It's snowing in Toronto this morning. Seriously. But irrespective of what I see outside my window, I know spring to be near at hand. Many of us have experienced the therapeutic power of caring for and raising plants and are looking forward to getting out into our gardens. I for one need to have some greenery around my office and home, which marks me as a bit unusual. In space, however, plants may eventually serve a more vital role. This week Alex examines the first green shoots researchers are cultivating along that pathway. Above, a zucchini plant is pictured on the ISS.
by Alex Innanen
Spring is arriving in fits and starts in Toronto, and that means it’s time to start this year’s seeds and get out in the garden. It would also normally be time to visit a nursery (or five) but this year is a bit different, and my gardening routine has been disrupted by isolation. Which got me thinking about growing plants in an even more isolated location – that’s right, it’s time for space plants!
I know that taking care of my plants, spending time in the garden, can be very relaxing and grounding. The same is true for astronauts. But we haven’t been growing plants in space just because they’re nice to work with. One big reason to grow plants in space is for food. On the ISS, it’s relatively simple to resupply the astronauts with fresh food but think back to early explorers spending months at sea and getting scurvy. If only they had had a grow light and some arugula! Plants have also been suggested as a from of life support – we know that plants recycle carbon dioxide into oxygen, the kind of reverse of what happens in animals where we breathe in oxygen and exhale carbon dioxide. In addition, wastewater can be used to grow plants, and the same plants then transpire, or release, clean water vapour, which can be condensed and used again.
Sounds great right? But first you have to get plants to grow in the space environment. So what do plants need to grow? Most plants need a light source, water, nutrients, some sort of substrate, like soil, and the correct temperature. But do they need gravity? Picture a plant – generally, the shoot grows up, above the soil, while the roots grow down. This happens no matter which way you orient the seed, the plant seems to “know” which way its roots should grow. It turns out that this has less to do with gravity and more to do with where the light source is coming from, so even on a space station where “up” and “down” don’t mean much, the shoot will grow towards the light source, while the roots will grow away.
Gravity does affect other aspects of caring for plants though: watering. On earth we rely a lot on drainage to ensure our plants don’t drown – my cactuses, which don’t like a ton of water, have looser soil and their pots sit in basins to collect the water that drains out. In micro-gravity the water isn’t pulled downward to drain out, so more creative water solutions are needed. In the Vegetable Production System (Veggie) on the ISS, the plants grow on “pillows”, which contain the required substrate and nutrients. The pillows sit in a reservoir, and wick water up to the plants’ roots, allowing them the mix of water and air they need to be healthy.
A zinnia, floating in its plant pillow.
Radiation is another factor in space plant growth – we know it affects living tissues, and even on the ISS, orbiting at around 400 km, the amount of ionizing radiation is far greater than here on Earth. The ISS is shielded, of course, but still astronauts who spend three months on the station exceed three times the maximum recommended yearly dose of ionizing radiation. While plants tissues tend to be less susceptible than those of animals, radiation can still cause genetic mutations in plant cells affecting photosynthesis, and the growth rate of the plant.
Thale Cress plants as part of the Plant Habitat-01 study
Despite the challenges, we’ve been happily growing plants in space for decades – plant growth experiments go back to the first space stations and have continued through today. Astronauts are able to supplement their meals with vegetables grown with Veggie and the Lada greenhouse, such as lettuce, cabbage and other greens. Alongside Veggie and Lada are other plant growth experiments. The Advanced Plant Habitat (APH) is a mini-fridge sized chamber which allows more area for plants to grow than was previously possible on the ISS. It’s nearly fully automated, with the crew only needing to add water and perform maintenance, and the plants themselves are isolated in a closed environment.
We’ve even dabbled in space trees – in a sense. Apollo 14 flew a number of tree seeds, in order to compare them with control seeds on Earth. While no formal study was done on the “moon trees” due to an accident in decontamination, many of the seeds did in fact germinate. The CSA also flew two Advance Plant Experiments (APEX) – APEX-Cambium and APEX-CSA2, which flew willow and white spruce seedlings, respectively. The seedlings grew for 30 days on board the ISS before being stored and returned to Earth in order to study their tissue development in comparison to their Earth-bound counterparts.
The incubator used for APEX-Cambium, with willow seedlings
So what about more distant pastures? Will we see greenhouses on the moon, or potatoes growing on Mars? That is certainly the hope, but for now, to paraphrase Voltaire, we can tend to our own gardens, whether they’re in our houses or orbiting 400 km above our heads.
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