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Research in plant physiology consists mostly of studies on plant growth because almost everything a plant does is done by growing. Most aspects of plant growth are strongly influenced by the earth's gravity vector. Research on those phenomena address scientific questions specifically about how plants use gravity to guide their growth processes.

The use of centrifugation as a tool for exploring qualitative and quantitative features of plant responses to gravity and to other body forces can be traced to plant scientists' early 19th century experiments. To study how plants perceive and respond to gravitational stimuli requires experimental manipulation of the force or acceleration vector direction and magnitude. As technology advanced, especially during the past half-century, so did the sophistication of experimental designs and of the scientific questions that could be addressed. The most significant improvement in methodology probably will prove to be attainment of experimental access to the hypogravity range of inertial accelerations by the combination of centrifugation and spaceflight.

This workshop on Gravity and the Cell was modeled on a 1968 conference on Gravity and the Organism. Each conference tried to identify the most salient scientific questions about how gravity is important to living systems. In the roughly two decades between the conferences there have been impressive advances in experimental methods, but the major scientific questions that have driven their applications to problems of gravitational biology, i.e., our broad research goals, remain much the same. In the case of plant research, improvements and extensions of biochemical techniques for investigating the ways organisms use environmental (g-force) information in salubrious ways has kept pace with progress in plant biochemistry. We now know much more about the roles of organic and inorganic substances that plants employ for information transfer and growth regulation and about the avenues and mechanisms for transport of those key substances within organisms. We have seen the acceptance of a "local control" concept that regulation of g-responses depends critically on plasmodesmata and gap junctions which allow plants and animals to throttle the transport of growth regulators across tissue boundaries often, especially in the case of plants, far removed from the morphological regions of concentration of statocytes that were once thought to be the exclusive bioaccelerometers used by plants. NASA's sponsorship of ground-based research in gravitational biology has served as important underpinning for orbital flight programs in space biology. The currently most noteworthy research area of ignorance is the mechanism by which the physical event of g-force susception becomes the biological process of g-force perception. Only rarely has it been possible to perform a definitive test of a theory of mechanism of gravity perception. Therefore, experimental research efforts in gravitational physiology still are essentially dependent on exploratory studies for which stimulus/response experiments require experimental manipulation of test subjects' g-force environment. Many research efforts have focused narrowly on contrasting the responses of test subjects to only two gravitational environments, 1 g versus real or simulated zero g. Much less effort has been devoted to studies at other g levels. For experiments in gravitational physiology three rotating machines have made, and no doubt will continue to make, important contributions: the centrifuge, the clinostat, and, most of all, the orbiting space vehicle. Some research trends, viewpoints, and shifts of emphasis are discussed.

Dark-grown, 4-day old, Helianthus annuus seedlings were rotated for 20 hr on horizontal clinostats to minimize the amplitude of circumnutation. Then a Plexiglas sheet was placed gently against the tip of the cotyledons. By time-lapse video imaging (using intermittent IR illumination to which the plants were insensitive) movements of the clinostatted plants were observed before, during, and after the period of mechanical contact. Immediately after the Plexiglas sheet was removed residual nutation increased in amplitude almost three-fold, then declined over the next 7 hr to the prestimulation level. This demonstration of enhancement of circumnutation by mechanical contact is consistent with the model of an endogenous oscillator that can be stimulated by factors other than gravity.

This introduction to the Topical Meeting on Gravity Sensing Organs specifically addresses the topic's specific descriptors: origin, evolution, and comparative physiology.

Growth process generate plant form and relate to most physiological functions. The Earth's gravity force affects plant growth in both obvious and subtle ways. It is a major environmental influence on morphology and physiology of plants. Gravity is less important as an agent for plant stress than as an environmental signal to guide growth. The plant's bioaccelerometers are remarkably sensitive, especially in hypogravity. Simulation (clinostat) studies and experiments in satellite laboratories are needed to understand the sensing, transduction, and response characteristics of g related mechanisms. By examining how plants alter growth processes to accomplish developmental or physiological "objectives" we may find it pragmatically desirable to ask ourselves how we might design a plant to achieve such responses to environmental influences. Examples of this design engineering approach for gravity related effects are described as an aid to experimentation.

The kinetics of hypocotyl nutations in Helianthus annuus L. were measured on plants which were rotated on clinostats with axes of rotation inclined at various angles, alpha, away from the vertical. The g-force component acting in the direction of the plant axis was taken as g cos alpha. The average period and average amplitude of nutation were constant for all such axially directed g-forces between 1.0 and 0.2 g (vertical to about 80 degrees inclination). On the horizontal clinostat (90 degrees inclination) nutation was neither initiated nor sustained. The g-force just sufficient fully to activate nutational oscillations should be sought in the range of the g-force parameter, 0 < g < 0.2.

For over half a century and especially since the 1960's a number of plant physiologists, seeking to explain the impressively ubiquitous mechanism that drives and regulates circumnutation in all growing plant organs, have been unable to agree on whether the differential growth process that leads to circumnutational oscillations is gravity dependent. There has been fairly general agreement that the question might be answered, if test plants could be deprived of all significant gravitational stimuli as would be possible in the near weightlessness or free fall environment of satellite orbit. Such an experiment was carried out during the Spacelab 1 mission. Circumnutational oscillations were observed which demonstrated that a protracted input of gravitational information from the environment was not required for initiation or maintenance of circumnutation in sunflower hypocotyls.

Circumnutations of hypocotyls of sunflower (Helianthus annuus L. cv. Californicus) were studied under 1 g and 3 g conditions. Root mean square values of the hypocotyl deviation from the plumbline and period of the movements were determined from calculations of the autocorrelation functions of the movements. The amplitude and the period of the circumnutations increased under 3 g as compared to 1 g. A transition from 3 to 1 g or vice versa also caused changes in period and amplitude of the movements. The results are interpreted as a support for the idea that gravity influences the circumnutation parameters in this sunflower variety. A comparison is made with published results on the dwarf sunflower cv. Teddy Bear where the force influence is very small or negligible. Simulations of a model for circumnutations show movements which are in qualitative agreement with the experimental results, provided adaptation to g-levels is included in the model. Finally, the results are discussed with the recent Spacelab-experiment (SL1) as a background.