Living plant systems: How robust are they in the absence of gravity?

The following hierarchical levels can be recognised in plant systems: cells, organs, organisms and gamodemes (interbreeding members of a community). Each level in this 'living hierarchy' is both defined and supported by a similar set of sub-systems. Within this framework of plant organization, two complementary questions are relevant for interpreting plant-oriented space experiments: 1) What role, if any, does gravity play in enabling the development of each organizational level? and 2) Does abnormal development in an altered gravity environment indicate sub-system inefficiency? Although a few representatives of the various organizational levels in plant systems have already been the subject of microgravity experiments in space laboratories-from cells in culture to gamodemes, the latter being found in some Closed Environment Life Support Systems-it would be of interest to investigate additional systems with respect to their response to microgravity. Recognition of the sub-systems at each level might be relevant not only for a more complete understanding of plant development but also for the successful cultivation and propagation of plants during long-term space flights and the establishment of plants in extra-terrestrial environments. (C)1999 COSPAR. Published by Elsevier Science Ltd.