SEED GERMINATION AND EARLY PLANT MORPHOLOGY - RESULTS FROM THREE MICROGRAVITY MISSIONS

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.

Plants grown in darkness and microgravity had overall lower mass and were smaller than the corresponding ground control samples, especially after longer growth periods (>5 days). Hypocotyl hook development proceeded normally for alfalfa and clover. In lettuce, hook formation did not occur for some plants depending on the orientation of the seed inside the substratum.