Plants: The Fellow Travelers of Space Voyage
In the last 50 years, scientists have been growing more than 40 cultivars in space, including Arabidopsis, edible crops and ornamental plants in order to find out what influence does the space have on plants' growth, how plants will impact astronauts' mental health and how plants sustain lives by providing oxygen, water and food.
These researches mainly took place in the near-Earth orbit with a focus on micro gravity's influence on plant growth. The water conductivity and gas exchange in micro gravity are essentially different than what they appear on Earth. Over irrigated and oxygen-deprived roots are common in these experiments. Thanks to the water absorption capacity and surface tension in the micro gravity, water in the grow media are arranged in concentric circles uniformly. This makes it difficult for plants to exchange components with the media. So it's essential to develop the technology that controls water and nutrient supply in micro gravity. Researchers have tried a variety of grow media: ion-exchange resin, solid agar, rock wool and vermiculite. In addition, perforated pipes, some of which were wrapped with thin ropes for capillary, were embedded in the grow media. But all the experiments on the near-Earth orbit are just a start. Humanity, when taking on farther and more time-consuming space voyages, need to depend on the space ship for life-sustaining materials and energy. Thus we need to build ourselves a controlled environment that provides oxygen, water and food stably. Plants will be the key element in such environment.
The key technologies of Controlled Ecological Life Support System, a.k.a. CELSS, are being tested and developed on Earth. The entire system is based on the relation among producer, consumer and decomposer: Plants, through photosynthesis, produce oxygen with the CO2 emitted by human beings, animals and microorgainisms. The condensate water produced via transpiration can be drank by space travelers and used for hygiene maintenance. The urine and waste water can be looped back in to supply water and nutrients for plant growth. Plants also produce all the life sustaining food for the travelers and animals on the space ship. Lives on Earth are depending on plants for oxygen, water and food. Humans need to utilize plants to sustain lives in controlled environment up in space. Thus it's crucial for us to understand how plants grow in space, screen out the cultivars that fit in the space ecology and develop the technologies that recycle resources.