Selenium and sulfur have similar chemical structures. This allows Se to be absorbed and incorporated in the same assimilation pathways as S. Onions (Allium cepa L.) are a crop with unique S metabolism, responsible for growth and flavor intensity. Because of the antagonistic behavior of the two ions, the effects of Se on S and Se nutrient depletion and tissue accumulation were investigated. `Granex 33' onions were grown in nutrient solutions with one concentration of S and increasing Se concentrations. Selenium was applied as sodium selenate (Na2SeO4) at concentrations of 0, 0.5, 1.0, 1.5, and 2.0 mg·L-1. Selenium depletion from the nutrient solution increased linearly with increasing Na2SeO4 treatment concentrations. Sulfur depletion increased and then decreased with increasing Na2SeO4 treatment concentrations. Selenium and S accumulation were highest in leaf tissues, less in root tissues, and lowest in bulb tissues at plant maturity. Selenium accumulation increased linearly with increasing Na2SeO4 for all tissues analyzed. Sulfur accumulation in leaf and bulb tissues was quadratic in response to increasing SeO4 -2, while S in root tissues decreased linearly with increasing Na2SeO4. Low concentrations of Na2SeO4 in our study enhanced S uptake and accumulation. Previously, Se was thought to competitively inhibit S uptake and metabolism.
Dean A. Kopsell and William M. Randle
Mark G. Lefsrud and Dean A. Kopsell
Controlled plant growing systems have consistently used the standard earth day as the radiation cycle for plant growth. However, the radiation cycle can be controlled using automated systems to regulate the exact amount of time plants are exposed to irradiation (and darkness). This experiment investigated the influence of different radiation cycle periods on plant growth and carotenoid accumulation in kale (Brassica oleracea L. var. acephala DC.). Plants were grown in a controlled environment using nutrient solutions under radiation cycle treatments of 2, 12, 24 and 48 hours, with 50% irradiance and 50% darkness during each cycle. The radiation cycles significantly affected kale fresh weight, dry weight, percent dry matter, and the accumulation of lutein, β-carotene, and chlorophyll a and b. Maximum fresh weight occurred under the 2-hour radiation cycle treatment, whereas maximum dry weight occurred under the 12-hour treatment. Maximum accumulation of lutein, β-carotene, and chlorophyll a occurred with the 12-hour radiation cycle at values of 14.5 mg/100 g, 13.1 mg/100 g, and 263.3 mg/100 g fresh weight respectively. Maximum fresh weight production of the kale was not linked to increases in chlorophyll, lutein, or β-carotene. Consumption of fruit and vegetable crops rich in lutein and β-carotene carotenoids is associated with reduced risk of cancers and aging eye diseases. Increased carotenoid concentrations in vegetable crops would therefore be expected to increase the value of these crops.
Mark W. Farnham and Dean A. Kopsell
Carotenoids are secondary plant metabolites in vegetables known to be essential in the human diet and reported to confer various positive health-promoting effects when consumed. Brassica oleracea L. vegetables like kale, cabbage, and broccoli are recognized as excellent sources of dietary carotenoids. Broccoli has emerged as the most important B. oleracea crop in the United States and it likely supplies more carotenoids to the U.S. diet than the other crops of this species. However, very little is known about the general carotenoid profile of this important vegetable or the levels of specific carotenoids and how they might vary among genotypes. Thus, the objectives of this study were to assess carotenoid profiles of different inbred broccoli heads; to assess chlorophyll concentrations measured simultaneously during carotenoid assays; to determine the relative effects of genotype versus environment in influencing head carotenoid levels; and to examine phenotypic correlations between carotenoid levels and other traits. Results show lutein to be the most abundant carotenoid in broccoli heads ranging from 65.3 to 139.6 μg·g−1 dry mass (DM) among nine inbreds tested in three environments. Genotype had a highly significant effect on lutein levels in broccoli heads and the ratio of σ2 g/σ2 p for this carotenoid was 0.84. Violaxanthin also exhibited a significant genotype effect, but it was found at lower levels (17.9 to 35.4 μg·g−1 DM) than lutein. β-carotene and neoxanthin were detected at levels similar to violaxanthin, but genotypic differences were not detected when all environments were compared. This was also true for antheraxanthin, which was detectable in all genotypes at lower levels (mean of 13.3 μg·g−1 DM) than the other carotenoids. Significant genotypic differences were observed for both chlorophyll a and b among the studied inbreds; however, no environment or genotype-by-environment effects were observed with these compounds. Results indicated that most carotenoids measured were positively and significantly correlated with one another, indicating that higher levels of one carotenoid were typically associated with higher levels of others. This study emphasizes the relative importance of lutein in broccoli heads and the key role that genotype plays with this compound, ultimately indicating that breeding cultivars with increased levels of this particular carotenoid may be feasible.
Dean A. Kopsell and William M. Randle
Beneficial effects of selenium (Se) can be delivered to humans through enriched plant foods. Plants in the Brassicaceae are good sources of sulfur (S) and can be enriched with Se. Breeding plants to be more efficient at Se accumulation may complement enrichment efforts. Because Se and S are chemically similar and can compete in plant metabolic pathways, S levels must be considered when attempting to manipulate Se, and vice versa. The objectives of this study were to establish genetic variances for S and Se accumulation, and to determine if simple recurrent selection could be used to manipulate Se accumulation in a rapid-cycling (Brassica oleracea L.) population. Progeny from a North Carolina Design II mating scheme were grown in two seleniferous environments and expressed variability for Se and S accumulation. Narrow sense heritability estimates for Se and S accumulation were moderate (0.55 to 0.75), which suggested progress was possible. However, standard errors were large and may influence expected progress during improvement efforts. Plants of a rapid-cycling B. oleracea were also subjected to two cycles of divergent selection for Se accumulation in leaf tissues. Realized heritabilities were high during selection for both high and low Se accumulation. Simultaneous evaluation of all populations revealed actual gains from selection to be 4.8% and 4.0% per selection cycle for high and low Se accumulation, respectively. Predicted gains for Se accumulation in the plants were 6.8%. Selection for Se accumulation was successful and indicates population improvements for such traits are possible within the B. oleracea analyzed. Breeding plants that are more efficient at accumulating Se could be a useful tool towards Se enrichment.
Mark G. Lefsrud and Dean A. Kopsell
Chlorophyll and carotenoid pigments were measured with high-performance liquid chromatography (HPLC) during leaf development in kale (Brassicaoleracea L. var. acephala D.C). Lutein and β-carotene are two plant-derived carotenoids that possess important human health properties. Diets high in these carotenoids are associated with a reduced risk of cancer, cataracts, and age-related macular degeneration. Kale plants were growth-chamber grown in nutrient solution culture at 20 °C under 500 μmol·m-2·s-1 of irradiance. Pigments were measured in young (<1 week), immature (1-2 weeks), mature (2-3 weeks), fully developed (3-4 weeks) and senescing (>4 weeks) leaves. Significant differences were measured for all four pigments during leaf development. Accumulation of the pigments followed a quadratic trend, with maximum accumulation occurring between the first and third week of leaf age. The highest concentrations of lutein were recorded in 1- to 2-week-old leaves at 15.1 mg per 100 g fresh weight. The remaining pigments reached maximum levels at 2-3 weeks, with β-carotene at 11.6 mg per 100 g, chlorophyll a at 251.4 mg per 100 g, and chlorophyll b at 56.9 mg per 100 g fresh weight. Identifying changes in carotenoid and chlorophyll accumulation over developmental stages in leaf tissues is applicable to “baby” leafy greens and traditional production practices for fresh markets.
Mark G. Lefsrud and Dean A. Kopsell
Plant growing systems have consistently utilized the standard Earth day as the radiation cycle for plant growth. However, the radiation cycle can easily be controlled by using automated systems to regulate the exact amount of time plants are exposed to irradiation (and darkness). This experiment investigated the influence of different radiation cycles on plant growth, chlorophyll and carotenoid pigment accumulation in kale (Brassica oleracea L. var. acephala D.C). Kale plants were grown in growth chambers in nutrient solution culture under radiation cycle treatments of 2, 12, 24, and 48 h, with 50% irradiance and 50% darkness during each time period. Total irradiation throughout the experiment was the same for each treatment. Radiation cycle treatments significantly affected kale fresh mass, dry mass, chlorophyll a and b, lutein, and beta-carotene. Maximum fresh mass occurred under the 2-h radiation cycle treatment. The maximum dry mass occurred under the 12-h radiation cycle treatment, which coincided with the maximum accumulation of lutein, beta-carotene, and chlorophyll a, expressed on a fresh mass basis. The minimum fresh mass occurred during the 24 h radiation cycle treatment, which coincided with the largest chlorophyll b accumulation. Increased levels of chlorophyll, lutein and beta-carotene were not required to achieve maximum fresh mass production. Environmental manipulation of carotenoid production in kale is possible. Increases in carotenoid concentrations would be expected to increase their nutritional contribution to the diet.
Dean A. Kopsell and William M. Randle
Four cultivars of onion (Allium cepa L. `Primavera', `Granex 33', `Pegasus', and `Sweet Success') were grown to maturity in modified nutrient solutions with or without 2.0 mg·L-1 Na2 SeO4 (1.51 mg·L-1 SeO4 -2). Selenium did not affect total flavor precursor content (ACSO) in `Granex 33', `Pegasus', and `Sweet Success'. However, Se affected several individual ACSOs and precursor intermediates. Selenium decreased γ-L-glutamyl-S-(1-propenyl)-L-cysteine sulfoxide and trans(+)-S-(1-propenyl)-L-cysteine sulfoxide content in all four cultivars. (+)-S-Methyl-L-cysteine sulfoxide content was higher while (+)-S-propyl-L-cysteine sulfoxide content was lower with the added Se for two cultivars. Selenium lowered total bulb S content in all cultivars, and increased the percentage of total S accumulated as SO4 -2 in three cultivars. The effect of Se on the flavor pathway was similar to that found when onions were grown under low S-concentrations. Flavor changes can be expected when onions are grown in a high Se environment, however, specific changes may be cultivar dependent.
Dean A. Kopsell and Carl E. Sams
Microgreens are specialty leafy crops harvested just above the roots after the first true leaves have emerged and are consumed fresh. Broccoli (Brassica oleacea var. italica) microgreens can accumulate significant concentrations of cancer-fighting glucosinolates as well as being a rich source of other antioxidant phytochemicals. Light-emitting diodes (LEDs) now provide the ability to measure impacts of narrow-band wavelengths of light on seedling physiology. The carotenoid zeaxanthin has been hypothesized to be a blue light receptor in plant physiology. The objective of this study was to measure the impact of short-duration blue light on phytochemical compounds, which impart the nutritional quality of sprouting broccoli microgreens. Broccoli microgreens were grown in a controlled environment under LEDs using growing pads. Seeds were cultured on the pads submerged in deionized water and grown under a 24-hour photoperiod using red (627 nm)/blue (470 nm) LEDs (350 μmol·m−2·s−1) at an air temperature of 23 °C. On emergence of the first true leaf, a complete nutrient solution with 42 mg·L−1 of nitrogen (N) was used to submerge the growing pads. At 13 days after sowing, broccoli plantlets were grown under either: 1) red and blue LED light (350 μmol·m−2·s−1); or 2) blue LED light (41 μmol·m−2·s−1) treatments for 5 days before harvest. The experiment was repeated three times. Frozen shoot tissues were freeze-dried and measured for carotenoids, chlorophylls, glucosinolates, and mineral elements. Comparing the two LED light treatments revealed the short-duration blue LED treatment before harvest significantly increased shoot tissue β-carotene (P ≤ 0.05), violaxanthin (P ≤ 0.01), total xanthophyll cycle pigments (P ≤ 0.05), glucoraphanin (P ≤ 0.05), epiprogoitrin (P ≤ 0.05), aliphatic glucosinolates (P ≤ 0.05), essential micronutrients of copper (Cu) (P = 0.02), iron (Fe) (P ≤ 0.01), boron (B), manganese (Mn), molybdenum (Mo), sodium (Na), zinc (Zn) (P ≤ 0.001), and the essential macronutrients of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S) (P ≤ 0.001). Results demonstrate management of LED lighting technology through preharvest, short-duration blue light acted to increase important phytochemical compounds influencing the nutritional value of broccoli microgreens.
Dean A. Kopsell and William R. Randle
The affects of selenium (Se) on sulfur (S) uptake and metabolism were evaluated in `Granex 33' onions. Plants were grown in a half-strength Hoagland's solution and modified with increasing Se fertility. Selenium was added as sodium selenate. During growth, plants were sampled biweekly and divided into root, bulb, and foliar tissue. Tissues were dried and ground for total S, and wet-ashed for total Se (GFAA). Selenium increased S uptake by onions. As Se increased in concentration, S utilization first increased then decreased in a quadratic trend.
William M. Randle, David E. Kopsell, and Dean A. Kopsell
A major decision in producing onions with mild flavor on low sulfur soils is determining when to stop applying SO4 -2 to the crop. Sulfate (SO4 -2) is necessary for good early growth, but high levels of available SO4 -2 late in the season increase bulb pungency. The objective of this research was to determine how sequentially reducing the availability of SO4 -2 during onion growth and development would affect flavor intensity and quality of Granex-type onions. Starting 77 days before harvest, SO4 -2 concentrations were lowered from 1 mm to 0.05 mm on different blocks of onions in a greenhouse experiment at bi-weekly intervals. Total leaf and bulb S were measured at harvest to monitor S accumulation as SO4 -2 fertility was sequentially reduced. Bulbs were harvested and analyzed for flavor precursors and their biosynthetic intermediates, gross flavor intensity as measured by enzymatically developed pyruvic acid (EPY), and soluble solids content. As SO4 -2 fertility reductions were delayed during the experiment, total leaf and bulb S increased linearly. In addition, bulb EPY concentrations increased linearly as SO4 -2 reduction was delayed, indicating increases in overall flavor intensity. While the total concentration of flavor precursors did not significantly change in response to lowering SO4 -2 fertility during the experiment, the concentrations of MCSO to 1-PRENCSO did. MCSO concentration decreased and then increased in a quadratic manner. MCSO produces fresh onion and cabbage like flavors. 1-PRENCSO, on the other hand, increased linearly as the high SO4 -2 fertility level was extended through bulb maturation. Increasing concentrations of 1-PRENCSO causes onions to have significantly more heat and mouth burn when eaten. Reducing available SO4 -2 49 days prior to harvest coincided with a reduction in EPY and a change in the flavor biosynthetic pathway that appeared to be associated with the metabolic changes occurring with the onset of bulbing. Chemical names used: enzymatically developed pyruvic acid (EPY); methyl cysteine sulfoxide (MCSO); 1-propenyl cysteine sulfoxide (1-PRENCSO).