Further in vitro studies of light-induced root nodulation in the cycadales

Abstract

Seedlings of Cycas revoluta (Cycadaceae), Encephalartos altensteinii and Zania furfuracea (Zamiaceae) were grown aseptically in light and darkness. In the dark, C. revoluta seedlings produced a tap root system with ageotropic or plagiotropic secondary roots. Callus formation occurred when secondary roots grew out of the agar, where lateral roots emerged from the primary root and where the primary root joined the shoot. Massive callus also developed along the upper primary root axis. Results with Z. furfuracea were similar except that callus occurred only where secondary roots emerged. With E. altensteinii secondary roots produced near the root-shoot junction were apogeotropic but were not nodular in the dark. More acropetally formed roots had pointed apices and were ageotropic or plagiotropic. Callus formation occurred where secondary roots emerged from the primary root, but otherwise was not observed in darkness. Exposure to fluorescent light caused lateral root nodulation in all species. Light induced subapical callus formation by primary and secondary roots of C. revoluta and with secondary roots of Z. furfuracea but not with E. altensteinii roots. Otherwise, callus formation by primary roots of all species was similar to that observed in darkness. Light also severely inhibited secondary root production and root elongation.