Seasonal and Genotypic Variation in Apple Rootstock Cold Temperature Tolerance

Xylem cold temperature tolerance in Oct. and Nov. 2014 was measured in 2-year old shoot pieces 6-8 cm in length from 13 apple rootstock cultivars in the Geneva series, 3 in the Vineland series, ‘M.9’ and ‘B.9’.  Following exposure to temperatures as cold as -35 °C, injury was measured as xylem browning using a rating scale of 0 to 10 with 0 indicating no browning and 10 indicating browning in the entire tissue.  Nonlinear regression was performed to estimate the inflection point and upper asymptote of a logistic sigmoid function of xylem browning vs. temperature.  We used the 95% confidence interval of the upper asymptote to categorize and compare genotypes since the inflection point did not generally represent hardiness.

In late Oct., most genotypes did not have significant discoloration until a temperature of -25 to -30 °C.  G.935, G.3902 and G.4011 had the least amount of xylem injury at the coldest temperature, -35 °C, with a discoloration rating of 2.0 or lower. G.4013, G.4814 and G.5087 had a similarly low level of injury, but a maximum rating that was greater than 2.0. The greatest injury occurred in G.41 and G.214, and these had a rating greater than 7.0. G.30, V5, V6 and V7 had maximum levels of browning at -35 °C in the range of 5.0 to 7.0, and were not significantly different from G.41. Intermediate browning occurred in M.9, B.9, G.4292, G.4814 and G.5257.

In mid Nov., G.935, G.4011, G.5087 had very little injury at -35 °C and a maximum rating of 2 or lower.  M.9, B.9, G.214, G.3902, G.4013, V5 and V6 also had less injury than the most tender genotypes, but a maximum rating greater than 2.0.  The greatest level of injury occurred in G.30, G.41, G.4292, G.4814, G.5257 and V.7 with a maximum injury rating greater than 5.0, although in G.4814 and G.5257, injury was minimal below -35 °C.