Growth of Pottia intermedia protonemata in altered gravity.

Plants are immobile; therefore, they are oriented in space due to growth movements--tropisms. The latter occur in response to environmental stimuli such as gravity (gravitropism), light (phototropism), chemical compounds or water (chemo- and hydrotropisms). Gravity is the only force that was impossible to control. The moss protonemata are among the limited group of plant objects with tip growth. What is unique about this structure is that protonemal apical cells both sense and respond to gravity. It is considered that the apical cell perceives gravity through amyloplasts (Sack, 1993; Chaban, 1996). Although the dynamics of protonemata negative gravitropism in different moss species was studied in detail, the role of gravity in both the structural polarity of apical cells and the formation of protonematal mat with circular symmetry is completely unexplored. Using the unique possibility to fly the moss on the space shuttle (STS-87) we aimed in this study to analyze the character of the interaction of gravity with light and endogenous factors in the pattern of protonemata space orientation.