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Thermally induced changes in shoot morphology of Hancornia speciosa microcultures: evidence of mediation by ethylene.

A shoot microculture protocol for the tropical fruit tree Hancornia speciosa Gom. was developed based on high temperature stimulation of axillary branching. An increase in the culture temperature from 25 to 31 degrees C (continuous over a 4-week period) stimulated the elongation of newly subcultured micro-shoots. Increases in culture temperature from 31 to 35 degrees C suppressed elongation but induced branching of all shoots. Increasing temperature to 37 degrees C reduced both shoot elongation and branching severely. Thermoperiodic regimes involving daily exposure for 8 or 16 h at 35 degrees C followed by 31 degrees C for the remainder of the day were only moderately effective in stimulating branching. However, maintaining cultures at 35 degrees C for 2 weeks followed by growth at a constant temperature of 31 degrees C led to vigorous branching. We hypothesize that thermally induced branching is mediated by a reduction in ethylene biosynthesis. In cultures of nodal segments grown at 31 degrees C, ethylene evolution peaked 12 to 16 days after subculture, and then gradually decreased until the end of the culture cycle. Compared with cultures held at 31 degrees C, those grown at 35 degrees C showed an earlier peak and a reduced rate of ethylene evolution throughout most of the culture cycle, and less 1-aminocyclopropane-1-carboxylate (ACC) synthase mRNA transcript and ACC oxidase activity. An inhibitor of ethylene biosynthesis (L-(2-aminoethoxyvinyl)-glycine) at 4.5 &mgr;M mimicked the effect of elevated temperature (35 degrees C) in stimulating lateral branching. The branching pattern of shoots grown at 35 degrees C in the presence of 1 &mgr;l l(-1) ethylene gas resembled that of shoots grown at 31 degrees C. We conclude that a reduction in ethylene evolution has a role in thermal induction of branching in H. speciosa. Heat induced release of axillary buds may be useful in the microculture of trees with strong apical dominance that cannot be overcome by cytokinin treatment.