ENVIRONMENTAL AND CULTURAL PRACTICES TO OPTIMIZE THE GROWTH AND DEVELOPMENT OF THREE MICROGREEN SPECIES
Allred, Jonathan Asher
The recent popularity of edible microgreens (young seedlings of vegetable and herbs harvested shortly after emergence of the first true leaf) has resulted in increased interest of greenhouse growers to cultivate them for local markets. Their cultivation in hoop-houses, greenhouses and indoor production vertical farms (plant factories), permit growers control over growth parameters. This in turn allows growers to influence crop yield, morphology, days to harvest (DTH) and secondary metabolites, which hold perceived health benefits with consumers. Recommendations for their germination and growth parameters vary widely by seed supplier leaving growers to determine their own ideal cultural practices best suited for their operation. The use of horticultural lighting systems to hasten growth and promote the development of aromatic compounds in microgreens has also received little attention in the published literature. Therefore, it was the objective of this study to quantify the effects of four common cultural practices, four daily light integrals (DLI) and four carbon dioxide (CO2) concentrations on the growth, morphology, DTH and secondary metabolite production of three microgreen species: arugula (Eruca sativa L.), mizuna (Brassica rapa L. var. japonica) and mustard [Brassica. juncea (L.) Czern. ‘Garnet Giant’]. We began by evaluating four seed densities (1.1, 1.65, 2.2, 2.75 and 3.3 seeds•cm-2); five fertilizer concentrations (0, 50, 100, 150 or 200 mg N•L-1); four substrate depths (1.8, 3.3, 4.3 and 5.8 cm); and four air temperatures (14, 16, 20 and 22 °C) on fresh weight (FW), fresh weight per plant (FWPP), dry weight (DW), plant height and DTH. FW and FWPP were influenced in equal but opposite quadratic fashions as seed density increased from 1.1 to 3.3 seeds•cm-2 where total FW increased while FWPP decreased. FW increased in a quadratic fashion as both fertilizer concentration and substrate depth increased from 0 to 200 mg N•L-1 and 1.8 to 5.8 cm. DTH decreased linearly as air temperatures increased from 14 to 22 °C. After parameters were established for optimal cultural practices, we sought to quantify the effects of four DLI and four CO2 levels on the growth, morphology and secondary metabolite content of microgreens. Four levels of DLI (3, 6, 9 and 12 mol•m-2•d-1) by four levels of CO2 (400, 600, 800 and 1000 ppm) were evaluated under a full factorial design. FW increased linearly for mizuna and mustard as DLI and CO2 increased from 3 to 12 mol•m-2•d-1 and 400 to 1000 ppm. Arugula FW increased in a quadratic fashion as DLI increased from 3 to 12 mol•m-2•d-1 and linearly as CO2 increased from 400 to 1000 ppm. Dry weight increased linearly for all species as DLI and CO2 increased from 3 to 12 mol•m-2•d-1 and 400 to 1000 ppm. For mizuna and mustard, DTH decreased in a quadratic fashion while arugula DTH decreased linearly as DLI increased from 3 to 12 mol•m-2•d-1 with no observed influence from CO2. Total phenolics and total flavonoids increased linearly as DLI increased from 3 to 12 mol•m-2•d-1 where the effect of DLI on phenolic content was dependent on the CO2 level. The results of these studies can help growers determine optimal cultural practices to maximize yields, minimize production time and achieve a target crop size based on individual market demand. In addition, results can help growers conclude what combination of DLI and CO2 can achieve maximum yields at the lowest lighting energy input. Growers can then determine the importance of achieving maximum phenolic and flavonoid compounds and adjust light and CO2 as needed.
Horticulture; CO2; Controlled Environment; Cultural Practices; DLI; Greenhouse; Microgreens; Plant sciences
Mattson, Neil S.
Timmons, Michael B.; Gomez, Miguel I.
MS of Horticultural Biology
Master of Science
dissertation or thesis