Studies in Abscission: I. FACTORS AFFECTING THE INHIBITION OF ABSCISSION BY SYNTHETIC GROWTH-SUBSTANCES

1. The general method is described for experimenting on applepedicelsor petioles from which the terminal organ has beenremoved.
2. Two quantitative techniques for applying growth-substancesaregiven in detail, one employing lanolin emulsion as the carrier,the other agar gel.
3. By these methods certain factors affectingthe inhibition ofabscission by the growth-substances -naphthylaceticacid (NAA)and 2:4-dichloro-phenoxyacetic acid (DPOA} have beeninvestigated. A relation between concentration of the growth-substanceandpersistence of the pedicel was established which was usuallylinear. In an experiment with NAA, for instance, the relationwas represented by— Persistence in days=6·52+0·0127xconcentrationin p.p.m.
4. The acid and the sodium salt of both NAA and DPOAgave similarresults when applied in lanolin emulsion.
5. Onboth young pedicels and young shoot internodes NAA was foundto exert a ‘formative’ influence resulting in increasein diameter which was not exhibited by DPOA to the same extent.
6. At least for concentrations of either growth-substance upto100 p.p.m., these were effective only when applied distallyto the abscission zone. At 1,000 p.p.m. there was an effectof application on the proximal side of the abscission region,but it was small compared with the effect of the same concentrationapplied distally.
7. The length of the path of the growth-substancefrom the pointof application to the abscission zone affectedthe degree ofinhibition inversely, the longer the path theless the effectof a given concentration in delaying abscission.The contrastbetween these findings for pedicels, and thoseof other authorsfor petioles, is briefly discussed.
8. A criticalperiod occurs shortly after removing the leaf-bladefrom itspetiole during which the abscission process is initiated,andto be effective in inhibiting this process growth-substancemust reach the abscission zone within this period. It is notat present established whether the pedicel shows a similar modeof behaviour.
9. A working hypothesis is given to account forthe control ofabscission by natural hormones, and extensionsare describedto embrace the results of experiments with syntheticgrowth-substances.