Gravitropism of oat (Avena sativa L.) and wheat (Triticum aestivum L.) 
coleoptiles was investigated in relation to the displacement angle or to the 
initially set stimulation angle (SA). We measured curvature rates at the early 
phase of curvature, before it was affected by the drop in SA resulting from the 
curvature response itself. The plot of the rates against the sines of initial 
SAs revealed similar curves for oats and wheat, which approached saturation as 
the sine increased to unity. The two species and previously analyzed rice [Iino 
et al. (1996) Plant Cell Environ. 19: 1160] appeared to have similar 
gravisensitivities. Initial SAs below and over 90 degrees yielded comparable 
rates when the sine values were the same, indicating that the extent of 
gravitropism is determined by the gravity component perpendicular to the organ's 
long axis. Long-term curvature kinetics at different SAs indicated that the net 
curvature rate dropped sharply before the tip reached the vertical position and 
then the tip approached the vertical slowly, with clear oscillatory movements in 
the case of wheat. During this late curvature phase, the coleoptile straightened 
gradually, although none of its parts had yet reached the vertical. When rotated 
on horizontal clinostats or displaced upwards to reduce SA in the late curvature 
phase, coleoptiles bent in the opposite direction. These results demonstrated 
that autotropism counteracts gravitropism to straighten coleoptiles.