Influence of the g-force on the circumnutations of 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.