The role of the leaves in the regulation of internode elongation in Phaseolus vulgaris

Abstract Time‐course patterns of leaf and internode elongation were studied in bean plants. Each leaf started its main elongation period when the leaf below reached half of its final length. The onset of leaf unfolding was nearly synchronous with the initiation of the elongation of the subjacent internode. Excision of young leaves increased the rate of stem elongation as a result of an earlier unfolding of the next upper leaves and the concomitant advancement in the elongation of their subjacent internodes. IAA or NAA (1% in lanolin) suppressed the enhancement effects of leaf excision on leaf and internode elongation. The excision of a young leaf increased the final length of internodes located below it, and at the same time decreased the final length of the internodes located above the excised leaf. The reduction was greater the younger the internode. Differences in internode elongation after leaf excision were related to changes during internode ontogenesis in their relative response to the availability of assimilates on the one hand, and on the other hand to hormonal factors transported acropetally from the young leaves to the growing internodes.