EVIDENCE OF ROOT ZONE HYPOXIA IN BRASSICA RAPA L. GROWN IN MICROGRAVITY

A series of experiments was conducted aboard the U.S. space shuttle and the Mir space station to evaluate microgravity‐induced root zone hypoxia in rapid‐cycling Brassica (Brassica rapa L.), using both root and foliar indicators of low‐oxygen stress to the root zone. Root systems from two groups of plants 15 and 30 d after planting, grown in a phenolic foam nutrient delivery system on the shuttle (STS‐87), were harvested and fixed for microscopy or frozen for enzyme assays immediately postflight or following a ground‐based control. Activities of fermentative enzymes were measured as indicators of root zone hypoxia and metabolism. Following 16 d of microgravity, ADH (alcohol dehydrogenase) activity was increased in the spaceflight roots 47% and 475% in the 15‐d‐old and 30‐d‐old plants, respectively, relative to the ground control. Cytochemical localization showed ADH activity in only the root tips of the space‐grown plants. Shoots from plants that were grown from seed in flight in a particulate medium on t...