PLANT GROWTH AND LIFE SUPPORT IN SPACE 929
cle [21,24]. An Earth-orbiting space station would typically have a 90-min orbital cycle, providing
about 60 min of light and 30 min of dark [39]; a planetary transit vehicle could capture uninterrupted
solar light, but the intensity would continually drop off with distance. Mars receives about 45% of the
solar radiation that Earth does (~600 W m^2 total) and has a relatively favorable photoperiod of ~25
hr [21,23]. But large dust storms occur at some latitudes on Mars, which would affect solar light col-
lection systems [24,40].
Depending on the crop growing area, electric lighting could represent the single largest energy de-
mand in the life support system. This includes not only making the light from electricity but also remov-
ing the heat generated by the lamps. If 40 m^2 of crop area and a PAR input of 1000 mol m^2 sec^1 (~200
W m^2 ) is needed to support one human (see later), then 200 W m^2 PAR/20% (electrical conversion ef-
ficiency for lamping systems) or about 1 kW of electrical power would be required for each m^2 of crop
area. Then, 1 kW m^2 40 m^2 person^1 , or 40 kW would be required per person for the electric light-
ing. This power requirement might be doubled to accommodate heat rejection, water pumping, air circu-
lation, etc., indicating that perhaps 100 kW of electrical power would be required per person to grow the
food just to meet caloric needs.
Thus if electric lighting is used, it will be critical to use efficient electric lamps to minimize the power
requirements. High-intensity discharge (HID) lamps, such as high-pressure sodium and metal halide
lamps, have high electrical conversion efficiencies and have been used extensively for plant research [41].
Yet these are relatively hot, point sources of light that require a sufficient distance from the plants to
achieve adequate distribution and avoid plant damage. Fluorescent lamps have a larger radiating surface
but are less efficient and shorter lived than HID lamps. Innovative lighting technologies, such as light-
emitting diodes (LEDs), are relatively cool light sources that can be positioned much closer to the plants
[42,43]. In addition, LEDs can have very long life spans, which could provide substantial savings for sys-
tem maintenance [42,43]. Recently developed microwave lamps have very high electrical conversion ef-
ficiencies (35–40%) and hold promise as sources for plant lighting [44]. Light collection and delivery
technologies are also being studied in which either sunlight or light from a bright, artificial source is col-
lected and delivered to plants via conduits or optical fibers [37,38]. These light conduits might even be
used for intracanopy lighting to increase system efficiency [45,46].
PHOTOPERIOD Long photoperiods have worked well for growing wheat and lettuce in life support
studies [31,47–49], whereas short photoperiods have been best for soybean, potato, rice, and sweetpotato
[50–55]. Thus, there will probably be a range of optimal photoperiods for different crops in life support
systems. Tomatoes and some cultivars of potatoes are intolerant (show injury) to very long photoperiods
[51,52,56], and some short-day crops are sensitive to night breaks or dim light during the dark periods.
For example, dim day length extensions of 5 mol m^2 sec^1 PAR effectively blocked tuber develop-
ment in potatoes [51], and light leakage even as low as 0.4 mol m^2 sec^1 during a dark period can de-
lay or inhibit tuber initiation [57]. Thus, sensitive short-day species might require light barriers to sepa-
rate them from areas on different lighting cycles, or crops might be grown in separate long- and short-day
chambers.
As an alternative to adjusting the lighting environment, day-neutral species and cultivars might be
selected to avoid photoperiod complications. Despite being short-day plants, some potato cultivars grow
and tuberize under continuous light, provided temperatures are kept sufficiently cool or cycled on a
diurnal basis [51,58]. Some sweetpotato and soybean cultivars also tolerate continuous light [34,59]. Yet
even with day-neutral cultivars, growth can still be affected by photoperiod: for example, early-season
potato cultivars and day-neutral rice cultivars can still show reductions in yield and harvest index under
long photoperiods [52,55].
SPECTRAL QUALITY The spectral balance of the light also needs to be considered for achieving op-
timal plant growth. If solar light is used, this would provide a broad spectrum to which the plants are well
adapted. If electric lamps are used, an acceptable photomorphological spectrum must be provided [41,60].
For example, the lack of blue light in red LEDs or low-pressure sodium lamps (monochromatic 589 nm)
could affect phototropic orientation and stomatal functions in many species [60,61]. Even with high-pres-
sure sodium lamps, which have a broader spectrum, there still might be insufficient blue light to prevent
excessive stem growth [34,61,62]. In studies with red LEDs, it is usually necessary to supplement the red
light with small amount of blue to get acceptable growth [43]; this can be done with broader spectrum
lamps or by including blue LEDs in the lighting arrays.