This CHROMEX flight experiment was a continuation of the experiment conducted on CHROMEX-01, which flew on STS-29. There were two primary objectives: 1) to test whether the normal rate, frequency and patterning of cell division in the root tip can be sustained in microgravity; 2) to determine whether the fidelity of chromosome partitioning is maintained during and after flight.
Twenty-five shoots derived from aseptic suspension cultures of the monocot daylily (Hemerocallis cv Autumn Blaze) and 75 tissue-cultured plantlets of the dicot Haplopappus gracilis were co-cultured in the Plant Growth Unit with the Air Exchange System. All plants had their roots severed prior to flight and were grown aseptically for 4 days in six Plant Growth Chambers on horticultural foam containing growth medium. The major change in protocol between this flight and CHROMEX-01 was the use of extensively washed horticultural foam, which necessitated the addition of a wetting agent to the medium to facilitate its uptake into the foam matrix. Upon recovery, space-grown root tips were fixed and subsequently examined for rates of cell division and chromosomal aberrations. Specimens were also dedicated to overall shoot and root growth measurements. Selected individuals were successfully rescued for multi-generational postflight studies on the ground.
Root growth occurred randomly in all directions in space. In contrast, growth was uniformly positively gravitropic in ground controls. Space-flight and ground-control plants produced an equivalent amount of tissue, and maintained their characteristic root-production patterns. Seedling-derived plantlets produced roots that were numerous but relatively short. Capitulum-derived plantlets produced fewer roots, but they were on average longer than those of the seedling-derived plantlets. Thus, the clonal root phenotype was not changed in space, at least for the short duration of the experiment. However, plantlets from both sources? exhibited total root-production values that were 67-95% greater than those obtained in their ground-control counterparts. The investigator proposed that microgravity brought about an altered moisture distribution pattern in the foam growth substrate, giving a more moist and thereby more favorable environment for root formation.