Gravitropic responses of oat coleoptiles were measured in different growth media; humid air, natural soil and
artificial soil (glass beads). The oat coleoptiles in soil and glass beads were monitored by NMR imaging, while
those in humid air were imaged in darkness with an infrared-sensitive charge-coupled device (CCD) camera. The
present study shows for the first time that gravitropic experiments can be performed in artificial soil using NMR
imaging as a convenient and suitable recording method. Not only was it possible to follow the gravitropic
curvatures in natural soil, but the artificial soil allowed plant images of sufficient spatial and temporal resolution
to be recorded. The advantages of using artificial soil in magnetic resonance imaging studies are that the iron
content of glass beads is very low compared with natural soil, and that the artificial soil matrix can easily be
standardized with regard to particle size distribution and nutrient content. Two types of glass beads were used,
the diameter of the small and the large beads being 300–400 and 420–840 μm, respectively. The growth rate of the
coleoptiles in soil and in big beads was roughly the same and only slightly lower than in humid air, whereas small
beads reduced the growth rate by approx. 16%. The bending rate of the coleoptiles during the gravitropic response
was reduced by c. 65% in soil and 75% in bead mixtures relative to bending in air. It should be noted, however,
that the maximum curvature of the coleoptile tip was of the same order in all cases, about 35°. This value may
represent the largest possible curvature of the organ. The potential of NMR imaging to study how plant organs
penetrate the soil under the influence of gravitropism, mechanical impedance and thigmotropism is also discussed.