An important factor affecting the uniformity of in vitro cultures is the topophysical position of the original explant. We investigated this phenomenon in Handroanthus guayacan, a tropical woody tree species. Shoots from a stock culture were separated into upper, middle and basal sections and transferred to a modified MS medium containing meta-topolin-riboside and indole-butyric acid. After 8 weeks, the middle section produced the most shoots, the longest shoots and the highest number of nodes per plant. Shoots derived from the upper section were elongated, but had the shortest internodes, while those from the basal section formed the largest callus. None of the three types of explants rooted during the proliferation phase. The topophysically dependent spatial distribution of endogenous cytokinins and auxins was determined. The topophysical effect observed could not be explained solely by analyzing the endogenous isoprenoid and auxin. However, the metabolism and distribution of the aromatic cytokinin could provide an explanation. The concentration of the meta hydroxy-substituted topolins was highest in shoots derived from the middle section. Aromatic N- and O-glucosides were much more concentrated in the leaves than in the stems. In conclusion, it is recommended to consider the explant's topophysis when developing a multiplication protocol to avoid heterogeneity in an in vitro culture.
Keywords: Handroanthus guayacan; N-glucosides; O-glucosides; cytokinin metabolites; micropropagation medium; topophysis.