COLD HARDINESS EVALUATION OF APPLE ROOTSTOCKS
Resistance to winter injury is important to the selection of rootstocks for the apple growing regions of British Columbia. Evaluation of rootstock hardiness has been conducted by freezing root pieces collected from 4- to 5-year-old trees grown in test plots. Root pieces were frozen at 1°C hr-1 to a series of test temperatures then evaluated for injury by measuring conductivity and estimating browning. A critical value based on regrowth of potted apple trees after freezing in test chambers was selected to interpolate the plots of injury vs. temperature. The ion leakage (40% specific conductivity) and amount of browning (above a trace) associated with decrease in regrowth were used to estimate the lowest temperature to which the roots survived without affecting growth (i.e., the minimum survival temperature, MST). Measurements of MST on the rootstocks Malling (M.) 26 (-10.0°C), Merton Malling (MM.) 106 (-7.2°C), and M.7 (-6.7°C), budded to 'Golden Delicious' and 'Heyer 12', agreed with field observations on their rank in cold hardiness. Although hardiness varied with date, the rank in cold hardiness among rootstocks measured in December, January, and February was the same. In another study using 'Summerland McIntosh' as a scion cultivar, the mean MST of P.2 (-13.3°C), Ottawa (O.) 3(-13.2°C), Budagovsky (B.) 9 (-12.3°C), Jork 9 (-11.8°C), Alnarp 2 (-11.2°C) was determined to be significantly lower than M.9 (-9.6°C). MST of M.7 (-7.6°C) was significantly higher than that of M.9. These measurements agree with reports of field survival. Rankings in hardiness among these rootstocks in December were similar to those in late February. In one study, in which hardiness was measured by scoring visual browning, Robusta 5 imparted greater hardiness to the scion ('Summerland McIntosh') than M.7, M.26, and B.9.
Quamme, H.A. and Brownlee, R.T. (1997). COLD HARDINESS EVALUATION OF APPLE ROOTSTOCKS. Acta Hortic. 451, 187-194
freezing injury, cold resistance, acclimation, conductivity