Influence of environmental and nutritional factors on the development of lettuce (Lactuca sativa L.) microgreens grown in a hydroponic system: A review

Lettuce microgreens are one of the most popular vegetables due to them being perceived as a "healthy food", with high concentrations of nutrients, beneficial vitamins, and minerals. With a short vegetation period, they can be cultivated with minimum investment, and they are increasingly accepted by consumers, as they are healthy and easy to prepare. Lettuce has high ecological plasticity, but, despite this, its phenotypic expression, morphology, physiology, and anatomy are significantly influenced by environmental conditions. Lettuce microgreens contain higher quantities of phytonutrients and minerals and lower quantities of nitrates at the early stage of development than at the completely developed stage. The environmental conditions that influence the development of lettuce microgreens (and their quality) in a hydroponic system are as follows (average ideal values): light (400 W), photoperiodicity (12 h), light intensity (400 mu mol m-2 s-'), colour spectrum (440-460 nm), temperature (20 +/- 2 degrees C), and humidity (80 +/- 5 %). The nutritional solution in a hydroponic system must be carefully monitored, by checking certain essential parameters such as the following (average ideal values): pH (6.3 +/- 0.4), electrical conductivity (1.8 +/- 0.2 mS), dissolved oxygen (6 mg L-'), and temperature (18 +/- 2 degrees C). The analysis of expert literature reveals that there is a need to establish certain protocols for cultivating microgreens in hydroponic systems, to minimize the factors that can negatively influence the plants, in order to obtain higher concentrations of active substances.