity of greenhouse- and garden-grown kale from this study. Numbers on the x-axis represent days after harvest. Days with different letters on the same day indicate a significant difference in water activity between growing environments (P < 0.01). Error bars are ± SD (n = 36).
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Water activity of greenhouse- and garden-grown kale from this study. Numbers on the x-axis represent days after harvest. Days with different letters on the same day indicate a significant difference in water activity between growing environments (P < 0.01). Error bars are ± SD (n = 36).
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Mean values of (A) water activity, (B) water content, and (C) texture of kale cultivars for two growing environments. Numbers on the x-axis represent days after harvest. Error bars are ± SD (n = 7 for greenhouse-grown kale; n = 6 for garden-grown kale).
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Mean values of (A) water activity, (B) water content, and (C) texture of kale cultivars for two growing environments. Numbers on the x-axis represent days after harvest. Error bars are ± SD (n = 7 for greenhouse-grown kale; n = 6 for garden-grown kale).
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Mean values of (A) water activity, (B) water content, and (C) texture of kale cultivars for two growing environments. Numbers on the x-axis represent days after harvest. Error bars are ± SD (n = 7 for greenhouse-grown kale; n = 6 for garden-grown kale).
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
The visual score was significantly affected (P < 0.01) by cultivar on days 2, 4, and 6 and by the interaction of packaging type × cultivar on days 2 and 6 (Table 1). Packaging type did not affect visual score on any day. Visual scores decreased over time for all cultivars, an expected trend as reported in the literature (Albornoz and Cantwell 2016) (Table 2; Fig. 3). On day 6, ‘Darkibor’ had the highest visual score, followed by ‘Black Magic’ and ‘Lacinato’; cultivars Dwarf Green Curled, Curly Roja, and Meadowlark had the lowest visual scores (Table 2). Notably, the visual scores did not correlate with previously recorded mineral values (Reda et al., 2022; Thavarajah et al., 2021). Cultivar had a significant effect on color, specifically Δ a*, on days 4 (P < 0.01) and 6 (P < 0.0001) (Table 1). ‘Curly Roja’ underwent the greatest change in red: green coloration (Δ a*) over time, which is not surprising as it was the only red kale grown in this study. ‘Lacinato’ and ‘Black Magic’ underwent the greatest increase in green: red coloration over time (Table 2). The cultivar, packaging type, and cultivar × packaging type interaction did not significantly affect Δ L* or Δ b* values.
Mean visual scores of the raised bed and greenhouse-grown kale cultivars separated by cultivar and growing environment. Numbers on the x-axis represent days after harvest. The visual score ranged from 1 to 9, with 1 representing the worst-looking leaves and 9 representing the best-looking leaves. Mean separation for cultivars is provided in Table 3. Visual scores ranged from 1 to 9, with 1 representing the worst-looking leaves and 9 representing the best-looking leaves. Mean separation for cultivars is provided in Table 3. Vertical bars standard errors of the means with 36 replications.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Mean visual scores of the raised bed and greenhouse-grown kale cultivars separated by cultivar and growing environment. Numbers on the x-axis represent days after harvest. The visual score ranged from 1 to 9, with 1 representing the worst-looking leaves and 9 representing the best-looking leaves. Mean separation for cultivars is provided in Table 3. Visual scores ranged from 1 to 9, with 1 representing the worst-looking leaves and 9 representing the best-looking leaves. Mean separation for cultivars is provided in Table 3. Vertical bars standard errors of the means with 36 replications.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Mean visual scores of the raised bed and greenhouse-grown kale cultivars separated by cultivar and growing environment. Numbers on the x-axis represent days after harvest. The visual score ranged from 1 to 9, with 1 representing the worst-looking leaves and 9 representing the best-looking leaves. Mean separation for cultivars is provided in Table 3. Visual scores ranged from 1 to 9, with 1 representing the worst-looking leaves and 9 representing the best-looking leaves. Mean separation for cultivars is provided in Table 3. Vertical bars standard errors of the means with 36 replications.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
The force required to puncture a leaf was significantly affected by cultivar (P < 0.01) on days 0, 2, and 6 and by packaging type and the packaging type × cultivar interaction (P < 0.05) on day 2 (Table 1). The force required to puncture a leaf generally decreased over time, although this varied by cultivar (Table 3; Fig. 2C). ‘Curly Roja’ required the most force to puncture a leaf across all days (Table 3). In addition, this current study results indicated that the plant growing environment has a greater influence on leaf texture showing garden-grown kale needs higher force than greenhouse-grown kale with lower water activity and water content. These results correspond to curly red kale receiving one of the highest scores for the texture qualities “Crisp Crunch,” “Brittle,” and “Dense” in a sensory evaluation study and consumer panel of kale (Swegarden et al., 2019). ‘Curly Roja’ has a higher concentration of magnesium than the other cultivars grown in this study (Reda et al., 2022). Component 1 of the PCA (Fig. 4) accounts for 37.5% of the variation in the dataset. The largest eigenvalues in component 1 were associated with visual score (negative); and day, Δ L* and Δ b* (positive). These data suggest that visual score vs. Δ L* and Δ b* can explain almost 40% of the variation associated with cultivars, packaging, and random error in the dataset. The visual score was significantly influenced by genotype. The relationship between color demonstrates this (Δ L* and Δ b*) and visual score; these data suggest that as Δ L* and Δ b* values increase, the assigned visual score will decrease. We expect a lower visual score would lead to reduced marketability (Fig. 4). An additional 22.3% of the variation in the data set is accounted for by component 2 (Fig. 4). The largest eigenvalues (the numerical values associated with the arrows of individual dependent variables in the PCA) in component 2 were associated with water content and water activity (positive), and Δ a* and force (negative). These data suggest that water content and water activity vs. Δ a* and force can explain an additional 20% of the variation associated with cultivars, packaging, and random error in the dataset. These study data suggest that as water content and water activity increase, the force required to puncture a leaf will decrease (Fig. 4). Because reduced water content was observed in garden-grown kale, the increased force needed to punch a leaf may be partly due to the growing environment (Fig. 1). In a sensory study of kale, its robust texture was defined as one of its most essential features. A reduction in force required to puncture a leaf could indicate the decreased textural quality and subsequent consumer preference. (Swegarden et al., 2019).
Principal component analysis of all data points recorded in this study. aw = water activity; WC (%) = water content; Force (g) = force required to puncture the leaf; Δ L*, Δ a*, and Δ b* are measures of color.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Principal component analysis of all data points recorded in this study. aw = water activity; WC (%) = water content; Force (g) = force required to puncture the leaf; Δ L*, Δ a*, and Δ b* are measures of color.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Principal component analysis of all data points recorded in this study. aw = water activity; WC (%) = water content; Force (g) = force required to puncture the leaf; Δ L*, Δ a*, and Δ b* are measures of color.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Conclusions
Cultivar significantly affected water content, activity, Δ a*, visual score, and force required to puncture a leaf for kale grown in this study. Packaging type only significantly affected water content and force needed to puncture a leaf on one of the sampling days. In contrast, the cultivar × packaging type interaction significantly impacted day 4 water content, day 2 force required to puncture a leaf, and visual score for days 2 and 6. Kale cultivar is the primary determining factor over time for shelf life measured except for water activity. Greenhouse-grown kale generally exhibited higher water content, while garden-grown kale exhibited significantly lower water activity. Nitrogen-enriched packaging did not impact the shelf life of kale grown in this study. Future breeding efforts focused on the shelf life of kale should incorporate cultivars Darkibor and Curly Roja for their superior performance demonstrated herein in terms of visual score and texture over time correlated with their decreased color change over time and reduced water content and water activity.
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Raised bed used in the study.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Raised bed used in the study.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
Raised bed used in the study.
Citation: HortScience 57, 11; 10.21273/HORTSCI16770-22
