Report on nutrient and production parameters and light requirements for microgreen production in hydroponic units

Microgreens are young plants between sprouts and baby greens with a growing period of about 7 to 16 days, depending on the variety. They are gaining increasing attention because of their short growing cycle and their beneficial attributes such as high nutritional content, valuable secondary metabolite production, and gastronomic applications. However, although growing microgreens is not a novel practice, best methods have not clearly been established. Therefore, this report, which is a deliverable within the GOhydro research project, has undertaken a literature review from 69 different studies in order to collect, harmonize, and synthesize information about existing production practices and current research trends. We have presented the results in three main sections: the first section is on substrate selection and fertilization regimes; the second section reviews the key production parameters (e.g EC, pH, temperature, relative humidity, photoperiod, daily light integral etc.) under which different studies were carried out; and the third section details light quantity and quality, which can influence productivity, nutrient density, colour, taste, and beneficial plant compounds. The results of our literature review, which contains data on 30 different varieties of microgreens, have shown that there is much variety-specific response variation to environmental parameters, fertilization, and different lighting conditions. One key result from our literature review is that an increase in cumulative light integral (DLI * growing period) corresponds to an increase in dry matter production up to 300 mol/m² (R²=0.83), past which gains are minimal or even negative. Another key result is that there are important tradeoffs between secondary metabolite production and growth, which are inversely associated for fertilization, but are positively associated with light. Finally, although there is no single optimal light ratio, there are suggested light regimes that incorporate 5-15% blue light, 85-95% red light, with supplementary far red, UV, or even green/amber light, depending on the desired outputs and species responsivity. This report has synthesized and harmonized the data from all 69 papers, which have been collected and presented in two key tables (Table 8.1 and 8.2 in appendix) that includes information on the species and author, experimental methodology, and key results. This allows for anyone interested in microgreen production to have access to the historical and current practices derived from the published scientific literature.