Morphological Stages of Seedling Development in Mustard (Brassica rapa) Microgreens

Microgreens, have seen a tremendous increase in production in the last decade as consumers demand novel, fresh, and local products. Microgreens are currently defined as any plant harvested immaturely for the fresh and whole consumption of its early developed leaves, cotyledons, and hypocotyl, pending the point of third true leaf emergence. At this time, there are no standards to define developmental stages of microgreens and their corresponding harvestable maturity. Counting the number of days from sowing until harvest is the most commonly used method. However, this method often results in inconsistent outcomes depending on environmental conditions. The objective of this study was to describe developmental stages of microgreens based on morphology that in turn can be used to define and establish harvestable maturity. In this experiment, Brassica rapa (Suehlihung No. 2 Mustard.), a commonly produced microgreen species, was grown for 14.5 days in a controlled environment with an 18hr photoperiod (23.9 ˚C, 10.5% RH, 77.8 µmol∙m^-2∙s^-1). Variables were measured as a function of days after radicle emergence including total height, length of the hypocotyl and epicotyl, length of individual cotyledons and true leaves, and angles between the two cotyledons and the first two true leaves. All measurements were taken twice a day (8:00am and 8:00pm) between germination and the emergence third true leaf. Germination was 57.5%, with a maximum germination rate occurring four days after sowing. The first easily defined maturity stage was measured five days after germination, and was characterized by stability in overall height and cotyledon length. Eleven days after germination, a second stage of maturity was described as the distance between the tips of the two cotyledons equaled the distance between the tips of the first two true leaves; this stage lead to the emergence of the third true leaf. Correlation between morning and afternoon measurements and the rate of cotyledon and true leaf development revealed that measuring morphological traits during the afternoon may result in more accurate morphological evaluations. For “mustard-microgreens”, relationships observed between time and organ development, suggest that harvestable maturity could be assessed by identifying morphological stages. The study indicated that morphological stages may be used to better define harvestable maturity of microgreens, to enhance production practices, establish product prices, and improve communication between the producers and consumers.