B. Horticultural Considerations
Plant tests for bioregenerative technologies have utilized a range of nutrient/water delivery concepts, in-
cluding solution cultures [82,83], solid media [45,84], and nutrient film technique (NFT) [85,86]. For
space applications, it will be necessary to recycle (recirculate) nutrient solutions to conserve water and
nutrients [82,85]. NFT has been used to grow a wide range of species, including wheat, soybean, potato,
lettuce, tomato, peanut, sweetpotato, spinach, beet, and rice [85–88] and has the advantage of low mass
(low water volume) and relatively simple harvesting for root-zone crops. On the other hand, low water
volumes make the system more susceptible to failure if circulation is interrupted (e.g., pump failures). An
additional concern is that solution culture systems typically lack buffering capacity unless buffering
agents are added [89]. The lack of solution buffering requires close management of nutrient concentra-
tions and pH to avoid periods of nutrient depletion or pH variations [85]. In studies in which plants were
grown in recirculating NFT with nitrate-nitrogen, acid requirements for nutrient solution pH control could
exceed 1.0 mmol Hper gram of dry mass produced or over 40 mmol m^2 crop growing area day^1 [85].
These acid requirements could add substantial costs to operating hydroponic systems for life support but
might be reduced by manipulating the nutrient solution composition, e.g., substituting some NH 4 for
NO 3 salts [90]. For early missions with relatively simple control systems, it would seem most efficient
to select a universal nutrient solution formulation that could be used with a range of crops [85].
Most of the plant testing for life support has involved single plantings, after which the crops were
harvested and the system was cleaned [10,47,85]. Yet for actual life support applications, crop produc-
tion would have to be sustained on a continuous basis. Controlled environment tests with NFT-grown
potatoes showed that productivities could be maintained through four production cycles (418 days) by
planting directly back into harvested spots and continually managing the nutrient solution [91,92]. But
these tests also revealed that tuber “promoting” compound(s) built up in the nutrient solution over time,
which resulted in reduced shoot growth and early tuberization in successive plantings [92,93]. These in-
ductive effects could be removed by placing activated-carbon filters in the nutrient solution, but this il-
lustrates the challenges that can arise for sustaining crop productivities over long periods of time.
An alternative to hydroponic approaches for space applications would be to use media that are
preloaded with the essential nutrients [84,94]. These systems would not require the monitoring and con-
trol of a recirculated hydroponic culture, and condensed water could be returned directly to the rooting
media. A disadvantage is that the medium would eventually become nutrient depleted and require
recharge or disposal. In addition, the nutrient loading may have to be tailored for each species to optimize
growth. Clearly, the choice of culture system will depend on mission constraints and costs. Early efforts
to grow plants on planetary surfaces might involve simple, deployable systems that are relatively
autonomous and might be used only for one crop cycle. Later missions might employ more sophisticated,
human-tended systems where optimizing crop growth and continuous production are required.
C. Crop Improvements
With the exception of lettuce and tomato, most of the crop cultivars studied for life support testing were
developed for field settings [52,65,87]. These cultivars were then screened for their performance under
controlled environment conditions [47,52,65,87]. For wheat, however, breeding lines were established
specifically for controlled environment performance [22]. This led to the development of cv. ‘Apogee’,
which grows only to about 50 cm in height and produces exceptionally high yields in controlled environ-
ments [22]. Tests are also being conducted with dwarf soybean and rice cultivars that are well adapted for
space systems [34] (B. Bugbee, personal communication). Molecular techniques have been employed to
improve crops for life support applications (e.g., sweetpotato protein content) [95], and additional use of
molecular approaches could accelerate development of short-stature, high-yielding crops for space agri-
culture.
III. WASTE RECYCLING WITH PLANTS
The advantages of using plants for CO 2 removal while producing O 2 and food are obvious, but another
possible function of plant systems for life support is their capability for waste recycling, especially
wastewater. The phrase “plant growing system” is key here, in that the microbial communities associated
932 WHEELER ET AL.