Conclusion on the Vegetative Phase of Plant Development in Microgravity

The orientation of the primary axes that is vertical on the ground (see Figure 6-14) is variable in microgravity and these axes could be subjected to random walk (Johnsson et al. 1996) eventually after some nastic movements (due to asymmetrical growth) at the beginning of their development. The apical dominance of the primary root is reduced in actual and simulated microgravity in such a way that secondary roots elongate faster and are initiated very close to the primary root tip. According to Aarrouf et al. (1999), this could be due to the cessation of the activity of the root meristem provoked by a change in the hormonal balance (at least for IAA, ABA, and zeatin). Such analyses should be performed on shoots since it seems that apical dominance of the shoot meristem is also weakened.

In space experiments the environment was most often not controlled and the atmosphere was not monitored in plant growth chambers. As apparently microgravity has a slight but continuous effect on plant growth, it may happen that other space factors could become prominent in microgravity. That is the reason why experiments without an onboard 1-g centrifuge should be questioned.

Another problem deals with the data that must be numerous enough to be analyzed by statistics, which is obviously in opposition with the need of using very small volumes or masses in space. New hardware is being developed like EMCS, for European Multi-Cultivation System, which will provide gas control and the availability of growing a (small) plant from seed to seed. This instrument should be helpful for determining the mechanism of the action of gravity on Earth (Figure 6-21).

Figure 6-21. Astronaut Chiaki Mukai works with plant sprouts on board the Space Shuttle Discovery. Photo courtesy of NASA.
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