Genotypic Variation in Apple Rootstock Cold Temperature Tolerance

One and two-year old shoots from 14 apple rootstock cultivars and selections were collected in late October 2012 and subjected to temperatures of –15, –20, –25, –30, and –35 °C, and in December to temperatures of –25 to –40 °C.  M.7 was included for comparison. Cold temperature injury was measured as xylem browning using a rating scale of 0 to 5 with 0 indicating none and 5 indicating severe browning.   In October, xylem injury was relatively minor until a temperature of –25 °C when G.4011, G. 4292 and G.4814 had significantly greater injury than M.7.  Injury was significantly greater in G.4292 than all other genotypes tested at –25 °C.  V.5, V.7, G.935, and G.4288 had less injury, and V.6, G.214, G.4013, G.5257, and G.6874 had similar injury as M.7.  At –35 °C, V.5, V.7, G.4011, G.4013, G.4292, G.4814, and G.6874 had a similar level of injury compared to M.7, whereas, V.6, G.214, G.4288, G.93,5 and G.5257 had less injury.  G.5257 had the least amount of xylem browning at –35 °C, which was significantly less than all other genotypes.  In December, injury was relatively minor until a temperature of –40 °C when V.5, V.6, V.7, G.214, G.3902, G.4013, G.4292, and G.5257 had a similar level of xylem injury as M.7, but G.935, G.4011, G.4288, G.4814, and G.6874 had less injury. G.935 and G.4288 were consistently hardier than M.7, whereas, G.4292 incurred the greatest level of tissue browning in both October and December. In October, shoot age had a relatively minor effect on xylem browning at temperatures of –25 and –30 °C, but a significant effect at –35 °C with a rootstock interaction.  One-year-old shoots of V.7 and G.214 had greater xylem injury than 2-year-old shoots, and 1-year shoots of G.4011 had less injury than 2-year shoots.  In December, xylem injury was generally greater in 2-year-old shoots compared to 1-year-old with no rootstock interaction.  The 'Geneva' rootstocks tested in these experiments belonged to a full sib family that segregates for many traits.  These results indicate that the genetic variation for early and mid-winter cold hardiness observed in these experiments may be harnessed for genetic mapping and understanding the inheritance of cold hardiness in apple rootstocks.