- Temperature
Many of the crops considered for life support (rice, peanut, soybean, and sweetpotato) prefer warmer tem-
peratures (e.g., 25 to 30°C), whereas potatoes and wheat do well at cooler temperatures (e.g., 15 to 20°C)
[47,50,53,71]. Wheat is tolerant of warmer temperatures (20–25°C), but the life cycle decreases and
yields tend to drop [22,47]. Potatoes also tolerate warm temperatures but do not tuberize well in controlled
environments when temperatures are 20°C [71]. Lettuce and tomatoes do well at intermediate temper-
atures, e.g., 23°C [72,73], whereas other species considered for life support studies, such as cabbage,
chard, and carrot, typically grow better at cooler temperatures (e.g., 16 to 18°C) [74]. This range of
temperature optima suggests that it may be most efficient to partition plant production systems into at
least warm and cool growth areas. In all cases, freezing temperatures would be lethal for most actively
growing crops and must be avoided.
- Volatile Organic Compounds
Life support habitats for space must be tightly closed to minimize atmospheric gas losses and associated
resupply costs. This tight closure could cause volatile organic compounds (VOCs) to accumulate, which
could affect both humans and plants in these systems [14,70,75]. These VOCs emanate from a variety of
sources, including plastics, paints, glues, caulking, and the plants themselves [75–77]. Closed-system
tests with plants have documented the presence of compounds known to be associated with plant
metabolism (Table 2), and the effects that these have on plants and humans is not well studied [75–79].
In addition to VOCs, volatile nitrogenous compounds such as N 2 O can accumulate in closed systems with
plants as a result of bacterial denitrification in root zones [80].
With plants in closed systems, there is a special concern with ethylene gas [81]. Closed-system stud-
ies conducted at NASA’s Kennedy Space Center showed that ethylene is a natural product of plant stands,
with the rates of production varying with species and developmental stage [78]. Somewhat surprisingly,
the highest ethylene production typically occurred during rapid vegetative growth [78]. Environmental
stresses, such as using continuous light with potato, or periods of fruit ripening with tomato also resulted
in rapid production of ethylene (B. V. Peterson et al., unpublished). High ethylene can result in leaf
epinasty, flower abortion, reduced stem elongation, and reduced seed set [81]. Current approaches for
controlling volatile organics in closed atmospheres of space habitats include the use of carbon filtration,
chemical oxidants such as potassium permanganate, or catalytic burners [14].
PLANT GROWTH AND LIFE SUPPORT IN SPACE 931
TABLE 2 Volatile Organic Compounds (VOCs)
from Humans and Plants
Humansa Plantsb
Acetaldehyde Benzaldehyde
Acetone 2-Butanone
Ammonia Carbon disulfide
n-Butyl alcohol Ethylene
Carbon monoxide 2-Ethyl-1-hexanol
Caprylic acid Heptanal
Ethanol Hexanal
Ethyl mercaptan 2-Hexen-1-ol acetate
Hydrogen Isoprene
Hydrogen sulfide Limonene
Indole 2-Methylfuran
Methanol Nonanal
Methane Ocimene
Methyl mercaptan -Pinene
Propyl mercaptan -Pinene
Pyruvic acid -Terpinene
Skatole Tetrahydrofuran
Valeraldehyde Tetramethylurea
Valeric acid Thiobismethane
aReed and Coulter [70].
bStutte and Wheeler [77]; Stutte [75].