Evolving a Novel Controlled-Environment Gas-Exchange System

A computer-controlled, open gas-exchange system is being developed to monitor photosynthetic rates of small (0.26 m2) crop stands grown under controlled conditions of lighting, CO2, and temperature. Tall crops that form closed canopies are sole-source lighted with vertical LED strips that switch on sequentially, and a reflective cuvette ceiling is adjusted to a constant distance above the crop stand as it grows. For overhead lighting of low-profile crops, a horizontal LED bank also is vertically adjustable. Plant growth is observed via a USB webcam while temperature and humidity probes monitor the cuvette environment. Each 0.8 m3 cuvette contains a recirculating hydroponic system with an external reservoir for solution maintenance. Temperature is maintained independently in both cuvettes by a combination of cuvette heat exchange and thermal control of the walk-in growth room that houses the cuvettes. Carbon dioxide is injected via a mass-flow-control valve into an inlet air stream, and the mixture is humidified before injection into the cuvette(s). Absolute and differential CO2 analyzers located outside the growth room measure precise inlet and outlet CO2 concentrations, and a command & control computer calculates, displays, and logs instantaneous photosynthetic and respiratory rates of the crop stand on a growth-area basis (µmol m-2 s-1) throughout the life cycle of the crop. The LED array for close-canopy overhead lighting is a “smart” lighting system that switches on LEDs located only directly above plant tissue, and measures power and energy expenditures for crop lighting. Near-term investigations with this system will seek combinations of adjustable light parameters (irradiance, red/blue ratio, photoperiod, distribution), CO2 concentration, and temperature that minimize energy expenditure without compromising productivity at each stage of crop development. Longer-term efforts will automate optimization of crop productivity based upon gas-exchange and energy-consumption rates. This project is sponsored, in part, by NASA grant NNX09AL99G.