Red and red-far red combination resulted in the highest stem elongation but with stem fragility. When Lactucasativa of variety red curly lettuce was grown under different light spectrum, it was found that anthocyanin synthesis, protein content and phenylalanine ammonia-lyase enzyme activity were highest in combined radiation of blue and red-light treatment . In another study, where redand green basil microgreens were grown with blue and redLED, it was found that growth of microgreens was enhanced with predominantly blue illumination showing larger cotyledon area and higher fresh mass,enhanced chlorophyll a, and anthocyanin pigments contents. Stimulation of phenolic synthesis and free radical scavenging activity were improved by predominantlyred light in the green cultivar and blue light in the red cultivar,which indicates that LED light has an influence on the colour of the leaf.
Lettuce crops grown with red and blue LED lighting used50% less energy per unit dry biomass accumulated than under traditional light sources, which indicates that the significant reduction in energy consumption for plant-growth by using LED than traditional light sources . In an experiment on the indoor cultivation of basil and strawberry, it was found that the plants expressed increased biomass, fruit yield, antioxidant content and reduced nitrate content when treated with LED with highest energy use efficiency than traditional fluorescent lamps and spectral red: blue ratio of 0.7 was essential for proper plant growth with improved nutraceutical properties .From the literature review, it is found that when the microgreens are treated with red and blue LED combination, they showed highest photosynthetic activity but the exact ratio of blue and red-light combination at which the results are obtained are not evident.
Thus, it is essential to consider the production of microgreens at different ratios of red and blue combination under different light intensities. The environmental factors such as carbon dioxide, temperature and humidity play a crucial role in the growth and development of plants throughout their lifecycles. Their levels are specific and critical for different stages of plant growth. In an experiment conducted to determine the impact of free air carbon dioxide enrichment on development and progress in cotton , the results were found that carbon dioxide enrichment resulted in significant increase in photosynthesis and biomass of leaves, stems and roots, reduced evapotranspiration,changes in root morphology and increased soil respiration .When experiments have been conducted to determine the effects of atmospheric humidity on sugar beet, wheat and kale, the results were found that the plants showed increases in growth with increased atmospheric humidity.
The leaf area of sugar beet and kale plants was also increased with increased humidity due to the increase in number of cells . Each plant species will require optimum temperature to show maximum rate of plant growth and development. Both the atmospheric temperature and the soil temperature will influence different stages of plant development such as vegetative and reproductive stage. For instance, the vegetative growth of the plant requires comparatively higher optimum temperature than reproductive stage .There are many literatures pertaining to the effect of climatic factors such as carbon dioxide, temperature and humidity on plant growth and development both indoor and outdoor conditions but there are no such studies on microgreens. From the previous related works, it is important to consider their impact of climatic factors and their regulation to produce maximum growth and yield of microgreens which are grown indoor. Plants require both macro- and micro-nutrients for their growth and development.Vertical farming technology involves the soilless culturing of plants with application of nutrient solution, especially, inorganic fertilizer to the plants through soilless medium. The Hoagland solution developed by Hoagland and Arnon with definite composition is a hydroponic nutrient solution,which provides every essential nutrients for plant growth .
The composition of Hoagland solution is given in Table 2 .The commercial crops grown under soilless culture yield good quality produce when supplemented with the nutrient solution of specific composition based on the crop, the growth stage, the climatic conditions, the substrate, or hydroponic system used. Thus, the standard formula is not always applicable to all the crops . When experimenting the plants with continuous flow of nutrient solution, it was found that there is a certain minimum concentration below which uptake is no longer possible at the required rate. At the other extreme of concentration, luxury consumption may lead to internal toxicity . Thus, it is not necessary to apply nutrients unless required by the crop for its growth.