LAUSR

Abstract New apple rootstocks that are fire blight resistant are rapidly becoming available in recent years. Rootstock effect on vigor, yield, return bloom, branch angle, bud break, and hormone profile was assessed using ‘Honeycrisp’ as the scion cultivar. Three Budagovsky (B.9, B.10 and B.7-20-20), five Geneva (G.814, G.214, G.935, G.41 and G.11) and two Malling (M.9T337 and M.26) rootstocks were used for this trial. G.814 and G.214 were high productive rootstocks, more vigorous than M.9T337 that could be interesting for weak growing cultivars such as ‘Honeycrisp’. B.9. B.10, G.41 and G.214 performed pretty similar to M.9T337, but they had slightly higher cumulative yields. High return bloom values were observed on G.935, M.9T337, and G.814, whereas B.72020 was the rootstock with the lowest values. In addition, B.72020 was the stock with more upright branches, while B.10 and G.11 had the flattest. On average, a few rootstocks had a low biennial bearing index, which included B.10 and G.814. Indole-3-acetic acid was the most abundant auxin in the xylem, being B.72020 and B.9 the stocks with higher content, while G.935 had the least. Regarding cytokinins (CK), the highest content was observed on G.11, and the lowest on G.814 and G.214. The highest ABA/CK ratio was observed on B.9, whereas B.72020 had the lowest value. On the other hand, B.72020 had the highest AUX/CK ratio value; and G.11 and G.935 the lowest. Fruit size was associated with high ABA/CK values. Trunk cross sectional area, branch angle, gas exchange variables, and biennial bearing were correlated with auxins and AUX/CK concentrations in xylem. Yield efficiency, crop load, leaf temperature, return bloom and bud break were correlated with ABA, CK, and ABA/CK in xylem. Since G.814, G.935 and G.11 had a very uniform bud break, these rootstocks could be suitable to test in areas where irregular bud break might be a problem. The high levels of endogenous ABA observed in Honeycrisp grafted on B.9 and G.11 suggests the possibility of these rootstocks tolerating drought stress by slowing evapotranspiration of the scion.