Nuclear magnetic resonance imaging in studies of gravitropism in soil mixtures

Gravitropic responses of oat coleoptiles were measured in different growth media; humid air, natural soil and artificial soil (grass beads). The oat c oleoptiles in soil and glass beads were monitored by NMR imaging, while tho se in humid air were imaged in darkness with an infrared-sensitive charge-c oupled device (CCD) camera. The present study shows for the first time that gravitropic experiments can be performed in artificial soil using NMR imag ing as a convenient and suitable recording method. Not only was it possible to follow the gravitropic curvatures in natural soil, but the artificial s oil allowed plant images of sufficient spatial and temporal resolution to b e recorded. The advantages of using artificial soil in magnetic resonance i maging studies are that the iron content of glass beads is very low compare d with natural soil, and that the artificial soil matrix can easily be stan dardized with regard to particle size distribution and nutrient content. Tw o types of glass beads were used, the diameter of the small and the large b eads being 300-400 and 420-840 mu m, respectively. The growth rate of the c oleoptiles 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 app rox. 16%. The bending rate of the coleoptiles during the gravitropic respon se was reduced by c. 65% in soil and 75% in bead mixtures relative to bendi ng in air. It should be noted, however, that the maximum curvature of the c oleoptile tip was of the same order in all cases, about 35 degrees. This va lue may represent the largest possible curvature of the organ. The potentia l of NMR imaging to study how plant organs penetrate the soil under the inf luence of gravitropism, mechanical impedance and thigmotropism is also disc ussed.