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Accurate root zone moisture control in microgravity plant growth systems is problematic. With gravity, excess water drains along a vertical gradient, and water recovery is easily accomplished. In microgravity, the distribution of water is less predictable and can easily lead to flooding, as well as anoxia. Microgravity water delivery systems range from solidified agar, water-saturated foams, soils and hydroponics soil surrogates including matrix-free porous tube delivery systems. Surface tension and wetting along the root substrate provides the means for adequate and uniform water distribution. Reliable active soil moisture sensors for an automated microgravity water delivery system currently do not exist. Surrogate parameters such as water delivery pressure have been less successful.

Plant growth, and especially plant performance experiments in microgravity are limited by the currently available plant growth facilities (low light levels, inadequate nutrient delivery and atmosphere conditioning systems, insufficient science instrumentation, infrequent flight opportunities). In addition, mission durations of 10 to 14 days aboard the NSTS Space Shuttle allow for only brief periods of microgravity exposure with respect to the life cycle of a plant. Based on seed germination experiments (5 missions from 1992 - 1994), using the Generic BioProcessing Apparatus hardware (GBA), two new payloads have been designed specifically for plant growth. These payloads provide new opportunities for plant gravitational and space biology research and emphasize the investigation of plant performance (photosynthesis, biomass accumulation) in microgravity.

Alfalfa, clover, lettuce and periwinkle seedlings were grown from seeds during five Space Shuttle missions between 1992 and 1994. Germination was initiated on orbit. Selected plants were fixed on orbit by injecting a glutaraldehyde fixative. More than 1,000 seedlings have been grown for periods ranging from 2 to 12 days. Plants were germinated under low light conditions (1 mission) and in the dark (4 missions). The seedlings grown under low light conditions showed no significant differences in accumulated fresh mass or plant geometry between flight and ground. The plumular hook had developed both for alfalfa and clover plants. Hook opening and greening of plants occurred after the seedling penetrated the Rockwool™ substratum and were exposed to light. Microgravity seedlings exhibited an increase in curvature and bending and some plants were ‘disoriented’ in that the roots had grown into the air space above the growth medium.