Stressors of Spaceflight
Learn more about the types of stress that plants experience in space.
What is a spaceflight environment/where will we grow our plants in space?
Plants cannot grow or survive in the vacuum of space. There is no oxygen, no defense, very little gravity, and no opportunity for nutrient uptake. Growing plants in human spaceflight systems such as a shuttle or aboard the International Space Station helps take care of a few of these issues right off the bat. Firstly, human spaceflight systems provide a standard atmosphere for its inhabitants and can for plants as well. Secondly, spaceflight systems are shelters that can physically shield inhabitants from solar radiation. The things that we cannot yet protect our plants from are the effects of microgravity, ionizing radiation, and intense oxidative stress.
Microgravity
Many people believe that space is a zero-gravity environment and that a lack of gravity causes weightlessness. This is a very common misconception. The truth is gravity is present everywhere in space. Everything in the universe is falling and is affected by gravity. The orbit of the International Space Station is held in place by gravity. If it were to be used, the term zero-gravity implies zero acceleration which is not true because the ISS is constantly accelerating towards Earth. A more scientifically accurate term is microgravity. Microgravity is micro-acceleration and astronauts and objects in space appear weightless because the effect of gravity grows weaker with increased distance from a large planetary body. Microgravity affects plants just as it affects any other organism in space. Since plants are terrestrial organisms, astrobotanists are interested in discovering how this change in gravity manipulates plant biology.
Related: Plant Gravitropism
Ionizing Radiation/Galactic Cosmic Rays
Galactic Cosmic Rays are a type of ionizing radiation that is difficult to defend against. Protons (and other heavier elements) are accelerated to the speed of light after blasting off from exploding stars. This strong radiation bombards organisms and damages their molecular structure. Unlike solar radiation, GCRs are difficult to defend against physically. The Earth generates a magnetic field that can deflect this radiation to keep our planet safe. This strength comes from sheer size. Obviously, a spacecraft is much, much, much smaller than a planet so it cannot generate a magnetic field that strong. This secondary radiation poses a huge health problem for astronauts as well as plants.
Oxidative Stress
There is a lack of oxygen in space, but that isn’t the reason that plants have been shown to experience oxidative stress there. In spacecraft systems, oxygen and carbon dioxide recycling systems are already in place to sustain humans, and do the same for plants. The type of oxidative stress plants experience in space is related to metabolism. For organisms on earth, oxidative stress is an unavoidable consequence of metabolism. The biochemical process of converting molecules and transforming energy creates oxidative byproducts and reactive oxygen species (ROS) that are harmful to cellular structure. In space, this oxidative stress is amplified in plants and the causes are currently under investigation. Two genetic series being explored are genes involved with the RBOH family as well as the peroxidase family of enzymes.