IntroductionLong-duration missions in outer Space will require technologies to regenerate environmental resources such as air and water and to produce food while recycling consumables and waste. Plants are considered the most promising biological regenerators to accomplish these functions, due to their complementary relationship with humans. Plant cultivation for Space starts with small plant growth units to produce fresh food to supplement stowed food for astronauts’ onboard spacecrafts and orbital platforms. The choice of crops must be based on limiting factors such as time, energy, and volume. Consequently, small, fast-growing crops are needed to grow in microgravity and to provide astronauts with fresh food rich in functional compounds. Microgreens are functional food crops recently valued for their color and flavor enhancing properties, their rich phytonutrient content and short production cycle. Candidate species of microgreens to be harvested and eaten fresh by crew members, belong to the families Brassicaceae, Asteraceae, Chenopodiaceae, Lamiaceae, Apiaceae, Amarillydaceae, Amaranthaceae, and Cucurbitaceae.MethodsIn this study we developed and applied an algorithm to objectively compare numerous genotypes of microgreens intending to select those with the best productivity and phytonutrient profile for cultivation in Space. The selection process consisted of two subsequent phases. The first selection was based on literature data including 39 genotypes and 25 parameters related to growth, phytonutrients (e.g., tocopherol, phylloquinone, ascorbic acid, polyphenols, lutein, carotenoids, violaxanthin), and mineral elements. Parameters were implemented in a mathematical model with prioritization criteria to generate a ranking list of microgreens. The second phase was based on germination and cultivation tests specifically designed for this study and performed on the six top species resulting from the first ranking list. For the second selection, experimental data on phytonutrients were expressed as metabolite production per day per square meter.Results and discussionIn the final ranking list radish and savoy cabbage resulted with the highest scores based on their productivity and phytonutrient profile. Overall, the algorithm with prioritization criteria allowed us to objectively compare candidate species and obtain a ranking list based on the combination of numerous parameters measured in the different species. This method can be also adapted to new species, parameters, or re-prioritizing the parameters for specific selection purposes.
<p>For Space exploration, the realization of long-term manned missions requires the possibility to grow plants in extra-terrestrial environments. Indeed, life support in Space will be based on the <em>in situ</em> regeneration of resources (e.g. air, water and food) needed by the crew that can be achieved in plant-based closed artificial ecosystems. At the same time the cultivation of edible plants can be useful to integrate astronauts&#8217; diet with fresh food directly produced onboard of Space platforms.</p><p>In this context the production of microgreens is gaining popularity as easy &#8216;vegetal systems&#8217; that can be grown in a few days, in small volume, providing high nutritional values.</p><p>&#160;However, one of the main constraints for the <em>in-orbit</em> production of fresh food of vegetable origin is the establishment of scientific requirements for a flight apparatus dedicated to the production of such species.</p><p>In this study we used a multidisciplinary approach to understand the effects of the environmental factors on morpho-functional and biochemical aspects of different species of microgreens.</p><p>To do so, we set-up various growth chamber experiments to test different type of substrate, nutritional solutions, light intensities and VPDs (vapour pressure deficits) on <em>Brassica oleracea </em>var. sabauda cv. Vertus and <em>Raphanus raphanistrum </em>subsp. sativus cv. Saxa microgreens. In additional experiments, we evaluated the effect of different light qualities (red, blue and optimum spectrum) on the biometric, qualitative and anatomical parameters of <em>Petroselinum crispum.</em></p><p>More specifically, once obtained the optimum light spectrum, we tested two type of substrates (cellulose sponge and coconut fiber) and two nutritional solutions (quarter strength throughout the cycle vs. half strength for the first half of the cycle followed by osmotic water during the second half). Then, using the quarter strength nutrient solution throughout the cycle and the coconut fiber substrate, we tested two different light intensities of an optimum light spectrum (300 &#181;mol photons m<sup>-2</sup>s<sup>-1</sup> vs. 150 &#181;mol photons m<sup>-2</sup>s<sup>-1</sup>) and two different VPD levels (low VPD of 0.3 KPa and high VPD of 1.2 kPa).</p><p>To understand the best combination of environmental factors on microgreens growth in small controlled artificial systems, we compared the biomass production, morphological traits, visual quality parameters (through the leaf colorimetry coordinates) and biochemical traits including chlorophylls, anthocyanins, ascorbic acids, and soluble sugars content. Microgreens were then collected and subjected to the preparation for microscopy analyses to detect possible environmental factor-induced modifications to the anatomical structure.</p><p>The overall analysis showed that the microgreens-response is strictly influenced by environmental factors. Results suggested that the possible occurrence of positive outcomes (increments in antioxidant and biomass production) in microgreens can be severely influenced by environmental conditions: such a phenomenon should be taken into account in the design of plant-based modules for crop production in Space.The outcomes of this study will also be helpful to optimize microgreens production in controlled environment agriculture systems on Earth.</p><p>&#160;</p>
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