Abstract
In temperate climates, overwintering buds of trees are often less cold hardy than adjoining stem tissues or evergreen leaves. However, data are scarce regarding the freezing resistance (FR) of buds and the underlying functional frost survival mechanism that in case of supercooling can restrict the geographic distribution. Twigs of 37 temperate woody species were sampled in midwinter 2016 in the Austrian Inn valley. After assessment of FR, infrared-video-thermography and cryo-microscopy were used to study the freezing pattern in and around overwintering vegetative buds. Only in four species, after controlled ice nucleation in the stem (−1.6 ± 0.9°C) ice was observed to immediately invade the bud. These buds tolerated extracellular ice and were the most freezing resistant (−61.8°C mean LT50). In all other species (33), the buds remained supercooled and free of ice, despite a frozen stem. A structural ice barrier prevents ice penetration. Extraorgan ice masses grew in the stem and scales but in 50% of the species between premature supercooled leaves. Two types of supercooled buds were observed: in temporary supercooling buds (14 species) ice spontaneously nucleated at −20.5 ± 4,6°C. This freezing process appeared to be intracellular as it matched the bud killing temperature (−22.8°C mean LT50). This response rendered temporarily supercooled buds as least cold hardy. In 19 species, the buds remained persistently supercooled down to below the killing temperature without indication for the cause of damage. Although having a moderate midwinter FR of −31.6°C (LT50), some species within this group attained a FR similar to ice tolerant buds. The present study represents the first comprehensive overview of frost survival mechanisms of vegetative buds of temperate trees. Except for four species that were ice tolerant, the majority of buds survive in a supercooled state, remaining free of ice. In 50% of species, extraorgan ice masses harmlessly grew between premature supercooled leaves. Despite exposure to the same environmental demand, midwinter FR of buds varied intra-specifically between −17.0 and −90.0°C. Particularly, species, whose buds are killed after temporary supercooling, have a lower maximum FR, which limits their geographic distribution.
Highlights
In temperate woody plants, vegetative buds are often less cold hardy than adjoining stem tissues and evergreen leaves (Bannister and Neuner, 2001)
infrared differential thermal analysis (IDTA) indicated that the low-temperature exotherm (LTE) was localized and originated from within the bud itself
The LTE was triggered by an ice nucleation event inside the bud in all buds that exhibited temporary supercooling
Summary
Vegetative buds are often less cold hardy than adjoining stem tissues and evergreen leaves (Bannister and Neuner, 2001). Buds have been reported to survive freezing by various frost survival mechanisms (Sakai and Larcher, 1987). Buds of temperate conifers have been shown to survive by deep supercooling and extraorgan freezing (Ishikawa and Sakai, 1982; Sakai, 1982), except for Pinus species. Buds of pines were shown to exhibit extracellular freezing (Sakai and Eiga, 1985; Quamme, 1995; Ide et al, 1998). While reproductive buds of many angiosperms exhibit deep supercooling (Quamme, 1995), their vegetative buds, based upon previous studies, have been suggested to survive freezing temperatures 10 to 15◦C lower than reproductive buds, by undergoing extracellular freezing (Sakai and Larcher, 1987). Exceptions, have been reported (Pyrus syriaca: Rajashekar and Burke, 1978) and more recently added (Acer japonicum: Ishikawa et al, 1997; Malus domestica: Pramsohler and Neuner, 2013; Alnus alnobetula: Neuner et al, 2019)
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