Nutrition by Design: Boosting Selenium Content and Fresh Matter Yields of Salad Greens With Preharvest Light Intensity and Selenium Applications

Selenium (Se) is an essential mineral in multiple human metabolic pathways with immune modulatory effects on viral diseases including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and HIV. Plant-based foods contain Se metabolites with unique functionalities for the human metabolism. In order to assess the value of common salad greens as Se source, we conducted a survey of lettuce commercially grown in 15 locations across the USA and Canada and found a tendency for Se to accumulate higher (up to 10 times) in lettuce grown along the Colorado river basin region, where the highest amount of annual solar radiation of the country is recorded. In the same area, we evaluated the effect of sunlight reduction on the Se content of two species of arugula [Eruca sativa (E. sativa) cv. “Astro” and Diplotaxis tenuifolia (D. tenuifolia) cv. “Sylvetta”]. A 90% light reduction during the 7 days before harvest resulted in over one-third Se decline in D. tenuifolia. The effect of light intensity on yield and Se uptake of arugula microgreens was also examined under indoor controlled conditions. This included high intensity (HI) (160 μ mol−2 s−1 for 12 h/12 h light/dark); low intensity (LI) (70 μ mol m−2 s−1 for 12 h/12 h light/dark); and HI-UVA (12 h light of 160 μ mol m−2 s−1, 2 h UVA of 40 μ mol m−2 s−1, and 10 h dark) treatments in a factorial design with 0, 1, 5, and 10 ppm Se in the growing medium. HI and HI-UVA produced D. tenuifolia plants with 25–100% higher Se content than LI, particularly with the two higher Se doses. The addition of Se produced a marked increase in fresh matter (>35% in E. sativa and >45% in D. tenuifolia). This study (i) identifies evidence to suggest the revision of food composition databases to account for large Se variability, (ii) demonstrates the potential of introducing preharvest Se to optimize microgreen yields, and (iii) provides the controlled environment industry with key information to deliver salad greens with targeted Se contents.

Hydroponic production of selected crops.

While there is a wide range of potentially profitable crops which can be grown in hydroponics under protected cultivation, greenhouse production is dominated by fruiting crops such as tomatoes, cucumber, capsicum and strawberries, and vegetative species such as lettuce, salad and leafy greens, herbs and specialty crops like microgreens. This chapter summarizes information on a selected range of common hydroponic crops to give basic procedures for each and an outline of the systems of production. These crops include tomato, capsicum or sweet bell pepper, cucumber, lettuce and other salad greens, strawberry and rose. Information is given on their hydroponic production systems and environment, propagation, plant density, pruning, pollination, fruit growth, crop nutrition, pests, diseases, disorders, harvesting and postharvest handling.

Hydroponic production of selected crops.

While there is a wide range of potentially profitable crops which can be grown in hydroponics under protected cultivation, greenhouse production is dominated by fruiting crops such as tomatoes, cucumber, capsicum and strawberries, and vegetative species such as lettuce, salad and leafy greens, herbs and specialty crops like microgreens. This chapter summarizes information on a selected range of common hydroponic crops to give basic procedures for each and an outline of the systems of production. These crops include tomato, capsicum or sweet bell pepper, cucumber, lettuce and other salad greens, strawberry and rose. Information is given on their hydroponic production systems and environment, propagation, plant density, pruning, pollination, fruit growth, crop nutrition, pests, diseases, disorders, harvesting and postharvest handling.

Microgreens: A New Specialty Crop

Microgreens are young, tender greens that are used to enhance the color, texture, or flavor of salads, or to garnish a wide variety of main dishes. Harvested at the first true leaf stage and sold with the stem, cotyledons (seed leaves), and first true leaves attached, they are among a variety of novel salad greens available on the market that are typically distinguished categorically by their size and age. Sprouts, microgreens, and baby greens are simply those greens harvested and consumed in an immature state. This article offers production advice for greenhouse microgreen production.https://edis.ifas.ufl.edu/hs1164 This is a minor revision of Treadwell, Danielle, Robert Hochmuth, Linda Landrum, and Wanda Laughlin. 2010. “Microgreens: A New Specialty Crop”. EDIS 2010 (3). https://journals.flvc.org/edis/article/view/118552.

Yield of Leafy Greens and Microclimate in Deep Winter Greenhouse Production in Minnesota

The Deep Winter Greenhouse (DWG) uses passive solar technology to create an environment where cold tolerant crops like leafy greens can be grown during the winter to satisfy consumer demand for fresh local produce year-round and increase revenue for farmers. A cultivar trial consisting of 12 different salad greens was conducted in partnership with three farmers in order to determine suitable cultivars for these unique systems. Yield and days to maturity were recorded, and microclimate conditions such as light, temperature, and relative humidity were tracked throughout the season. This study included three sites from three distinct locations within Minnesota. Environmental differences among the DWG sites and within site replicates were observed. Yield also varied with micro-climate. The trials demonstrate that lettuces and greens such as ‘Florence’ (Lactuca sativa L.), ‘Salanova’ (Lactuca sativa L.), and ‘Vitamin Green’ (Brassica rapa L.) generally grow well. These cultivar trials, coupled with other horticultural trials and economic analyses, will better inform recommendations for farmers growing in these unique environments.

Production of Baby-leaf Salad Greens in the Spring and Fall Seasons of Northwest Washington

Baby-leaf salad green crops such as lettuce (Lactuca sativa), kale (Brassica oleracea), arugula (Eruca sativa), and mustard greens (Brassica juncea) thrive in the cool, humid climate of the maritime Pacific Northwest, particularly in the extended spring and fall seasons. To identify cultivars best suited for extended-season production in northwest Washington, nine leafy green cultivars were grown at two locations in the spring and fall seasons for 2 years. A high level of variability in crop performance was observed between seasons, locations, years, planting dates, and cultivars, indicating low-yield stability in baby-leaf salad crops across diverse environments and conditions. Overall, cultivars had a higher marketable weight in the spring than in the fall. Marketable weight was higher in Spring 2013 than in Spring 2014, and was higher in Fall 2013 than in Fall 2012. Days to harvest (DTH) were shorter in the spring than in the fall both years, and in both seasons DTH varied by ≈1 week between the two trial locations. Fresh weed biomass was almost 5.5 times higher in spring than in fall both years. Overall, pak choi ‘Joi Choi’ and mustard ‘Komatsuna’ had the highest marketable weight, lowest DTH, and lowest weed biomass across the widest range of environments and conditions, while beet ‘Bull’s Blood’ had the lowest marketable weight, relatively long DTH and highest weed biomass. These results suggest that baby-leaf salad crop cultivar selection differs for spring and fall seasons, and production can be highly variable between years and locations. Further, results suggest that growers should plant a diversity of crop cultivars each season to protect from crop loss and to achieve overall yield stability.