Bulb tissue cold-tolerance sensitivity differences in Lilium longijlorum 'Nellie White' and L. formosanum

As part of a breeding objective for selection and development of winter-hardy (USDA Z3-4), seed-propagated interspecific lily hybrids, this research focuses on the identification of cold-sensitive levels of lily bulb tissue(s). The objective of this experiment was to test two cold-sensitive Lilium spp. for tissue sensitivity to and damage by freezing temperatures. Bulbs of L. longiflorum 'Nellie White' and a L. formosanum hybrid population were acclimated at 2°C for 1,000 hours. In a separate experiment, 'Nellie White' bulbs were also acclimated at 2°C for 2,000 h. Cold tolerance was assessed using laboratory freezing tests at 0, -2, -6, -8, -10, -12°C with varying ramp time periods and a 2 hour soak time in a programmable freezer. Lethal temperatures at which 50% kill occurred (LT₅₀) of all bulb tissues (basal plate, roots, mother scales, daughter scales, meristem, pre-formed leaves), reduction and quantitative regrowth data (leaf, root numbers; shoot length) were determined for dissected bulbs tissue(s) after freezing. Significant changes in the interaction of genotype × temperature were detected. The LT₅₀=-6°C for all structures of 'Nellie White' bulbs (both 1,000 and 2,000 h) and roots of L. formosanum whereas an LT ₅₀=-8°C was found for L. formosanum mother and daughter scales, leaves, meristems, and basal plates. Cooling for 1,000 h resulted in significant regrowth differences for leaf number and shoot length of 'Nellie White' between -6 and -8°C. However, after 2,000 hours cooling the number of leaves and shoot length differed significantly between -2 and -6°C for 'Nellie White'. TTC tests did not correspond to laboratory freezing LT_50s and had insufficient or variable amounts that were difficult to detect. As a result, laboratory freezing and regrowth tests provide more substantive data for use in determining cold tolerance in geophytic Lilium.