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An automated nutrient replenishment system has been developed in order to provide a constant electrical conductivity (EC) value for the nutrient solution over the period of plant growth. A single nutrient film technique (NFT) system developed by the Tuskegee University NASA Center was equipped with the EC control system for growth trials with sweetpotatoes. The system is completely controlled and monitored by a PC through the use of LabView instrumentation and data acquisition software. A submersible EC probe driven by an EC controller measures the EC of the nutrient solution reservoir. EC values are passed from the controller to the PC through analog outputs. If the EC is outside a given range, the PC sends a signal to one of two solenoid valves that allow concentrated stock solution or deionized water to enter the reservoir to either raise or lower the EC respectively. For this application the set point is 1200μS cm-1, with a dead band from 1180 to 1220μS cm-1.

Several plant growth systems have been tested for crop production in microgravity and lunar/Mars environments in support of NASA's Advanced Life Support Program and long-term human space missions. These systems have incorporated such design features as the nutrient film technique (NFT), microporous plates, microporous tubes, and expandable boundary chambers and have been developed and used to support sweetpotato production at Tuskegee University. In the present research, the performance of different sweetpotato cultivars in a microporous tube system with lunar simulant medium was studied. The lunar simulant is an inert aggregate with an average particle size of about 3 mm. Buried in this solid medium is a microporous tube. Nutrient solution is circulated through the microporous tube under a slight negative pressure. This pressure is controlled to allow a slight seepage from the tube with some accumulation of water in the solid medium, but no free water.

Long-term manned space missions will require life support processes including food production. Porous plate and tube membrane systems have been identified to have potential for crop production in a microgravity environment. Of several systems tested, a stainless steel plate membrane system with a porous medium underneath has proven to be superior in terms of the uniformity of nutrient solution distribution. Several trials with sweetpotatoes, showed successful plant growth, with reduced foliage and storage root yield as compared to the nutrient film technique (NFT). These results can be attributed to reduced nutrient solution availability compared to NFT. It is expected that design improvements can increase sweetpotato yield..

A closed nutrient delivery chamber with expandable boundaries has been developed to support the growth of root crops, with potential applications in microgravity. The chamber is completely enclosed, separating the root zone from the foliage zone with a padded sealant through which the plant stem passes. The expandable boundary chamber (EBC) allows for expansion of the root zone volume, through longitudal pleats, as the plant grows. Two units have been evaluated with a trial crop of sweetpotato (Tuskegee Univ. breeding clone TU-82-155) for 120 days in a greenhouse environment. Storage root yield per plant in the EBC averaged 1.33 kg in comparison to 0.3 kg for the conventional Nutirent Film Technique (NFT) grown plants. This excellent yield warrants further design refinement and serious consideration of the system for earth use and microgravity applications.