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 specios a Gem, was developed based on high temperature stimulation of axillary bran ching. An increase in the culture temperature from 25 to 31 degrees C (cont inuous over a 4-week period) stimulated the elongation of newly subcultured micro-shoots. Increases in culture temperature from 31 to 35 degrees C sup pressed elongation but induced branching of all shoots. Increasing temperat ure to 37 degrees C reduced both shoot elongation and branching severely. T hermoperiodic 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 moderatel y effective in stimulating branching. However, maintaining cultures at 35 d egrees C for 2 weeks followed by growth at a constant temperature of 31 deg rees C led to vigorous branching. We hypothesize that thermally induced bra nching is mediated by a reduction in ethylene biosynthesis. In cultures of nodal segments grown at 31 degrees C, ethylene evolution peaked 12 to 16 da ys after subculture, and then gradually decreased until the end of the cult ure cycle. Compared with cultures held at 31 degrees C, those grown at 35 d egrees C showed an earlier peak and a reduced rate of ethylene evolution th roughout most of the culture cycle, and less 1-aminocyclopropane-1-carboxyl ate (ACC) synthase mRNA transcript and ACC oxidase activity. An inhibitor o f ethylene biosynthesis (L-(2-aminoethoxyvinyl)-glycine) at 4.5 mu M mimick ed the effect of elevated temperature (35 degrees C) in stimulating lateral branching. The branching pattern of shoots grown at 35 degrees C in the pr esence of 1 mu l l(-1) ethylene gas resembled that of shoots grown at 31 de grees C. We conclude that a reduction in ethylene evolution has a role in t hermal induction of branching in H. speciosa, Heat induced release of axill ary buds may be useful in the microculture of trees with strong apical domi nance that cannot be overcome by cytokinin treatment.