Controlling rootstock sprouts of Agonis flexuosa, (Willd.) Sweet at ambient and elevated CO₂ by multiple applications of low concentration NAA

Abstract

A method for grafting variegated scion material to green leafed Agonis flexuosa (Willd.) Sweet stock was developed to overcome the difficulty of striking cuttings. However sprouting of both seedling and cutting-grown A. flexuosa rootstocks was a significant problem. Microwedge grafting of actively growing leafy scions and stocks in fog at 32/22¦C gave 90-100% success and scion bud activity was stimulated within 2 weeks. Weekly or fortnightly spray applications of 100 mg NAA/L (napthaleneacetic acid), starting at the time of grafting, gave effective sprout control whereas a single pre-graft spray of 200 mg NAAL was not effective. CO2 enrichment of the fog environment was investigated as a means of enhancing scion growth. CO2 at 80 kPa increased scion dry weight (DW), leaf and branch numbers, but had no effect on rootstock sprout or stem DW or sprout numbers. Optimum NAA concentrations for rootstock sprout suppression under elevated CO2 with fog, were 50-100 mg/L, which were not deleterious to scion shoot length, when sprayed on the stock portion only. Stock sprout numbers, scion leaf and branch numbers were negatively correlated with NAA concentration. Sprout growth at 100 mg NAA/L was about 5% of control sprout growth. Concentrations >200 mg NAA/L caused leaf tip necrosis and excess stem callusing. Scion growth was inversely related to the degree of resprouting in control treatments.