Increasing plant productivity in closed environments with inner canopy illumination

Due to the high cost of habitable real estate associated with space travel and colonization, and the ultimate use of plants as the primary method of life support, it is necessary to develop cultivation methods whereby the highest sustainable level of productivity is achieved within the least amount of space. It is well known that in a dense plant canopy, lower leaves become shaded from above and eventually no longer contribute to carbon gain. In fact, they contribute to net respiratory carbon losses. One method of improving biomass production is to introduce light of suitable quantity and quality to the inner canopy, thereby utilizing unused photosynthetic capacity. By coupling microwave-powered lights to 100-mm-diameter glass tubes lined with 3M Optical Lighting Film, light with a spectral quality similar to that of sunlight was delivered to the inner canopy of a developing soybean crop. Results indicated that increases in productivity of 23-87%, as measured by CO2 assimilation, can be achieved in dense plant canopies (LAI approximately 6) when overhead lighting (40O-1200 micromoles m-2 s-1) is supplemented with inner canopy illumination.