would be needed (Figure 4). Depending on mission constraints, this might be acceptable if energy is lim-
ited but growing volume is available.
Crops grown in controlled environments for life support testing showed no major changes in proxi-
mate or elemental composition when compared with field-grown crops, but ash and protein can be higher
in controlled environment–grown crops [108–110]. Much of this may be the result of luxuriant uptake of
nutrients, especially nitrogen from hydroponic culture, which can artificially increase protein estimates
[108]. Hydroponically grown crops can also have a high P content in their tissues, which can interfere
with Ca nutrition in humans. Also, leafy vegetables can accumulate high levels of nitrate, which may pose
a health concern [108]. But these concerns might be managed through cultivar selection and careful con-
trol of the plant nutrient solutions, e.g., reducing nitrate concentrations prior to harvest.
B. Atmospheric Regeneration
Several investigators have studied photosynthetic gas exchange rates of crops using specially built or
modified chambers [8,9,11,47,64,111,112]. These chambers allowed direct measurements of CO 2 re-
moval by plant stands and could be used to estimate standing biomass throughout growth and develop-
ment [9,64,112]. In nearly all cases, CO 2 uptake was strongly affected by canopy cover, PAR, and CO 2
concentration and could be used to detect stresses or perturbations to the crop stands [9,64,112]. Oxygen
production was also tracked in some studies, which provided information on assimilation ratios of the
crop canopies [9]. In studies where canopy gas exchange was not measured, CO 2 uptake and O 2 produc-
tion could still be estimated from elemental analysis of biomass following crop harvests [10,111].
Although total O 2 production and CO 2 removal by plants is a function of the total biomass, only a
fraction of this biomass is edible—perhaps 50% for mix of species [9]. Thus, if enough edible biomass is
produced to meet human dietary needs, the O 2 requirements and CO 2 removal should also be met [9]. The
balance of O 2 and CO 2 in closed life support systems will also depend on waste recycling strategies; if
waste (inedible) biomass is oxidized, this would consume some of the O 2 produced in photosynthesis.
Alternatively, if waste biomass is discarded following nutrient extraction, then an equivalent amount of
carbon would need to be replaced, such as with stowed food. By using crops with a high harvest index
(i.e., percent edible biomass) the proportion of waste biomass can be reduced, which in turn can reduce
O 2 requirements for waste processing [9]. The composition of stowed foods must also be considered,
where, for example, consumption of foods containing fat would reduce the respiration quotient (CO 2 pro-
duced/O 2 consumed) of humans [2,3]. Likewise, production of fat by plants could change the assimila-
tion quotients, as could nitrate reduction requirements, which utilize energy from photosynthesis [2,3,9].
Precisely balancing O 2 and CO 2 through photosynthesis may be difficult, but these imbalances could be
managed with the use of supplemental physicochemical gas control technologies [2,4,5].
C. Spaceflight Testing
Because of the high cost of spaceflight testing, most plant studies for life support have been conducted in
“ground-based” settings, but spaceflight testing would provide a good first step for demonstrating the ul-
timate use of plants for life support. Numerous plant experiments have been carried out in space, but these
studies were generally focused on fundamental biological questions [25]. Nonetheless, results from these
studies suggest that plant growth and development can proceed in space if a good growing environment
is maintained—adequate light, water, nutrients, etc. [25]. A key step toward achieving this will be the de-
velopment of a reliable water and nutrient delivery system that operates in weightlessness [29,30]. Lim-
ited electrical power for lighting and cooling and relatively small growing volumes most likely will con-
tinue to impose constraints for spaceflight testing in the near future.
Of the food crops discussed for life support, wheat has been studied the most in space. These stud-
ies were carried out both on the U.S. Space Shuttle and the Russian Mir Space Station [26,113]. Initial ef-
forts to produce seeds from the wheat in the Mir studies were unsuccessful, and it now appears that this
was due to high background levels (~1 ppm) of ethylene in the growing environment [26,113]. Steps were
taken to reduce ethylene in subsequent studies, and wheat plants produced viable seeds in these tests
[113].
Spaceflight testing has also been carried out with potato but using explants (leaf cuttings) to study
early tuber development. Results from a 16-day Space Shuttle experiment in 1995 using leaf cuttings
showed that potato tubers could form and accumulate starch in spaceflight [27,28]. In addition, gas mon-
PLANT GROWTH AND LIFE SUPPORT IN SPACE 935