Arouna, Nafiou (2024) Impact of light quality and cultivation substrate on plant performance and nutritional value of potato (Solanum tuberosum L.) as candidate crop for BLSSs. [Tesi di dottorato]

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Abstract

The future of space exploration will depend on the development of bioregenerative life support systems (BLSSs), to recycle and regenerate resources to sustain human life. BLSSs are closed-loop ecosystems that use bacteria, algae, and plants to recycle waste into resources, such as food, water, and oxygen. Despite the significant progress in space farming with plant modules like Veggie and the Advanced Plant Habitat, demonstrating the feasibility of growing safe, fresh plant food under microgravity, human life support in orbit still heavily relies on Earth-based resupply missions, which are unsustainable for long-term missions to the Moon or Mars. The European Space Agency (ESA) launched the Micro-Ecological Life Support System Alternative (MELiSSA) initiative in the late 1980s to create a bioregenerative life support system that can sustain long-term space missions beyond Low Earth Orbit (LEO). In this system, higher plants show the greatest potential as bioregenerators due to their ability to produce fresh food, recycle carbon dioxide into oxygen through photosynthesis, purify water via transpiration, and mineralize waste. They also provide psychological benefits to astronauts by reducing stress associated with space missions. Crop selection is critical for space farming, requiring plants that meet both human nutritional needs, such as high energy density, essential nutrients, and storage stability, and technical criteria, including rapid growth, compact size, high productivity, and environmental adaptability. Growing plants in space requires systems that efficiently deliver water and nutrients under challenging conditions, such as microgravity and resource limitations. While hydroponics remains the dominant method due to its high efficiency and precise nutrient control, utilizing in-situ resources like regolith in Moon or Mars colonies offers significant advantages by reducing the need to transport large amounts of soil or growth media from Earth and significantly lowering launch costs. Regolith provides a medium for root anchoring and resource delivery but faces challenges such as poor water retention, lack of organic matter, low nutrient bioavailability, and high salinity. To overcome these limitations, regolith must be amended with organic materials like compost to improve its structure, water retention, and nutrient-holding capacity. In addition, artificial lighting is essential in BLSSs to support plant growth in space, where sunlight is limited and irregular. Light drives photosynthesis and regulates processes such as germination, growth, flowering, and plant architecture through photomorphogenesis, controlled by photoreceptors that respond to different light spectra. Within the visible spectrum, red (R) and blue (B) light are most effective for photosynthesis. R light promotes growth and biomass, while B light influences stomatal opening and promotes the biosynthesis of chlorophyll and bioactive compounds. Studies have demonstrated that combining R and B enhance the plant productivity and the nutritional value of edible biomass. Potato (Solanum tuberosum L.) is a candidate crop for space cultivation due to its high nutritional value, offering essential macronutrients like starch, protein, and dietary fiber, while being low in fat. However, it also contains toxic glycoalkaloids (TGA), which must be reduced to ensure food safety for astronauts, as excessive TGA levels can compromise the astronaut’s health. Our earlier studies showed that potato plants adapt well to Martian regolith simulant (MMS-1) amended with green compost (70:30 v/v), enhancing photosynthesis, growth, and yield compared to pure MMS-1. However, the effects of regolith on tuber nutritional value and TGA content remain unknown. Light quality also affects tuber characteristics, with red-blue and white fluorescent light influencing protein content and altering the ratio of glycoalkaloids (α-chaconine to α-solanine) differently between cultivars. Despite these findings, the specific relationship between light spectra ratios and tuber quality remains unclear. Based on these observations, in the first chapter, we investigated the effect of Martian regolith on the plant growth and nutritional quality of potato tubers. We grew potato plants on the MMS-1 Mojave Mars regolith simulant, pure (R100) and mixed with green compost at 30% (R70C30), in a pot in a cold glasshouse with fertigation. For comparison purposes, we also grew plants on fluvial sand, pure (S100) and amended with 30% of compost (S70C30), a volcanic soil (VS), and a red soil (RS). The main results show that plant growth and tuber fresh weight were greater in VS, R70C30, S70C30 and S100, while they were reduced on R100. Referring the tuber quality, the total protein content was higher in tubers of potatoes grown on amended substrates; instead, the starch content decreased with the addition of compost. The content of total glycoalkaloids (as the sum of α-solanine and α-chaconine) was lower in all substrates except in VS and R100, where the highest values were recorded. Specifically, tubers from R70C30 showed a 42% decrease in total glycoalkaloids (TGA) compared to those from R100. Potato tubers grown in regolith-based substrates showed higher levels of nutrients such as potassium, magnesium, and iron, which are essential for human health, especially in space environments where dietary supplementation is critical. In the second chapter, we investigated the influence of genetic material and light spectrum on the performance of two potato cultivars, ‘Colomba’ and ‘Libra,’ grown in a greenhouse. The effects of 100% natural light (CNT) and two lighting treatments, in which 30% of solar radiation was replaced by red and blue LED light, RB 1:1 and RB 2:1, were evaluated on plant growth, gas exchange, and tuber yield and quality. In CNT plants, the tuber yield was higher in ‘Libra’ compared to ‘Colomba.’ However, under RB 2:1 lighting, tuber yield in ‘Colomba’ increased by 21%, while in ‘Libra,’ it decreased by 19%. Supplemental lighting did not alter the mineral content of tubers, except for zinc and copper. However, the tubers of ‘Colomba’ contained higher concentrations of most minerals than ‘Libra,’ probably due to different genetic traits. In both cultivars, the contents of α-solanine, one of the main glycoalkaloids in potatoes, varied significantly under different light treatments. In 'Colomba,' RB 1:1 lighting increased α-solanine by 1.36-fold (from 84.98 to 115.45 mg/kg DW), while RB 2:1 reduced it by 2.24-fold (to 37.88 mg/kg DW). In 'Libra,' both treatments reduced α-solanine, with RB 1:1 achieving a 1.6-fold reduction (from 83.36 to 51.96 mg/kg DW) and RB 2:1 achieving a 2.35-fold reduction (to 35.45 mg/kg DW). ‘Colomba’ prioritised the accumulation of free amino acids, GABA, and polyphenols, enhancing its stress response and antioxidant capacity, and adapted well to variable light conditions, with significant increases in tuber yield under LED light treatments. However, it experiences a pronounced pigment degradation during senescence. Differently, ‘Libra’ focused on the synthesis of carbohydrates; essential amino acid content was lower compared to ‘Colomba,’ but it maintained better pigment retention during senescence. In the last experiment (chapter 3), we investigated the effect of supplemental red-blue LED lighting on plant growth, yield, mineral elements, and metabolite accumulation in tubers of potatoes grown in a vertical system in a greenhouse compared to only sunlight as a control. The system was designed to simulate a two-layer growing system, in which the upper level received only natural light, while the lower level, shaded by a fully opaque shelf, was equipped with an LED panel to supplement sunlight up to 300 µmol m-1s-1. Despite the lower daily light integral (DLI), plants under LED treatment demonstrated significantly higher photosynthetic activity and a slower decline in chlorophyll levels over time, which contributed to a higher tuber yield (+29%) compared to the control. Under LED lighting, plants emphasised their primary metabolism pathways, accumulating nitrogen and essential amino acids, polyamines, and sugars, while simultaneously reducing the presence of toxic compounds like glycoalkaloids. This resulted in an increased stress response and improved tuber quality. In all potato experiments, plants successfully completed the tuber-to-tuber cycle in pots and produced healthy tubers. The compost amendment coupled with fertigation improved the fertility of regolith-based substrate, enhancing the plant performance and tuber nutritional quality. However, additional studies should investigate the long-term effects of repeated compost amendments on regolith-based substrate fertility and crop productivity. Red-blue LED light promotes not only growth but also metabolic pathways that enhance tuber resilience and nutritional and nutraceutical values, accumulating health-promoting compounds, such as polyamines. These findings offer valuable insights into optimising crop production and food quality in space, where fresh foods play a critical role as countermeasures against degenerative diseases caused by space factors. However, future research should explore other wavelengths, such as green, beyond red and blue, to optimize plant growth and health-promoting metabolite production.

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