Nodulation in light- and dark-grown Macrozamia communis L. Johnson seedlings in sterile culture

The ability of aseptically cultured Macrozamia communis seedlings to form apogeotropic root nodules in light and darkness was examined. In darkness, apogeotropic root nodules developed at the junction of the primary-root and the cotyledonary node. More acropetally formed lateral-roots were not nodular and were typical ageotropic or plagiotropic secondary-roots. When these cultures were exposed to light, all of the roots became typical, apogeotropic cycad root nodules. Light stimulated dichotomous branching of nodules previously formed in the dark. Nodules exposed to light were more papillose than those maintained in darkness. Light also inhibited primary and secondary-root elongation.