Potential Anatomical Methods for the Determination of Weak Wood in Apple

Abstract

Two experiments were performed to study the anatomical traits related to the development of graft unions of relatively weak ('Honeycrisp'/'M.26 EMLA', 'Cripps Pink' cv. Maslin/'Geneva® 41', 'Scilate' (Envy™)/'Geneva® 41' and strong ('Honeycrisp'/'M.7 EMLA', 'Zestar!'/'M.26 EMLA', 'Zestar!'/'M.7 EMLA', 'Cripps Pink' cv.Maslin/'M.9 NAKB T337', 'Scilate' (Envy™)/'M.9 NIC29') scion/rootstock combinations of apple. The objective was to determine the cause of the weak unions so it may be used to develop a rapid screening tool to identify new potentially weak combinations. Fiber cell walls were thinner below and at the union in 'Honeycrisp' and 'Zestar!' when propagated on 'M.26 EMLA'. 'Honeycrisp' had significantly thicker cell walls at the union than 'Zestar!' combinations. 'Cripps Pink' and 'Scilate' combinations were thinner below and above the graft union on 'G.41' rootstocks. Trees propagated on 'M.26 EMLA' produced significantly less fiber tissues than those propagated on 'M.7' EMLA', and 'Honeycrisp' produced significantly less fiber and conductive tissues than 'Zestar!'. Laser ablation tomography (LAT) revealed weak and strong combinations both contained areas of poor xylem differentiation at the graft union. Xylem tissues at the graft union are highly variable, making it difficult to determine the strength of a scion/rootstock combination based off of anatomical features of the union alone.