Mechanical stress and gibberellin: Regulation of hollowing induction in the stem of a bean plant, phaseolus vulgaris l.

In pole bean plants, mechanical stress (MS) inhibited stem elongation and induced radial thickening of the stem. Application of uniconazole, an inhibitor of gibberellin biosynthesis, also reduced stem growth but had no effect on stem diameter. Both MS and uniconazole significantly reduced hollowing of the first internodes, but only the former increased ethylene evolution from the first internode. Application of GA₃ increased the length of the first internode and decreased its diameter in bush bean plants; this was accompanied by a significant promotion of stem hollowing. Aminooxyacetic acid (AOA) decreased ethylene evolution from the GA₃-treated internodes, though it did not reduce the GA₃-induced hollowing of the first internodes. Application of GA₃ affected neither ethylene evolution nor cellulase activity in the first internodes of bush bean plants. Application of GA₃ stimulated much greater cell elongation in the center of pith tissue than in the outer surrounding tissues, suggesting a possible physical breakage of the inner cells, which leads the hollowing of bean stems. These results suggest that gibberellin is a factor responsible for stem hollowing in bean plants. Because MS is known to reduce gibberellin content in bean plants [Suge (1978) Plant Cell Physiol. 21: 303] MS may inhibit stem hollowing by reducing the amount of endogenous gibberellin.