Cold Hardiness of Apple Rootstock Trunk and Root Tissues

Ungrafted clonal apple rootstocks were stored at 1 °C until controlled exposure of the trunk and shoots to -12, -15, -18, -21, -24 and -27 °C in Dec. 2009.  To protect roots from cold temperature injury, the roots were insulated by peat-based media inside a Styrofoam container.  Temperature of the media remained above -1 °C during exposure to subzero temperatures.  Internal browning of phloem, cambium and xylem was rated on a scale of 1 to 5 in the lower and upper sections of the tree trunk.  Browning of Bud.9 xylem was not affected by any of the temperatures, but purple coloring made visual rating difficult.  P.2 was also unaffected.  Browning of the xylem occurred at a temperature of -24 °C in G.11 and G.30, and at -21 °C in M.26.  Xylem browning was most severe in M.26, G.30 and G.11 which had a rating of 4.0, 3.3 and 2.8, respectively at the coldest temperature, -27 °C.  Browning of the phloem and cambium was less severe.  Trees were grown in a greenhouse for two months after which shoot growth was measured as an assessment of survival.  In Feb. 2010, the same rootstock cultivars were subjected to -10, -13, -15, -18, -21 and -24 ° C with the roots exposed to these temperatures.  Roots were placed inside plastic bags to protect them from dessication.  Relative electrical conductivity of fine root segments increased with decrease in temperature, but did not vary between the different rootstocks.  Trees were grown in a greenhouse to assess tree survival.