The influence of simulated low-gravity environments on growth, development and metabolism of plants.

Low-gravity environments may be simulated through appropriate horizontal clinostat rotation. This simulation is accomplished through a biological nullification of the directional component of gravitational force. Measuring biologically effective gravity force by organ response, it is readily demonstrated that biologically active gravitational force may be treated as a two-dimensional vector. Though the magnitude dimension of this vector remains virtually constant anywhere on earth, the biologically effective direction dimension may be quantitatively altered by clinostat rotation, provided appropriate angular velocities and angles of inclination of clinostat axes are employed. Using oat seedlings, a rotation rate of 2 rpm, and a horizontal axis clinostat, a 'zero g' environment may be simulated. This simulated 'zero g' condition is attested by the inability of plants to perceive unidirectional gravitational force of sufficient magnitude to elicit directional growth. Under such conditions, plants will grow in the direction imparted by the initial orientation of the plants in the system. Geotropic curvature responses to subsequent geostimulation are, however, greater in seedlings grown under these conditions, nullifying the direction dimension of gravitational force, than in seedlings grown with rotation but with normal unidirectional gravity loads. Root growth under simulated 'zero' gravity conditions is likewise enhanced as compared to plants grown with rotation but normal unidirectional gravity. These differences in magnitude of growth and response to subsequent geostimulation are inexplicable on bases of modified auxin economy or production. Respiration rates are, however, materially enhanced by the simulated 'zero g' environments. This enhancement of respiration, as well as growth, quantitatively diminishes as the unidirectional gravity load is increased. These results imply that the primary effect of low-gravity environments is likely that of modifying the magnitude of respiratory metabolism. Such a modification is partly or wholly explicable on a basis of a modified distribution pattern of metabolically active particulate cell inclusions.