The specifications for the Plant Research Unit (PRU), a plant habitat designed for Space Station operations are described. A prototype brassboard model of the PRU is described and the results of testing of its subsystems are outlined. Comparative testing was performed of the effects of long-term red LED illumination as the sole light source for plant development, red LEDs supplemented with blue wavelengths, and white fluorescent sources. Both wheat and Arabidopsis were able to complete a life cycle under red LEDs alone, but with significant differences in physiology and morphology. The differences noted were greatest for Arabidopsis where the time to flowering was increased markedly when red photons alone were provided. The addition of 10% of blue light was effective in eliminating the differences observed. The results of comparative testing of three nutrient delivery systems for the PRU are briefly discussed.