Cold hardiness in various organs and tissues of Rhododendron species and the supercooling ability of flower buds as the most susceptible organ

The freezing resistance of various organs and tissues was determined in 24 Rhododendron species (mainly Subgenus Tsutsutsi) having different ecological distributions. The order of hardiness for organ or tissue is as follows: leaf bud > wood ≧ bark > flower bud, and the flower bud is characterized as the most cold-susceptible organ. The relationship of killing temperature (KT) to northern distribution was the most significant in leaf buds compared to other organs and tissues. KTs of leaf buds for the most hardy species were -45 °C (or below) and those for the most tender species were about -23 °C, while KTs of flower buds were about -28 °C for the former and -16 °C for the latter. Although KTs of flower buds native to southwestern Japan were well correlated with the exothermic temperature distribution (ETD) of florets, those in the more northern species were generally lower than ETDs. The supercooling ability of flower buds appears to be sufficient to avoid the freezing stress since the extreme minimum temperature (EMT) at the northern limit of natural distribution for each tree species examined was not lower than the KT and ETD of the flower buds.