Physiological response to temporary changes in gravity conditions on plants.

Gravity is the main factor that influences the direction of growth of plant organs, and has also a direct effect on the plant metabolism. When an organ, mainly roots, is turned by between 0° (vertical) and 90° (horizontal), the change of orientation is perceived by its organs producing the so-called gravitropic reaction (Perbal and Driss- Ecole, 2003), which involves a strong metabolic response. In order to study these reaction in real microgravity conditions, some experiments have been set up during six ESA parabolic flight campaign. Oxygen concentration in the solution where roots of Zea mays were placed have been constantly monitored during normal, hyper- and microgravity conditions. An evident burst in oxygen fluxes started just 2.0 ± 0.5 s after the imposition of microgravity conditions. No significant changes were noticed neither in normal nor in hypergravity conditions. Moreover, oxygen bursts were detected only in the root apex zone The significance of these results is dramatic on the nature and location of the graviperception. This spike-like activation/deactivation of oxygen when growing roots were exposed to microgravity situation can be detected at each parabola with the same results, whereas the hypergravity situations, which are imposed inbetween, do not intefere with the microgravity-induced bursts. Concerning the different location of the selective oxygen microlectrodes, oxygen bursts happened only in the root apex, whereas no changes in the physiological activity was never identified in the mature zone of the root, showing a clear role of the root apex region in detecting environmental changes, which is the case of microgravity conditions. Moreover, control treatments without roots showed no responses in both the microelectrodes placed at root apex and mature region levels confirming the real and effective role of root apex. Concurrently measurments of oxygen consumption of root apex were done using oxymeters, a difference in oxygen consumption was aspected during the different gravity conditions, but measurments revealed the onset of long lasting oxygen bursts appearind only during microgravity, demonstrating once again the result obtained with the oxygen electrodes. Although the chemical nature of these oxygen bursts is sill unknown, they may implicate a strong generation of reactive oxygen species as they exactly match the microgravity situation. Thus, our data strongly suggest that the sensing mechanism is not related to a general mechano-stress, which was imposed also during hypergravity, but is very specific of the microgravity situation. Moreover, it is well-known that stress rapidly induces reactive oxygen bursts which are associated with oxygen influx and reactive oxygen efflux from stressed plant tissues.