But the experiments also reveal just how stressful it is for plants to grow in lunar regolith, or soil, which is wildly different from natural habitats on Earth.
Different types of plants, including food crops, have flown on the space shuttle and the International Space Station. Plant samples have even been used to prove that lunar samples are not harmful to life on Earth.
“Plants helped establish that the soil samples brought back from the moon did not harbor pathogens or other unknown components that would harm terrestrial life, but those plants were only dusted with the lunar regolith and were never actually grown in it,” said study coauthor Anna -Lisa Paul, research professor of horticultural sciences at the University of Florida Institute of Food and Agricultural Sciences.
Paul and study coauthor Rob Ferl, distinguished professor of horticultural sciences at the UF Institute of Food and Agricultural Sciences, wanted to take things a step further and see if seeds would grow in lunar soil.
“For future, longer space missions, we may use the Moon as a hub or launching pad,” Ferl said in a statement. “So, what happens when you grow plants in lunar soil, something that is totally outside of a plant’s evolutionary experience? What would plants do in a lunar greenhouse? Could we have lunar farmers?”
It’s an experiment that has been long in the making – 15 years have passed since the researchers made their first proposal and request for lunar samples. The request was granted 18 months ago.
The research team requested 4 grams of lunar material collected during the Apollo 17 mission from NASA. Ryan Zeigler, NASA’s Apollo sample curator, saw the scientific value in providing more from different Apollo missions. He sent them a total of 12 grams (2.7 teaspoons) from lunar samples collected from the Apollo 11, 12 and 17 missions.
“That made a big difference in enabling us to take a deeper look into the science and effects of lunar regolith on plants than we would have otherwise been able to do,” Paul said.
The researchers used wells the size of thimbles as pots. Typically, these plastic trays are used to culture cells.
The scientists filled each well with a gram of lunar soil, added nutrients and water, and poked in a few seeds of Arabidopsis thaliana, or thale cress, a small flowering plant native to Eurasia and Africa.
Thale cress is an attractive plant specimen to researchers because it is well studied and its genetic code has been mapped – which allowed the researchers to study how the alien soil affected the plant’s gene expression. And the plant has already been to space aboard the shuttle and space station, providing the team with space data to use for a comparison.
Arabidopsis seeds were also planted in a synthetic substance that simulates lunar soil, as well as volcanic ash and other substrates from extreme environments.
To the surprise of the researchers, almost all of the seeds planted in the dark lunar soil actually sprouted and began to grow.
“We did not predict that,” Paul said. “That told us that the lunar soils did not interrupt the hormones and signals involved in plant germination.”
The Arabidopsis sprouts, however, showed signs of struggle as they adjusted to the lunar soil.
The seedlings were smaller, grew slower and varied in size compared to plants grown in Earth soils. The roots were stunted. And the plants took longer to grow expanded leaves than the Arabidopsis plants grown in volcanic ash. Some of the lunar soil plants showed reddish black pigments in their leaves, an outward sign of stress.
On a genetic level, three of the smaller, darker plants expressed more than 1,000 genes that were largely related to stress.
“At the genetic level, the plants were pulling out the tools typically used to cope with stressors, such as salt and metals or oxidative stress, so we can infer that the plants perceive the lunar soil environment as stressful,” Paul said.
“Ultimately, we would like to use the gene expression data to help address how we can ameliorate the stress responses to the level where plants – particularly crops – are able to grow in lunar soil with very little impact to their health.”
The Arabidopsis plants that struggled the most were grown in mature lunar soil, and they turned purple as an oxidative stress response. These soils collected during the Apollo 11 mission were more exposed to the harsh space environment, and the plants grown in the Apollo 12 and 17 samples seemed to fare better.
The lunar surface, regularly bombarded by cosmic rays and solar wind, also includes iron particles and tiny fragments of glass. All of these elements could impair plant growth.
“Even the plants that looked healthy were struggling to stay looking healthy,” Paul said. “They grew in lunar regolith, but they had to deal with it at a metabolic level in order to maintain that kind of health.”
The researchers want to conduct follow-up studies to understand how growing plants in the lunar environment could actually alter the moon’s soil.
“The Moon is a very, very dry place,” said study coauthor Stephen Elardo, UF assistant professor of geology. “How will minerals in the lunar soil respond to having a plant grown in them, with the added water and nutrients? Will adding water make the mineralogy more hospitable to plants?”
On the moon, these plants could pull resources from the lunar soil, like metallic iron, and make them available in useful forms.
Further research could also help the researchers determine the most efficient way to grow plants in lunar soil and help them avoid the stressors identified during the study. And the team wanted to reach a clear understanding of the nutritional value of these plants – and if that’s affected by the soil.
While thale cress is edible, it’s not tasty or considered to be a food crop. But it’s in the same plant family as broccoli, kale, turnips and cauliflower.
Sharmila Bhattacharya, chief scientist for astrobionics at NASA, found the plant growth discovery to be “quite thrilling” and said the study presents many follow-up opportunities for scientists. Bhattacharya did not participate in this study.
“We need to work out how to make the plants grow even better in this regolith substrate,” Bhattacharya wrote in an email. “For example, do we need to add other components to help the plants along, and if so, what are they? Are there other plants that can adapt better to these regolith substrates, and if so, what traits make them more robust to these environments?
“That’s what’s so exciting about science; each new finding leads to more unique and transformative results down the road, which we can then use to help improve sustainability for our future space exploration missions!”
The scientists see their experiment as a positive result for what may be possible as humans push the boundaries of space exploration.
“It’s amazing that the plant still grows in this harsh environment,” Elardo said. “It’s stressed, but it does not die.”
“When humans move as civilizations to stay somewhere, we always take our agriculture with us,” Ferl said. “The idea of bringing lunar soil into a lunar greenhouse is the stuff of exploration dreams.”