Circumnutations of Sunflower Hypocotyls in Satellite Orbit

Abstract

The principal objective of the research reported here was to determine whether a plant's periodic growth oscillations, called circumnutations, would persist in the absence of a significant gravitational or inertial force. The definitive experiment was made possible by access to the condition of protracted near weightlessness in an earth satellite. The experiment, performed during the first flight of Spacelab on the National Aeronautics and Space Administration shuttle, Columbia, in November and December, 1983, tested a biophysical model, proposed in 1967, that might account for circumnutation as a gravity-dependent growth response. However, circumnutations were observed in microgravity. They continued for many hours without stimulation by a significant g-force. Therefore, neither a gravitational nor an inertial g-force was an absolute requirement for initation or continuation of circumnutation. On average, circumnutation was significantly more vigorous in satellite orbit than on earth-based clinostats. Therefore, at least for sunflower (Helianthus annuus L.) circumnutation, clinostatting is not the functional equivalent of weightlessness.

Circumnutation of rice coleoptiles: its relationships with gravitropism and absence in lazy mutants

“…On the other hand, Chapman & Brown (1979) and Brown & Chapman (1988) reported that the circumnutation did not disappear on horizontal and three-dimensional clinostats, although the number of circumnutating seedlings and the amplitude of circumnutation decreased. The latter group obtained qualitatively similar results under microgravity (Brown & Chapman 1984;Brown et al 1990). It has been suggested that the difference between the two groups is due to the difference in the sunflower cultivars used (Zachariassen et al 1987).…”

Circumnutation as a visible plant action and reaction

“…In this model, earth gravity does not play a crucial and decisive role in generating circumnutations. Studies on sunflower circumnutations in the circumterrestial orbit, which excluded the significance of gravity in generating circumnutations, contributed to the revival of Darwin's model, 12 in which the inner oscillator is an indispensable element in generating circumnutations.…”

“…19 In Helianthus annuus, changes in the direction may be evoked by a geotropic stimulus; 6 in the microgravity conditions of the circumterrestial orbit, the direction of circumnutations changes more frequently than on the earth. 12 Johnsson 22 shows changes in the direction in the sunflower accompanied by a change in the amplitude and frequency of circumnutations. The study of Israelsson and Johnsson 7 revealed that 60% of the sunflower population studied circumnutated in the ccw direction, and 25%-cw; the others displayed pendulum-like movement.…”

“…5 In experiments with the use of the clinostat and centrifuge as well as the microgravity in the circumterrestial orbit, it was repeatedly demonstrated that changes in gravity affected the intensity, period and amplitude of circumnutations. 7,12,13 Recent investigations of Arabidopsis thaliana and Pharbitis nil mutants scr i pgm (having defective differentiation of endodermal cells) reveal that gravitropic response is an essential component in plant shoot circumnutation. [38][39][40] Circumnutations as the Result of Actions of Endo-and Exogenous Factors.…”

“…Three concepts can be distinguished: one-circumnutations are of endogenous origin, the second-circumnutations are a result of earth gravity, thus they are exogenous, and the third and most recent idea-circumnutations stem from both endogenous and exogenous processes. 6,12,22,[32][33][34] Endogenous Nature of Circumnutations-An Inner Oscillator. Charles Darwin, 3 the author of the concept of endogenous character of circumnutations, published already in 1880, described circumnutations in numerous plant species and claimed that they were endogenous and other types of movement were their modification.…”