Role and Impact of Microgravity on Plant Growth, Lignin Content, and Plant Nutrition

Abstract

Space exploration and space-related research have expanded significantly in recent years. However, one of the most prevalent complaints from astronauts is the lack of fresh food. Thus, it is important to understand how microgravity affects plant growth and structure to not only fulfill the wants of astronauts but to also provide them with a continuous and sustainable source of food and vitamins. Despite the growing interest, there is limited research analyzing previous experimental studies on plants grown under microgravity, so a collective overview is necessary for understanding current research in plant growth and lignin and soluble sugar content for plants grown in space. This paper aims to review the role of microgravity on plant growth, lignin content, and nutrition through a comprehensive search for research articles and an analysis of their methodology and experimental data. Results found that plant growth, in terms of stem length, tended to decrease under microgravity conditions; however, results of fresh weight contradicted this general trend. Lignin content was reduced under microgravity: flight groups had around 25-100 μg less lignin per stem of each plant than control groups, which supported previous research. There was little research regarding soluble sugar content, but with the data present, results showed no general pattern. Therefore, more research should be done regarding microgravity’s effect on plant growth and soluble sugar. Moreover, research could be done on protein content, water and nutrient uptake, and others to further improve astronaut health and nutrition through plants in space. 

Keywords: space, microgravity, plant growth, lignin content, soluble sugar, nutrition, flight, clinorotation, astronaut

Introduction 

As endless advancements are being achieved on Earth, many have turned toward understanding the mysteries of outer space, one aspect being space exploration, which has recently been a booming field of study, with travel to Mars and life on other planets being some of the most prevalent goals. However, space travel has many negative side effects on humans: space radiation from electromagnetic waves can cause cancer and degenerative diseases; bone and muscle loss due to space flight conditions; and feelings of isolation and confinement^1,2. Furthermore, one of the lesser-known issues of space exploration is the lack of fresh food, which is often the biggest complaint of many astronauts³. Not only would fresh food provide a change from the preserved foods astronauts are used to, but it would also keep astronauts healthy by supplementing vitamins that may no longer be in prepackaged food due to breaking down over time⁴.

Growing horticultural crops on spacecraft for food seems like a simple solution; however, the microgravity environment presents several challenges that differ significantly from Earth cultivation. With the absence of gravity, the growth and structure of plants become disorientated with no direct direction to grow in. On earth, gravitropism, the directional control of plant organ growth in response to gravity, causes plants to grow opposite of gravity and roots to grow in the direction of gravity⁵. However, without gravitropism, plant roots may grow in random directions, resulting in insufficient nutrient and water absorption. Additionally, microgravity affects the movement of fluids and hormone accumulation, potentially causing irregular fluid distribution and changes in growth patterns⁶. As a result, numerous scientists are researching and experimenting with the effects of microgravity on plants grown in space.

One key aspect being studied is how plant structure and internal components are affected by microgravity. This encompasses the cell wall found in vascular plants, which protects the cell membrane by acting like a barrier and provides structural support and shape. The cell wall is composed primarily of polysaccharides such as cellulose, hemicellulose, pectin, and lignin, but also minerals like calcium and magnesium, each with specific components and properties in structuring and protecting plants⁷. 

Focusing on the structural polymer lignin, lignin is mainly deposited in the secondary cell wall of plants in tissues like the xylem and sclerenchyma, which conduct water and provide structural support, respectively⁸. Lignin’s polymer structure comprises of three main monolignol units: p-coumaryl, coniferyl, and sinapyl alcohols, which are polymerized through various chemical bonds⁹. Furthermore, lignin cross-links with cellulose and hemicellulose, creating a strong network that resists bending and compression¹⁰. These aspects of lignin lead to the overall strength and rigidity present in plant cell walls. Hence, lignin has many beneficial purposes on earth. For example, lignin protects plants by making them tough, allows the transportation of water and nutrients, enables upward growth, and makes plants resistant to chemical manipulation¹¹. Lignin’s structural property is most akin to bones in humans, which, as mentioned before, gradually lose density in space due to lower physical demands. Consequently, the formation of the lignin was seen to be reduced when grown from seeds in a microgravity environment. Although it seems harmful, the lack of lignin could potentially be advantageous for astronauts. Protection from predators and toughness is unnecessary for plants in space, and less lignin would allow more energy to be extracted, possibly providing better nutrient absorption and oxygen for astronauts⁴. 

Currently, despite the positive possibilities, this concept is still relatively unknown. Few studies have investigated the effects of microgravity on plants, and even fewer have specifically focused on the lignin content in plants. This may be because of the difficulty of performing such experiments and the lack of access to such equipment. Only a few researchers work at space centers or stations, and planning and conducting experiments in space can be costly and time-consuming. In addition, there are few to no comprehensive studies about microgravity’s effect on lignin and how plant rigidity and nutrition values are affected. A better understanding of such topics could be groundbreaking in future space travel. With the ability to grow and eat plants in space, missions in space could be longer and keep astronauts healthier. This study aims to understand how microgravity affects the structural polymer lignin in plants and its overall impact on plant rigidity and nutritious value. The study poses as a more concise understanding and analysis of lignin and how it is affected by microgravity. It also serves as a summary or overview of other findings regarding plant lignin in microgravity. This study also examined current solutions and methodologies for growing fresh food in space. 

This study conducted a literature review using the two sources, ScienceDirect and Google Scholar. Articles were selected based on relevance to plant lignin and microgravity, and any important findings or data were included. Limitations of this study include no access to the facilities, equipment, or budget necessary for growing plants in space (or a microgravity-like environment) and analyzing lignin content, which prevents the experimental possibilities. 

To summarize, fresh food is one of the issues regarding space travel that scientists and astronauts are trying to better understand. This is because the microgravity environment prevents regular plant growth, which can cause an irregular distribution of nutrients and hormones, causing altered growth patterns. To better understand plants in microgravity, lignin, a structural polymer in plant cell walls that provides rigidity and protection, can be studied. Previous research has shown that lignin content is reduced in microgravity, which seems harmful but can actually be beneficial by enhancing the nutrients and energy astronauts can consume. While current research is limited, this study aims to provide a comprehensive understanding of how microgravity affects lignin, plant growth, and nutrition. 

Results

Plant Growth Results 

The stem lengths in centimeters of different types of plants grown under microgravity and 1g were compared in a bar graph (see Figure 1). The x-axis includes the kinds of plants grown with a flight and control group for each type of plant, and the y-axis is the stem lengths (cm). Plant height or length is a general indicator of how well a plant grows, thus, these values were compared to better understand the growth of plants under microgravity. An overall pattern showed that control groups generally had a higher stem length than flight groups.

As shown in Figure 1, plant types mung bean (a), mung bean (b), and oat seedlings had a greater difference between the flight and control groups of around 2 cm. Mung bean (a) had an average stem length of 12.26 cm for the flight group and 14.67 cm for the control group; the data of mung bean (a) did not include standard error (SE). According to Cowles et al. (1994), it had a p-value ≤ 0.001 and the data was significant¹². Mung bean (b) had an average stem length of 10.09 ± 2.15 cm for the flight group and 12.91 ± 1.62 cm for the control group and had a calculated p-value ≤ 0.05, which was significant. Oat seedling flight and control group values were 14.47 ± 2.44 cm and 16.23 ± 3.59 cm, respectively, and its calculated p-value signified the data was not significant. Additionally, it should be noted that mung bean (a) and mung bean (b) were data from two separate experiments in two different research articles that experimented with the same type of plant. 

Dwarf wheat and pine seedlings had a smaller difference of around 1 cm, while white spruce seedlings and thale cress had no difference or a very slight difference of less than 0.5 cm. The flight and control group values for dwarf wheat were 14.23 ± 1.25 cm and 15.133 ± 1.61 cm, and 5.85 ± 0.84 cm and 6.79 ± 0.80 cm for pine seedlings, respectively. Their respective p-values were > 0.05 and ≤ 0.001, therefore, the data for dwarf wheat was not significant, while pine seedlings were significant. The flight and control group values for white spruce seedlings were 3.22 ± 0.27 cm and 3.14 ± 0.29 cm, respectively, and 1.34 cm and 1.01 cm for thale cress. White spruce seedlings had a p-value ≤ 0.05, which was significant, while thale cress, similar to mung bean (a), had no SE, and its p-value was not mentioned; hence, its p-value was inconclusive. 

Overall, a general pattern can be seen among the flight and control groups where the stem lengths of the control groups have a higher value than the flight group. This could be attributed to the fact that microgravity causes stunted growth in plants. However, white spruce seedlings and thale cress contradicted this general pattern, and both showed the flight group having slightly higher stem length measurements than the control group. The two plants showed the least growth, which could relate to the contradiction of the general pattern.

White spruce seedlings, mung bean (a), and pine seedlings had p-values ≤ 0.05 and were significant. Dwarf wheat, mung bean (b), and oat seedlings each had a p-value > 0.05 and were not significant (NS). The p-value for thale cress could not be calculated and was thus inconclusive. 

Note that mung bean (a) and mung bean (b) were two different experiments, and mung bean (a) and thale cress have no standard error, not a SE = 0 

Continuing with the physical growth of plants, the next bar graph compares the fresh weight in grams of different types of plants grown under microgravity and 1g (see Figure 1). The x-axis is the types of plants grown with a flight and control group for each type of plant, and the y-axis is the fresh weight (g). Similar to plant length, the weight or biomass of a plant is a common indicator of how well a plant grows; thus, these values were compared to better understand how plants grow when affected by microgravity. In general, no overarching pattern was seen. 

As shown in Figure 2, all plant types had a small difference of less than 1 g between the flight and control groups. Tobacco had a fresh weight of 4.8 ± 0.25 g and 4 ± 0.2 g for the flight and control group, respectively. However, the calculated p-value was ≤ 0.01, which was significant. The flight and control group values for soy seedlings were 15.8 g and 16.1 g, and its p-value was inconclusive. The respective values of common wheat on Salyut-7 for the flight and control groups were 8.97 ± 2.8 g and 9.24 ± 3.05 g, and 5.64 ± 1.3 g and 5.83 ± 1.83 on Mir. Both common wheats had a p-value > 0.05, hence, their data was not significant. 

The differences between the flight and control groups were generally small, under 1 g. The fresh weight data showed no clear pattern, and microgravity did not significantly affect plant weight. Thus, the two plant growth data contradict this since the fresh weights show that microgravity does not affect plant growth, while the stem length data shows it does. 

Tobacco had a p-value ≤ 0.05 and was significant. Common wheat (Salyut-7) and common wheat (Mir) had a p-value > 0.05 and were NS. The p-value for soy seedlings could not be calculated and was thus inconclusive. 

Note that common wheat (Salyut-7) and common wheat (Mir) were different experiments from the same research paper. 

Lignin Content Results 

The lignin content in micrograms (μg) per stem of different types of plants grown under microgravity and 1g were compared in a bar graph (see Figure 3). The x-axis includes the kinds of plants grown with a flight and control group for each type of plant, and the y-axis is the lignin content (μg per stem). Lignin is located in the cell wall and is a measure of plant rigidity; it correlates to plant nutrition since less lignin would allow plants to be more easily digested and more energy to be obtained. Thus, lignin content values were compared to better understand the structure of plants under microgravity. An overall pattern showed that control groups generally had a higher lignin content than flight groups. 

As shown in Figure 3, pine seedlings (a) and mung bean (a) had the largest difference between the flight and control groups of greater than 50 μg per stem. Pine seedlings (a) and mung bean (a) had an average lignin content of 668.68 μg and 729.78 μg, and 301.63 μg and 405.96 μg for their flight and control group, respectively; the data did not include SE. According to Cowles et al. (1994)¹², pine seedlings had a p-value ≤ 0.01, and mung bean (a) had a p-value ≤ 0.001, so plant types were statistically significant.  

Pine seedlings (b) and mung bean (b) had a smaller difference of around 25 μg per stem. Pine seedlings (b) and mung bean (b) had an average lignin content of 577.35 μg and 603.59 μg, and 100.39 μg and 122.47 μg for their flight and control group, respectively; the data also did not include SE. According to Cowles et al. (1984), pine seedlings (b) p-value ≤ 0.001 and mung bean (b) p-value ≤ 0.05; thus, both plant types were significantly significant¹³. It should also be noted that plants with (a) and (b) were from two separate research articles that experimented with the same type of plant. 

The remaining plant types, oat seedlings, rice, and common wheat, had a smaller difference between the flight and control groups, less than 10 μg per stem. Oat seedlings had an average lignin content of 32.25 μg and 32.33 μg for their flight and control groups; the data did not include SE. However, according to Cowles et al. (1984), its p-value ≤ 0.01, so the data was significant¹³. The respective values for the flight and control group of rice were 46.7 ± 0.7 μg and 53.7 ± 1.5 μg, and it had a calculated p-value ≤ 0.001, so it was significant. Common wheat had a flight value of 69 μg and a control value of 62 μg; its SE was not included in the data. According to Stutte et al. (2006), the p-value > 0.05, and the data was not significant¹⁴. 

Overall, there was a general trend that the lignin content for the control group was greater than the flight group. This was likely because microgravity reduces lignin content, which causes plants to be less rigid and protected in space. However, common wheat contradicted this general pattern while oat seedlings and rice had around the same values between its flight and control groups with no distinct difference.

Pine seedlings (a), mung bean (a), pine seedlings (b), mung bean (b), oat seedlings, and rice had a p-value ≤ 0.05 and were significant. Common wheat had a p-value > 0.05 and was NS. 

Note that pine seedlings (a) and pine seedlings (b) were from different experiments from separate research articles, as well as mung bean (a) and mung bean (b).  

Soluble Sugar Content Results

The soluble sugar content in micrograms (μg) per shoot of different types of plants grown under microgravity and 1g were compared in a bar graph (see Figure 4). The x-axis includes the kinds of plants grown with a flight and control group for each type of plant, and the y-axis is the soluble sugar content (μg per shoot). Because lignin content tends to decrease under microgravity, nutrition was expected to increase. Thus, the soluble sugar content of different plants was compared to better understand plant nutrition under microgravity. Generally speaking, no noticeable trend was seen between the flight or control groups. 

As shown in Figure 4, only tobacco had a big difference of 100 μg per shoot between its flight and control group. Tobacco had a soluble sugar content of 600 μg and 500 μg for its flight and control group, respectively; the data did not include SE. According to Soleimani et al. (2019), the p-value ≤ 0.05 and was significant¹⁵. On the other hand, rice and common wheat had little to no difference between their flight and control groups. The respective flight and control group values for rice were 112.6 ± 2 μg and 104.3 ± 3.9 μg, and 320.1 μg and 344.9 μg for common wheat. Rice had a calculated p-value ≤ 0.05, so it was significant. However, the data for common wheat did not have SE. According to Stutte et al. (2006), the p-value > 0.05 and was not significant¹⁴. 

Regarding the trends, the soluble sugar content for the tobacco flight group was much greater than the control group. However, the control group for common wheat was slightly greater than the flight group, while the two rice values were similar, with no clear trend. The data from the different types of plants contradict, thus, there was no clear pattern or conclusion for the soluble sugar content of plants grown under microgravity. 

Tobacco and rice had a p-value ≤ 0.05 and were significant. Common wheat had a p-value > 0.05 and was NS. 

Discussion

The goal of this study was to better understand the effect of microgravity on plants. Data from research articles regarding plant growth, lignin content, and nutrition were reviewed and analyzed to compare results from microgravity environments to 1g control conditions on Earth. Previous experiments often found reduced lignin content under microgravity, which could potentially increase the energy and nutrition extracted from the plants when eaten. Thus, this study aimed to confirm previous conclusions and provide new insights regarding plants grown under microgravity. 

The results of this study found that plant growth, in terms of stem length and fresh weight, decreased under microgravity, however, only some types of plants showed significant results. Furthermore, lignin content tended to decline in microgravity, and most plants showed significant results, meanwhile, the soluble sugar content had contradicting results, indicating that further research needs to be done to arrive at a conclusion. 

Plant Growth 

Stem length is a measure of plant growth, so it was evaluated to better understand how microgravity affects plant growth. As shown in Figure 1, the flight group of five of the seven plant types showed shorter stem lengths than the control group (see Figure 1). Additionally, two of the five plants were extremely statistically significant, meaning that the observed result was unlikely to have occurred by chance. However, the other three plants were not significant, while two plants, white spruce seedlings and thale cress, contradicted this general trend and had a higher value for their flight group than their control. In general, these two plants had the smallest flight and control group values, which may have caused the discrepancy in the overall trend. The smaller values for white spruce seedlings could be caused by their slower growth rate since white spruce is a coniferous tree while the other plants are annual plants that grow each year¹⁶. Although thale cress is a fast-growing plant, it was grown for the shortest amount of time, which may have caused the low stem lengths (see Table 1). Therefore, future experiments could grow white spruce seedlings and thale cress longer to see if they support this contradiction from the general trend. Thus, there was a general trend that the stem length values for the flight groups were shorter than the control groups, and it could be concluded that microgravity causes stunted growth in plants.

The shortening of plant stem length in microgravity conditions can be attributed to several physiological and biochemical changes, including reduced lignin deposition and disrupted water-soluble sugar metabolism. Reduced lignin caused by microgravity, as seen in Figure 3, can cause stunted growth and reduced plant length. Lignin plays a role in structural support; thus, lignin deficiency may be associated with dwarfism, breakdown of vascular tissue, and disruption of microstructure¹⁷. However, the relationship between lignin content and plant size is relatively complex and poorly understood. Repression of enzymes CAD and CCR in the lignin biosynthetic pathway, causing a decreased lignin content and change in lignin composition, may result in dwarfism and severely reduced plant growth¹⁸. Moreover, when subunits MED5a and MED5b of the transcriptional co-regulatory protein complex Mediator are disrupted, stunted growth and lignin deficiency seemed to be rescued, despite lacking essential subunits of lignin like guaiacyl and syringyl¹⁹. This suggests that the transcriptional process and signaling pathways responding to cell wall defects may play a vital role in stunted growth caused by lignin deficiency¹⁰. The regularity of cellulose fibrils within lignin mutants may also be an underlying contributor to shortened stem lengths under microgravity. Lowered lignin content disrupts the natural order of cellulose fibrils in cell walls, possibly depositing cellulose in ways that can easily be broken down by enzymes involved in wall restructuring¹⁷. Although the plant can still produce cellulose regularly, most of it may be digested and recycled, causing stunted plant growth. 

Water-soluble sugar concentrations under microgravity may also stunt plant growth. Despite Figure 4 showing no clear pattern of soluble sugar content in flight groups compared to control groups, the tobacco flight group had a greater amount of soluble sugar content compared to its control group¹⁵. Previous research has also found a substantially greater concentration of soluble sugars, glucose, fructose, and sucrose in the stems of flight samples than those in ground-based samples²⁰. This higher concentration of sugar can significantly stunt plant growth. Because soluble sugars are required for the growth and regulation of osmotic homeostasis in root cells, excess sugars can downregulate genes necessary for plant growth, disrupt water uptake, and cause nutrient deficiencies²¹. Moreover, excess sugars can interact with growth-regulating hormones, potentially leading to imbalances that further affect growth patterns. Research has shown that high sugar concentrations can reduce auxin distribution and efficiency in plant tissues, impairing the regulatory growth and development function of auxin and reducing plant growth²². Therefore, reduced lignin content and potential increases in water-soluble sugars may cause reduced stem lengths in plants grown under microgravity.  

Fresh weight is also an indicator of plant growth. In Figure 2, the weights of different plants grown under microgravity and 1g showed contradicting results with no overall pattern (see Figure 2). Three of the four plants had a higher fresh weight value for their control group than their flight group, but there was a small difference between the two groups. Furthermore, the only result that did not follow this general pattern was significant, so microgravity may not affect the fresh weight of plants. However, many grasses like bamboo or wheat exhibit rapid growth in length with relatively lightweight and hollow stems²³. This structural aspect minimizes the weight increase but allows the plant to grow linearly, which could explain the small difference between flight and control values since two of the plant types were common wheat. 

Nevertheless, regardless, both stem length and fresh weight are measures of plant growth and generally correlate with each other; if stem length increases, so does weight. The result of stunted stem lengths and no clear pattern in fresh weight data show that a clear conclusion cannot be made about plant growth. However, this discrepancy can be caused by altered resource allocation in plants. In plant tissues, the allocation of auxin, a plant hormone crucial in shaping several aspects of plant development, is regulated by gravity²⁴. Under microgravity, it is reported that auxin distribution and transport are altered in several plant species²⁵. This change can influence cell elongation and lead to shorter stems but increased biomass accumulation in other parts of the plants such as leaves or roots, accounting for the discrepancy in fresh weight data. Plants may also shift carbon allocation from stem elongation as it is not as necessary for structural components like lignin, which is reduced under microgravity and allows biomass to accumulate in other plant organs²⁶. Some studies have also found that cell wall remodeling occurs throughout plants, which can affect plant morphology of leaves, roots, and hypocotyls in ways that do not correlate with overall biomass²⁷. Certain plants grown in space had higher elasticity moduli and viscosity coefficients, which resulted in stiffer cell walls and increased resistance to deformation, suggesting that cell walls may have a lower capacity to expand²⁸. Thus, reduced cell wall expansion stunts plant growth, with minimal effects on fresh weight, showing that plant size is not directly related to biomass under microgravity conditions. 

Furthermore, constituent differences between plant types could result in the discrepancy. Certain plants grown for stem-length experiments including white spruce, pine, mung beans, and oats, generally contain higher amounts of lignin. These woody and fibrous plants rely more on rigid cell walls for support and growth. Under microgravity, lignin production is often reduced, making these plants structurally weaker with shorter stem lengths. While the plants studied for fresh weight, specifically tobacco, rice, and common wheat, tend to have lower lignin content⁴². These soft-stemmed plants depend more on cell expansion and water content for growth rather than lignin. Thus, a reduction in lignin and differences in growth habits may not have a major impact on their overall fresh weight, creating the difference between the plant growth factors. 

Ultimately, although the data of this paper did not find clear results, other research has found a pattern amongst fresh weight. One paper by Rivera et al. (2006) that studied the impact of microgravity on rocket seedlings or arugula through clinorotation found that the control group had a 19% higher fresh weight than the simulated experimental group²⁹. Despite this, only a few other papers focused on how fresh weight is affected by microgravity, so further research still needs to be done to understand how microgravity affects plant growth. 

Lignin Content

Lignin content relates to plant rigidity and correlates to plant nutrition since less lignin allows plants to be more easily digested and more energy to be acquired when eaten, which may be useful for aiding astronauts. Therefore, lignin content values were analyzed to evaluate plant structure under microgravity. As shown in Figure 3, six of the seven plant types showed a general pattern where lignin content in flight groups was less than in control groups (see Figure 3). Moreover, three were extremely statistically significant, two were highly statistically significant, and one was significant, meaning that the observed result was unlikely to have occurred by chance. Thus, microgravity likely causes reduced lignin in plants. Other papers have found results supporting the idea that microgravity causes reduced lignin content in plants, similar to how space also causes a loss of bone density in astronauts³⁰. However, this pattern did not apply to one plant, common wheat. This may be attributed to the fact that the overall values for common wheat are small since the plant is not very rigid and its structure and function do not require much lignin³¹. Also, although oat seedlings and rice results supported the overall pattern, they had small data values. Like common wheat, this may result from their plant structure not requiring much lignin. There was also an inconsistency between the two mung bean plants. A large difference existed between their values even though the same plant was grown, and they had similar methodologies (see Table 3). However, their difference may be caused by different light intensities during plant growth or varying ways of measuring lignin content. Despite the variability amongst the different types of plants, there is a general pattern of smaller lignin content for flight groups, resulting in the conclusion that microgravity reduces lignin content. 

This lignin reduction can be caused by changes in PAL activity and reactive oxygen species (ROS) production. Lignin is synthesized through the phenylalanine/tyrosine metabolic pathway where enzymes like phenylalanine ammonia-lyase (PAL) and tyrosine ammonia-lyase (TAL) are polymerized into three main types of lignin monolignols¹⁰. Under microgravity, the gene expression of enzymes like PAL may be downregulated due to lower abiotic stress caused by the absence of gravity in space³². Cowles et al. found that PAL activity in pine seedlings under microgravity reduced, which could cause lignin content to change. However, Wakabayashi et al. and Grudzińska et al. have found PAL activity to increase significantly in white clover sprouts and rice shoots grown in microgravity but not in pine seedlings. At the same time, lignin remained reduced, thus suggesting that PAL activity in microgravity-grown plants is not necessarily linked with the accumulation of lignin^33,34. However, this difference may be attributed to pine seedlings being woody plants while white clover and rice shoots are herbaceous plants³⁵. Woody plants, which require high lignin content for structural support, may lower PAL activity due to reduced stress, causing less lignin. More flexible herbaceous plants rely less heavily on lignin, potentially resulting in plants detecting stress differently; PAL activity may increase to produce secondary metabolites like flavonoids, which can protect against oxidative stress in microgravity and reduce the need for lignin³⁶. 

Altered lignin content can also be attributed to reactive oxygen species (ROS) that are naturally produced byproducts of metabolism during plant photosynthesis and respiration³⁷. Studies have shown that oxidative stress increases in plants under microgravity, likely due to increased ROS production³⁸. Lignin biosynthesis is intricately linked to oxidative stress, and lignin accumulation often occurs in response³⁹. However, microgravity causes ROS-scavenging enzymes that remove harmful ROS to be highly upregulated; this disruption can prevent monolignols from being polymerized efficiently into lignin⁴⁰. Moreover, MAP kinase (MAPK) pathways, which regulate cellular functions like phenylpropanoid metabolism, and WRKY transcription factors involved in stress responses were activated⁴⁰. Increased activity of these stress-signaling pathways may cause fewer monolignols to be available for lignin synthesis, thus altering the lignin content in plants under microgravity. 

Soluble Sugar

Water-soluble sugar content is related to the nutrition and energy obtained when eaten. By understanding microgravity’s effect on soluble sugar content, scientists can better evaluate its impact on food grown in space and its potential implications for astronaut health, which is why soluble sugar content was measured. In Figure 4, the soluble sugar content of the different types of plants showed contradicting results and no overall pattern (see Figure 4). Tobacco and rice plants showed a higher soluble sugar content for the flight group than the control. Moreover, they were statistically significant, so it was unlikely that the observed result occurred by chance. However, rice had a small difference between the two groups. The other plant, common wheat, had a higher control value than flight but was not statistically significant. Ultimately, microgravity could potentially increase the soluble sugar content in plants, however, there was not enough data to support the conclusion. Some research papers have found similar results, though. Specifically, an article by Mortley et al. (2010) that experimented with sweet potatoes grown under microgravity found that the concentration of soluble sugars was substantially greater in the stems of flight samples than in control samples²⁰. This increase in soluble sugar content could be attributed to gravitational stress: the absence of gravity may alter plant metabolism, resulting in an accumulation of soluble sugars⁴¹. 

Moreover, similar to the lignin changes, the higher amounts of soluble sugar in tobacco in the flight conditions can result from increased ROS production. Soluble sugars act as nutrient and metabolite signaling molecules that activate specific transduction pathways, which can modify gene expression and protein complexes⁴². Specific metabolic reactions and regulations directly link soluble sugars with ROS production and with anti-oxidative processes like the oxidative pentose-phosphate (OPP), which contributes to ROS scavenging or detoxifying⁴². Thus, oxidative stress from the accumulation of ROS can lead to more soluble sugar forming as a protective mechanism that counteracts ROS damage. Specifically, increased glucose content can feed the OPP pathway to enhance NADPH production, a major cofactor for ROS scavenging pathways like the ascorbate-glutathione cycle which can detoxify hydrogen peroxide (H₂O₂), an ROS produced as a byproduct of metabolism⁴³. 

Higher soluble sugar content in tobacco can also be caused by altered carbon allocation or distribution of photosynthesis products to different parts of the tobacco plant²⁶. Sugar, a product of photosynthesis and a primary source of carbon for plants, gets allocated to different plant organs such as the leaves, stems, and roots, and various functions like growth and reproduction⁴⁴. However, under microgravity, plant growth as well as cell wall and lignin formation tend to decrease, leading to less carbon being used for structural components and growth. In turn, higher amounts of carbon could remain in the form of soluble sugars. Therefore, this increase in soluble sugar content for tobacco could be a result of higher ROS production and altered carbon location and distribution of photosynthetic plant parts. 

Environmental Variables

While microgravity is a major factor influencing plant growth and development, other environmental conditions should be considered, including light intensity and temperature. In the absence of gravity, plants use light as a primary cue to orient their growth⁴⁵. Thus, in darkness or low-intensity lights, plants may exhibit irregular growth without a sense of direction. Light intensity variations can also alter photosynthetic efficiency and potentially mask microgravity-induced effects. Plants under microgravity exposed to light were found to normalize changed gene expression, maintain normal hormonal levels, and have cell growth and division similar to those under regular gravity conditions⁴⁶. Higher-intensity light could correlate to further benefits that counteract the disruption in plant growth caused by microgravity. However, Figure 3 containing lignin content contradicted this pattern. Rice was grown under darkness and had smaller contents of lignin, while pine seedlings, mung beans, and oat seedlings were grown at a light intensity of around 75 μmols and generally had higher lignin content. However, common wheat, which was grown under a light intensity of around 280 μmols, had a small amount of lignin content. It should be noted, though, that these values cannot be accurately compared as these plants contain different amounts of lignin. Despite this, further research should be done investigating the effects of light intensity in plants in relation to microgravity. 

Temperature may also play a role in affecting plants under microgravity. Kitaya et al. (2001) found that under microgravity, the surface temperature of plant leaves increases while the net photosynthetic rate decreases when compared to 1g conditions, suggesting that excess leaf temperature may disorder leaf physiological processes⁴⁷. Furthermore, low net photosynthetic rates would reduce plant growth at both the vegetative and reproductive stages. In Figure 1, the role of temperature and how it interacts with microgravity to affect plant growth can be seen as the majority of the plants in the flight group had shorter stem lengths than the control groups (see Figure 1). Moreover, in Figure 2, three of the four plants had lower fresh weights in flight groups compared to control, potentially showing the impact of temperature on plant growth in microgravity. However, the difference is minimal, and the fourth plant shows contradicting data, though, this can be attributed to altered resource allocation in plants, cell wall remodeling, etc. Ultimately, air movement, which enhances the heat and gas exchange between plants and the air surrounding them, and thus the growth of healthy plants, should be further studied to understand how temperature is interlinked with microgravity effects⁴⁸. 

Future Implications

Currently, plants being grown under microgravity have not been extensively researched, and some show contradicting results that cannot lead to clear conclusions. Data for stem length and fresh weight, which are indicators of plant growth, have different results, which shows an issue in current research. Previous research on lignin agreed with the results of this study; however, it should be further studied in correlation to plant nutrition or soluble sugar content since there is little research on the soluble sugar content of plants grown under microgravity. Soluble sugars in plants directly influence the nutrition and available energy obtained from plants when consumed. Thus, by better understanding how microgravity affects lignin and plant nutrition or soluble sugar amounts, astronauts could have access to fresh food, which would keep them healthier and provide them with immediate energy. More nutrient-dense crops could also improve taste, which would help maintain astronauts’ appetite and morale, allowing them to stay physically and mentally healthy⁴⁹. Additionally, soluble sugars can increase essential minerals like calcium and iron, which are crucial for astronauts⁵⁰. Microgravity causes bone density loss, so maintaining adequate calcium levels is important for mitigating this effect. Likewise, iron is vital for oxygen transport, which is particularly important for astronauts since oxygen levels are tightly regulated. Therefore, research in the future could focus more on how microgravity’s effect on lignin content alters the number of soluble sugars, which is important for the health of astronauts. 

Beyond lignin and soluble sugar content, further research could be done on how microgravity affects plant protein content and amino acid composition. Proteins and amino acids are crucial for human nutrition and could further benefit the health of astronauts⁵¹. Moreover, the root and nutrient uptake of plants could also be researched. Microgravity disrupts normal root orientation, which may alter water and nutrient absorption, which affects plant health or yield⁵². Hence, future research could focus on how root structure and function adapt to microgravity and how its efficiency is affected. Research into hydroponic or aeroponic systems that could be used for microgravity could improve plant nutrition and astronaut health in space. These systems are soilless plant cultivation techniques that can optimize space, limited resources on spacecraft habitats, and water efficiency. Recent advancements in LED lighting technology and controlled-environment agriculture can be integrated into spacecraft to maximize plant growth under limited conditions^53,54. Future research could further investigate incorporating AI and automation to create monitoring systems for self-sustaining plant growth modules, ensuring astronauts have a reliable food supply and life support system⁵⁵. Current issues in hydroponics and aeroponics should also be addressed such as altered fluid behavior and nutrient delivery due to microgravity, and pathogen vulnerability and inconsistent oxygen supply caused by the closed environment⁵⁶. Ways to counteract or make up for these discrepancies caused by microgravity should be further researched. Other research regarding plants and microgravity that could be done in the future include photosynthesis, hormone regulation, and the overall growth cycle or plant reproduction. 

Ultimately, this study found that microgravity tended to stunt plant growth regarding stem length, while fresh weight was unaffected. Lignin content decreased under microgravity and supported previous research and experiments. Soluble sugar content had contradicting results with no overall pattern. Therefore, further research needs to be done to better understand how microgravity affects plant growth and soluble sugar content. Such knowledge could positively impact astronauts and their health when aboard space missions. Research about the effect of microgravity on plant protein, water and nutrient uptake, reproduction, and others could also be done in the future. By addressing these research areas, scientists can work toward engineering and cultivating plants that are viable for supporting astronaut health and nutrition in space. 

Methods

A systematic review was conducted following a PRISMA flow diagram. The databases ScienceDirect and Google Scholar were identified to gather sources for literature review and data analysis. Starting with ScienceDirect, a comprehensive search was done by entering the keywords “microgravity” and “lignin” in the search bar (the exact search was “‘microgravity’ ‘lignin’”). Thus, papers not including those words were excluded. The results were then filtered by article type for only research articles, resulting in 66 results from 2024 to 1983. The articles were reviewed for relevance; the highlights and abstracts were screened for keywords like microgravity and lignin. Papers were omitted if they had little to no correlation with plants grown in microgravity or lignin content. If no relevant data related to lignin, plant growth, or nutrition was present, the paper was also excluded. After this process, a total of 9 research articles from ScienceDirect remained. 

The same search was conducted on Google Scholar with the same keywords and exact search. Google Scholar could only filter review articles, so all default search settings were kept, resulting in 1210 articles. Because Google Scholar could only sort articles by relevance, the first 70 articles were reviewed, and 11 were found relevant. However, 6 studies were found to be repeated from ScienceDirect; therefore, only 5 articles were selected from Google Scholar. In total, 14 research articles from ScienceDirect and Google Scholar were analyzed and used for data. Research was completed on August 4, 2024, so any relevant articles published after that date were excluded. 

The information from the research articles was then organized into three concise sections: Plant Growth, Lignin Content, and Nutrition. The section Plant Growth discusses how microgravity affects the growth of various plants in space. This involves organic compounds found in plants and plant structures, specifically measurements of stem lengths and fresh weights. The Lignin Content section focuses on microgravity’s impact on lignin with measurements of lignin content in various plants. The third section, Nutrition, explains how microgravity impacts plant nutrition values, mainly the amounts of soluble sugar.

Plant Growth Methods

Regarding the research articles analyzed for the Plant Growth section, all the articles that were involved with stem lengths grew different types of plants, except for Cowles et al. (1994) and Cowles et al. (1984) which both included mung beans (see Table 1)^12,13. Most plants were grown using a plant growth unit (PGU), which was then transferred to a space shuttle for the experimental group. On the other hand, Rioux et al. (2015) conducted both flight and control experiments using the Advanced Biological Research System (ABRS), a specialized laboratory system primarily for space research, which has growth chambers capable of controlling temperature, illumination, relative humidity (RH), CO2, ethylene, and volatile organic compounds⁵⁷. Furthermore, their experiment was conducted on the International Space Station (ISS). Soga et al. (2002) used Biological Research in Canisters (BRIC-60) hardware, a standardized aluminum cylinder that has been flight-tested and approved for Space Shuttle experiments ⁵⁸. Similar to the research articles using PGUs, Soga et al. (2002) also transferred the plants to a space shuttle. 

Other aspects of the methodology, like duration, temperature, and light intensity, were relatively the same. However, Rioux et al. (2015) conducted the experiment for a longer period (around three times longer)⁵⁷. Measuring methods were generally unavailable since stem lengths could simply be measured with a measuring tool. However, Rioux et al. (2015) mentioned that photos of the plants were taken with a gridded whiteboard of 1 cm² squares behind it, which was used as a scale for measuring. Additionally, samples from Soga et al. (2002) were frozen and then measured using a scale⁵⁸. 

Articles	Type of plant grown	How was it grown?	Location of experimental group	Duration	Temperature	Light Intensity	Measuring Method
Rioux et al. (2015)13	White spruce seedlings	Advanced Biological Research System (ABRS)	International Space Station (ISS)	30 days	24 – 25 C	70 – 75 μmols	Gridded white- board with 1 cm2 squares to use as a scale for measuring
Levine et al. (2001)14	Dwarf wheat	Plant Growth Unit (PGU)	Space Shuttle Discovery (STS-51)	10 days	22 – 26 C	30 – 60 μmol	NA
Cowles et al. (1994)12	Mung beans	Plant Growth Unit (PGU)	Shuttle Flight STS-51F	8 days	24 – 28 C	NA	NA
Cowles et al. (1984)15	Pine seedlings, mung beans, oat seedlings	Plant Growth Unit (PGU)	Space Shuttle Columbia (STS-3)	8 days	22 – 28 C	Around 75 μmols	NA
Soga et al. (2002)16	Thale cress	Biological Research in Canisters (BRIC-60) hardware	Space Shuttle STS-95	6 days	22 – 24 C	NA	Samples were frozen and measured using a scale

The methodologies for the research papers with weight measurements and data all grew different types of plants, however, it should be noted that Laurinavicius et al. (1994) had two experiments, both growing common wheat (see Table 2)⁵⁹. Soleimani et al. (2019) and De Micco et al. (2006) both grew their experimental groups on Earth through clinorotation, a method involving a clinostat that continuously rotates a sample around a horizontal axis to replicate microgravity by averaging out the effects of gravity^15,60. On the other hand, the experiments of Laurinavicius et al. (1994) were conducted on orbital stations Salyut-7 and Mir in space. Moreover, the duration of these experiments was much longer than the other two research papers’ methods. Although it was not specified why this experiment was conducted for so long, it was likely attributed to how the plant was grown. Clinorotation is generally used for shorter experiments (a few days) to understand immediate responses to microgravity while orbital stations are in space for much longer, i.e. the ISS, which has been in space since 1998. The light intensity was not mentioned in any research articles, however, the plants from Laurinavicius et al. (1994) were grown in darkness⁵⁹. 

It should be noted that clinostats may not fully replicate all aspects of microgravity. Due to the earth’s gravity, plant fluids still settle, which can lead to differences in aeration, root hydration, and other factors when compared to actual space conditions⁶¹. Clinostats also induce fluid motion and shear stress, which does not occur in microgravity and can lead to different physiological responses in plants⁶². Therefore, despite being able to simulate some aspects of microgravity in plants like altered auxin distribution and cell expansion, clinorotation cannot fully model the effect of plants in space. Ultimately, differences in how the plants were grown may affect the results. 

Articles	Type of plant grown	How and where was it grown?	Duration	Temperature
Soleimani et al. (2019)17	Tobacco	2-D Clinorotation (20 rpm) on Earth	7 days	23 – 27 C
De Micco et al. (2006)18	Soy seedlings	Uni-axial clinostat (Cl) on Earth	8 days	18 – 24 C
Laurinavicius et al. (1994)19	Common wheat	Orbital station Salyut-7 in Space	63 days	26 C
Laurinavicius et al. (1994)19	Common wheat	Orbital station Mir in Space	72 days	26 C

Lignin Content Methods

The research articles by Cowles et al. (1994) and Cowles et al. (1984) were both used for plant growth (stem length) and had similar methods (see Table 1). In contrast, the experiment of Wakabayashi et al. (2015) was conducted in Measurement Experiment Units (MEUs), small compartments where the rice was grown (see Table 3)³³. These MEUs were placed in a Cell Biology Experiment Facility (CBEF), an incubator that could regulate proper environmental conditions and had already been used in space experiments. Stutte et al. (2006) used a Biomass Production System (BPS), a type of plant growth unit with four plant growth chambers (PGCs) that have independent control of air temperature, relative humidity, light level, and CO₂ concentration¹⁴. The duration and temperature across all research articles were relatively the same, however, Stutte et al. (2006) had a much longer duration of 73 days, the reason for which was not specified. Furthermore, Stutte et al. (2006) had a higher light level, which may be because it was measured at the top of the chamber, near the direct source of the light. 

Regarding their methodology for determining lignin content, Cowles et al. (1994) measured the lignin spectrophotometrically after tissue digestion and lignin solubilization, where lignin solubilization makes lignin more soluble and easier to break down its tissues¹². Conversely, Cowles et al. (1984) used a modified spectrophotometric procedure where a single stem section was first sequentially extracted with acetone and deionized water¹³. These samples were dehydrated, oven-dried, digested with acetyl bromide, and diluted with glacial acetic acid. Afterwards, they were treated with hydroxylamine-HCI and quantified spectrophotometrically. The methods for Stutte et al. (2006) and Wakabayashi et al. (2015) followed similar procedures involving acetyl bromide and glacial acetic acid, where they were added to a dry insoluble residue and heated^33,14. A NaOH solution was then added, and the samples were centrifuged. Using a spectrophotometer, the absorbance values were determined at 280 nm to measure the total lignin. 

Articles	Type of plant grown	How was it grown?	Location of experimental group	Duration	Temperature	Light Intensity	Method for Determining Lignin Content
Cowles et al. (1994)12	Mung beans, pine seedlings	Plant Growth Unit (PGU)	Shuttle Flight STS-51F	8 days	24 – 28 C	NA	Spectrophotometry after tissue digestion and lignin solubilization
Cowles et al. (1984)15	Pine seedlings, mung beans, oat seedlings	Plant Growth Unit (PGU)	Space Shuttle Columbia (STS-3)	8 days	22 – 28 C	Around 75 μmols	Modified spectrophotometric procedure using acetyl bromide and glacial acetic acid
Wakabayashi et al. (2015)21	Rice	Cell Biology Experiment Facility (CBEF) and Measurement Experiment Unit (MEU)	Sent on Space Shuttle STS-132 and in orbit on International Space Station (ISS)	5.3 days	20 – 22 C	Darkness	Samples dissolved with acetyl bromide in glacial acetic acid and measured based on absorbance
Stutte et al. (2006)20	Common wheat	Biomass Production System (BPS)	International Space Station (ISS)	73 days	24 C	Around 280 μmol	Spectrophotometric procedure using acetyl bromide and glacial acetic acid

Soluble Sugar Content Methods

The research articles for soluble sugar content were mentioned previously in Plant Growth and Lignin Content Methods. However, there were slight differences in their methodologies for finding sugar content (see Table 4). Soleimani et al. (2019) and Wakabayashi et al. (2015) both used a phenol-sulfuric acid method originally mentioned in DuBois et al. (1956)^15,33,63. This procedure involved adding phenol and sulfuric acid to the sample and incubating it at room temperature. The intensity of the color produced was then measured using a spectrophotometer, and this absorbance level was directly proportional to the sugar concentration of the sample. On the other hand, Stutte et al. (2006) mixed the sample with sulfuric acid, let it sit, and then heated the mixture at 100 C for three hours¹⁴. After, barium carbonate and sulfate were added and the sample was centrifuged. Monosaccharides were converted to alditol acetates and analyzed by a Gas Chromatography Flame Ionization Detector (GC-FID) using SP-225 capillary column and a temperature program of 215 – 230 C. 

Articles	Type of plant grown	How was it grown?	Location of experimental group	Duration	Temperature	Light Intensity	Method for Determining Sugar Content
Soleimani et al. (2019)17	Tobacco	2-D Clinorotation (20 rpm)	Earth	7 days	23 – 27 C	NA	Double beam spectrophotometer using phenol-sulfuric acid method
Wakabayashi et al. (2015)21	Rice	Cell Biology Experiment Facility (CBEF) and Measurement Experiment Unit (MEU)	Sent on Space Shuttle STS-132 and in orbit on International Space Station (ISS)	5.3 days	20 – 22 C	Darkness	Phenol-sulfuric acid method using glucose as a standard
Stutte et al. (2006)20	Common wheat	Biomass Production System (BPS)	International Space Station (ISS)	73 days	24 C	Around 280 μmol	Samples mixed with sulfuric acid and heated, cooled, and dried; converted to alditol acetates and analyzed by GC-FID using SP-225

The combination of data from different plant species allows for a broader understanding of how microgravity affects plant growth and development. Rather than analyzing and comparing differences between the species, the study focuses on the changes observed between flight and control conditions within each species. Thus, the biological differences among the species provide insight into how various plant types respond to microgravity. For example, the experiment performed by Cowles et al. (1984) purposely chose different plant species to understand how their physiological and developmental traits influence their responses to gravity¹³. Pine was grown because it is a gymnosperm capable of synthesizing large amounts of lignin, and gymnosperms were believed to be universally affected by gravity⁶⁴. Additionally, mung beans and oats were selected to represent dicotyledons and monocotyledons, respectively. By having a diverse selection of plant species, the study provides a more comprehensive assessment of how various plant types respond to microgravity and whether microgravity-induced changes are universally shared among plants.

Statistical Analysis

Quantitative data of mean values and their standard error (Mean ± SE) for a flight and control group was collected from the research papers. The flight group was an experimental group grown under microgravity in space or using clinorotation. The control group was grown on Earth in a greenhouse or laboratory location.  

The data was organized into four measurements or categories: stem length, fresh weight, lignin content, and soluble sugar content. All data was then scaled to the same dimensions. Stem lengths not measured in centimeters were converted to centimeters, as well as fresh weight measured in grams. The majority of the lignin content was measured in micrograms per stem and was taken directly from the original data. Some data presented as percentages of lignin present throughout the plant were excluded, as the specific amount of lignin could not be calculated from only a percentage. All data including soluble sugar content was in micrograms per shoot.

Using Excel, comprehensive bar graphs were created to understand the impact of microgravity better. Each type of plant had two data bars, the flight and control group, and error bars. The p-value was found using a type 1, 2-tailed t-test for flight and control groups. Because a t-test requires two arrays of data with at least two values, the standard error was subtracted or added to the mean data value, creating a total of six data values for one plant type and three in each array, the two arrays being flight and control. The p-value was used to determine whether the data was significant or not. Any p-value > 0.05 was not significant while any p-value ≤ 0.05 was significant. In some cases, the p-value was taken directly from the papers reviewed. While for certain papers, the standard error was not included with the quantitative data. Thus, the p-value of some plants could not be calculated and was considered inconclusive. Ultimately, by calculating the p-value between an individual plant type’s flight and control group, the p-value was more focused on comparing flight and control differences, rather than differences amongst varying plant types. 

It should be noted that the statistical analysis in this review was based on estimated data points derived from mean ± SE, as limitations such as raw data being unavailable and being unable to reach out to the authors of the original papers. Although this approach allowed for comparative analysis, it introduces pseudo-replication and does not fully capture the variability of the original studies. Therefore, the results should be interpreted with caution. 

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