Achieving maximum plant yield in a weightless, bioregenerative system for a space craft.

Limitations to maximum plant yield are photosynthesis, respiration, and harvest index (edible/total biomass). Our best results with wheat equal 97.5 g total biomass m-2 day-1. Theoretical maximums for our continuous 900 micromoles photons m-2 s-1 = 175 g carbohydrate, so our life-cycle efficiency is about 56%. Mineral nutrition has posed problems, but these are now nearly solved. CO2 levels are about 80 micromoles m-3 (1700 ppm; ambient = 330 ppm). We have grown wheat plants successfully under low-pressure sodium lamps. The main factor promising increased yields is canopy development. About half the life cycle is required to develop a canopy that uses light efficiently. At that point, we achieve 89% of maximum theoretical growth, suggesting that most parameters are nearly optimal. The next important frontier concerns application of these techniques to the microgravity environment of a space craft. There are engineering problems connected with circulation of nutrient solutions, for example. Plant responses to microgravity could decrease or increase yields. Leaves become epinastic, grass nodes elongate, and roots grow out of their medium. We are proposing space experiments to study these problems.