For scientists and engineers who are trying to design a real system, the devil is in the details. The ideal space-plant would have short stalks to save room, would have few inedible parts, would grow well in low light, and would be resistant to microbial diseases. Research is underway at KSC to choose varieties of wheat, rice, lettuce, potatoes and other plants that meet these criteria. In an orbiting greenhouse, freely-falling plants don't feel the constant downward pull of gravity. As a result, water spreads out evenly in the soil-like material around their roots, which makes it harder for both air and water to reach the roots.
Designers have to provide fans to keep the air moving.
Researchers caution that ironing out these sorts of details won't guarantee a working system when all the pieces are assembled. For microbes, with their short generation times, you're talking about real evolutionary time scales for prolonged missions. To test how the humans, plants, and microbes fare when sealed together for extended periods of time, JSC is building a test chamber called BIO-Plex. For example, foreseeable advances in biotech and nanotech could make it possible to alter plants' genes so that their cells produce little molecular sensors, transmitters, and receivers. These would monitor the plants internally and report on their health to ensure a good crop, and could even make the plants controllable, sprouting and flowering on cue.