Axillary Shoot Abscission in Pecan and Its Relationship to Growth Regulators

Abstract

The relatively low incidence of axillary shoot development in pecan [Carya illinoensis (Wangenh.) C. Koch] and its detrimental impact on nut productivity prompted: 1) an evaluation of methods to induce axillary shoot morphogenesis, and 2) estimation of levels of endogenous phytohormones in abscising and nonabscising axillary shoots. Retention of developing shoots from 1-year-old terminals was increased three-fold by ringing. Application of benzyladenine (BA) delayed, but did not prevent, shoot abscission, whereas abscisic acid (ABA), indoleacetic acid (IAA), and gibberellic acid (GA3) applications had no detectable effect. Normal morphogenesis of axillary shoots was independent of the presence of developing catkins. Phytohormones tentatively identified in pecan axillary shoots and catkins were IAA, ABA, dihydrozeatin riboside (DHZR), t-zeatin riboside (ZR), trans-zeatin (Z), and dihydrozeatin (DHZ). A possible influence of these substances on budbreak and shoot morphogenesis is discussed. Axillary shoots eventually abscising exhibited a substantially higher level of IAA-like and ABA-like substances than did shoots completing morphogenesis. Chemical names used: N-(phenylmethyl)-1H-purin-6-amine(BA); abscisic acid (ABA); 1H-indole-3-acetic acid (IAA); gibberellic acid (GA,).