Prevention of auxin-induced vascular differentiation in wound callus by surface-to-surface adhesion between calluses of stock and scion in cactus grafts

Abstract

The relationship between formation of CVB, which connects vascular cut ends of stock and scion, and the condition of tissue where CVB is expected to be induced was studied using a cactus graft combination of Notocactus submammulosus var. pampeanus young seedlings on Hylocereus trigonus plants. When a scion with its basal part removed by a transverse cut was put onto a transverse cut surface of a stock with a distance of 1 mm between the vascular cut ends of both graft components, CVB was formed in only 13% of the grafts in the callus mass made by adhesion between calluses of stock and scion. The percentage arose to ca. 100% when 100 ppm NAA was applied to the scion apices. This promotive effect of NAA decreased with increasing time from grafting to NAA application. When wound calluses of stock and scion were prevented from contact only at the portion between the vascular cut ends of stock and scion by introducing a piece of aluminium foil into the graft union, CVB was formed in 80% of the grafts in the callus of the stock without NAA application. In the case where the scion was grafted at right angle onto an additional longitudinal cut surface made 1 mm from the vascular bundle of the stock with remaining intact parenchyma between this cut surface and the vascular bundle of the stock, CVB was not formed even when NAA was applied. These results suggested that wound response was a prerequisite for CVB differentiation and that surface-to-surface adhesion of calluses inhibited CVB formation by making callus cells differentiate into quiescent parenchyma cells rapidly. The mechanism of this phenomenon is discussed.