- Carbon Dioxide
With the assistance of physicochemical control systems, adequate CO 2 concentrations (i.e., CO 2 partial
pressures) should be achievable for plant production systems in space. Studies of canopy gas exchange typ-
ically show increased photosynthesis as CO 2 is increased from 0.035 to 0.07 kPa (350 up to ~700 ppm at
101 kPa total pressure) [63], with maximum rates for C 3 species occurring near 0.10–0.15 kPa [10,47,64].
Likewise, biomass yields of C 3 crops usually reach a maximum at 0.10–0.15 kPa [33,54,65]. The effects
of CO 2 partial pressures greater than this are less clear, however [54,65]. Total biomass production in radish
and lettuce was reduced by superelevated CO 2 (e.g., 1.00 kPa), whereas no effects were observed on
biomass yields of soybean, potato, and wheat [65–68] (Figure 3). Others have reported no effects on
biomass but decreased seed yield in wheat and rice [54,69]. For potato, soybean, radish, and sweetpotato,
water use increased substantially at superelevated CO 2 because of increased stomatal conductance [66,68].
This finding was surprising in light of CO 2 ’s tendency to reduce conductance as it increases across lower
concentrations (e.g., 0.04 to 0.10 kPa) [63]. If plant growth systems are closely linked with atmospheres
in human habitats in space, then CO 2 pressures may become superelevated as a result of human respira-
tion, ranging from 0.2 to 0.6 kPa [14,70]. In addition, if plants were grown in enclosures on Mars where
local CO 2 was used as a pressurizing gas, then CO 2 partial pressures would probably exceed 1 kPa.
930 WHEELER ET AL.
Figure 3 Possible effects of carbon dioxide (CO 2 ) on plant growth and water use. Superelevated CO 2 levels
(0.2 kPa) could be toxic for some species and/or result in increased water use [54,65,68].