to optimize crop performance. The gravitational environment must also be considered: for example, or-
biting space stations and transit missions to planets would be nearly weightless, and surface settlements
on the moon would have about ^16 gnand on Mars about ^13 gn[21,23,24]. Evidence to date suggests that even
under near weightlessness, acceptable plant growth and development should be possible provided the
other environmental needs are satisfied [25–28]. Yet providing all these environmental needs in weight-
lessness can be difficult. For example, watering plants in weightlessness will require closed plumbing sys-
tems to prevent water from escaping and a distribution system to maintain both adequate water and oxy-
gen throughout root zones [29,30]. On the moon or Mars, the gravitational fields should provide sufficient
mechanical advantage for moving water and nutrients and allow use of such conventional recirculating
hydroponic approaches. Plants might be grown on centrifuges in spaceflight to impose an artificial grav-
ity and potentially alleviate root-zone drainage problems, but this would present additional engineering
challenges, especially for large-scale systems.
- Light
PHOTOSYNTHETICALLY ACTIVE RADIATION Of all the environmental factors for growing
crops for life support, light (photosynthetically active radiation, PAR) is perhaps the most important with
regard to crop yields and system costs. At low to moderate light levels, crop yields are near-linear func-
tions of total PAR [10,31,32], and for some species (e.g., wheat), yields continue to increase even at very
high PAR levels [31] (Figure 2). These findings are encouraging for life support and suggest that grow-
ing areas could be reduced significantly with higher light intensities. Whether productivities of broad leaf
crops would continue to increase at such high PAR levels needs further study, but results with potato and
soybean suggest that maximum yields might be achieved near 800 to 1000 mol m^2 sec^1 [33,34]. For
some crops, high PAR levels may be undesirable because of injuries such as tipburn (e.g., lettuce) [35]
and leaf chlorosis, particularly under high-intensity discharge lamps [36].
Most of the testing to date with plants for life support has used electric lighting. The results from
these studies should be applicable to space habitats where sufficient electrical power is available. Al-
ternatively, incident solar lighting might be used to grow plants to reduce large electrical power re-
quirements, but this would require transparent materials with the appropriate pressure and thermal in-
tegrity, or light collection/conduit systems [37,38]. Solar light collection systems would also be subject
to the local photoperiods and the native solar intensity. For example, the solar radiation constant for the
moon is similar to that just outside the Earth’s atmosphere (1370 W m^2 total radiation with ~600 W
m^2 PAR), but the light cycle at most latitudes is ~14.7 days with a corresponding ~14.7 day night cy-
928 WHEELER ET AL.
Figure 2 Crop growth rate of wheat versus photosynthetically active radiation (PAR). (Data from Refs. 10,
11, and 31.)