Bulb Tissue Cold-Tolerance Sensitivity Differences In Non-Hardy and Winter Hardy Lilium

The genus Lilium contains ~80-100 species in seven sections distributed across the northern hemisphere between 10°N and 60°N. lat. Many species are important commercial floricultural crops. Not all lily species and cultivars are winter hardy in northern latitudes (USDA Z3-Z4) such as Minnesota. For instance, Lilium longiflorum (Easter lily) and L. formosanum only marginally survive in Z4 with adequate snow cover and mulch while others are extremely cold tolerant (L. regale, L. martagon). As part of a breeding objective for selection and development of winter-hardy, seed-propagated interspecific lily hybrids this research focuses on the identification of which bulb tissue(s) are the most cold-sensitive. This information will provide selection criteria in assessing and selecting for cold tolerance. Lily germplasm evaluated in this study included Z3-4 hardy (2-Oriental, 28-Asiatics, 1-Aurelian, 3-Trumpets, 4-Orienpets, 8-LAs, 6 L. martagon, L. regale) and Z5+ non-hardy (L. davidii var. unicolor, L. henryi, L. longiflorum, L. formosanum, L. leichtlinii, L .speciousum album, L. tsingtauense) genotypes. Bulbs were acclimated to 2°C for 1,000 hrs prior to conducting programmed laboratory freezing tests of intact bulbs in soilless medium. Cold tolerance was assessed at 0°C, -2°C, -6°C, -8°C, -12°C with varying ramp time periods and a 2h soak time. After each treatment, samples were placed into a cooling chamber at 2°C for 20d in darkness until completely thawed. Subsamples were dissected and the remainder were moved to greenhouse conditions of 20/17°C(day/night) temperature with a 16h photoperiod. Three weeks later those samples were dissected. For non-hardy species, e.g. L. longiflorum ‘Nellie White’, LT50 =-2°C for <1/3 damage to the mother scales, whereas LT50 =-12°C for 50% kill of mother scales, 33-50% kill in daughter, and 100% of the meristems. Leaves were less sensitive (LT50 =-8°C for <33% kill). However, an LT50 =-6°C for was found for the basal plate leading to bulb lethality in most cases; at -8°C, virtually all bulbs were dead and did not regrow. The order of tissue sensitivity was mother scales first, followed by the basal plate, leaves, and then daughter scales and meristem. Hardy and non-hardy groups differed in tissue sensitivities.