Physiol, Plant, 49: 161-168, 1980 LEAF REGULATION OF INTERNODE GROWTH 161 The role of the leaves in the regulation of internode elongation in Phaseolus vulgaris By JAIME KIGEL The Hebrew University of Jerusalem, Dep, of Agricultural Botany, Rehovot, Israel (Received 13 August. 1979; revised 28 December, 1979) 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 ef- fects of leaf excision on leaf and internode elongation. The exci- sion of a young leaf increased the final length of internodes lo- cated 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 in- ternode ontogenesis in their relative response to the availability of assimilates on the one hand, and on the other hand to hor- monal factors transported acropetally from the young leaves to the growing internodes. Key-words: Bean, elongation, leaf-excision, auxin, correlative growth. Introduction During the vegetative growth of caulescent plants, the elongation of the shoot takes place in an orderly, sys- tematic manner. Successive internodes develop stepwise, one internode approaching maturity before the next en- ters its period of rapid growth (Sachs 1882). Relatively little attention had been given to the factors controlling this repetitive sequential pattern of development. Nevertheless, growth correlations between leaves and their adjacent internodes had been frequently studied. Excision of the apical bud (decapitation) and/or the young leaves reduces the elongation of the subjacent growing internodes in Coteus (Jacobs and Bullwinkel 1953), sunflower (Wetmore and Garrison 1966, Jones and Phillips 1966) and bean (Lockart 1964, Phillips 1971), but increased it in pea (Arney and Mancinelli 1967), A similar reduction in internode elongation was obtained in apple shoots, but excision of still younger leaves increased the length of internodes just below the excised leaves, while it reduced the length of those above them (Barlow and Hancock 1956), On the other hand, the excision of young primary leaves in bean enhanced the elongation of the stem above them (Valio and Schwabe 1978), From these "surgical" investigations it is evident that young leaves can promote or inhibit inter- node elongation. In the present work, an analysis of in- ternode elongation after the excision of leaves at different developmental stages, was used to clarify further the role of the leaves in the regulation of stem growth. Abbreviations: IAA, indoleacetic acid; NAA, naphthaleneacetic acid. Material and methods Dwarf French bean plants {Phaseotus vutgaris L. cv, Bul- garian) were raised from seed and grown singly in 1 liter drained plastic pots, in a substrate of 1:1 (by vol,) mixture of vermiculite and basalt gravel. The plants were irrigated each morning with half-strength, modified Hoagland nutrient solution (Went 1957), and flushed each after- noon with tap water, to prevent salt accumulation. Plants were grown at 22/17°C in 16-h photoperiods. Natural daylength was extended with incandescent light (10 mJ cm"^s~' at plant level) from 0400 to dawn, and from dusk to 2000. Plants for experiments were 2 weeks old, with nearly fully expanded primary leaves, just before the unfolding of the first trifoliate leaf. At this stage the cotyledons abscised, and the lengths of the first trifoliate leaf and subjacent internode were 15-20 and 4—6 mm, respec- lt Physiol, Plant, 49:2 0031-9317/80/060161-08 $03,00/0 © 1980 Physiologia Plantarum