Studies in Abscission

Abstract

The general method is described for experimenting on apple pedicels or petioles from which the terminal organ has been removed. Two quantitative techniques for applying growth-substances are given in detail, one employing lanolin emulsion as the carrier, the other agar gel. By these methods certain factors affecting the inhibition of abscission by the growth-substances α-naphthylacetic acid (NAA) and 2:4-dichloro-phenoxyacetic acid (DPOA} have been investigated. A relation between concentration of the growth-substance and persistence of the pedicel was established which was usually linear. In an experiment with NAA, for instance, the relation was represented by— Persistence in days=6·52+0·0127×concentration in p.p.m. The acid and the sodium salt of both NAA and DPOA gave similar results when applied in lanolin emulsion. On both young pedicels and young shoot internodes NAA was found to exert a ‘formative’ influence resulting in increase in diameter which was not exhibited by DPOA to the same extent. At least for concentrations of either growth-substance up to 100 p.p.m., these were effective only when applied distally to the abscission zone. At 1,000 p.p.m. there was an effect of application on the proximal side of the abscission region, but it was small compared with the effect of the same concentration applied distally. The length of the path of the growth-substance from the point of application to the abscission zone affected the degree of inhibition inversely, the longer the path the less the effect of a given concentration in delaying abscission. The contrast between these findings for pedicels, and those of other authors for petioles, is briefly discussed. A critical period occurs shortly after removing the leaf-blade from its petiole during which the abscission process is initiated, and to be effective in inhibiting this process growth-substance must reach the abscission zone within this period. It is not at present established whether the pedicel shows a similar mode of behaviour. A working hypothesis is given to account for the control of abscission by natural hormones, and extensions are described to embrace the results of experiments with synthetic growth-substances.