Does winter desiccation account for seasonal increases in supercooling capacity of Norway spruce bud primordia?

Abstract

Bud primordia of Picea abies (L.) H. Karst. remain ice free at subzero temperatures by supercooling. Once ice forms inside the primordium, it is immediately injured. Supercooling capacity increases seasonally from ~−5 °C to as much as −50 °C by currently unknown mechanisms. Among other prerequisites, dehydration of tissues over the winter months has been considered to play a key role in freezing tolerance. In this regard, the water content of bud primordia may be crucial, especially in reference to supercooling. In order to assess the role of dehydration in supercooling capacity, seasonal changes in supercooling capacity and the water potential of bud primordia of Picea abies (L.) H. Karst were measured at two sites that differed by 1298 m in elevation, after artificial frost hardening and dehardening treatments and after controlled bench drying. The extent of supercooling of bud primordia varied from −7 °C in summer to −24.6 °C in winter, a difference of 17.6 –19.3 K. Total actual water potential (Ψtact) of bud primordia was −2 MPa in summer and decreased to a mean of −3.8 MPa in midwinter. The decline involved dehydration, and to a lesser extent, osmoregulation. At decreased Ψtact values (<3.0 MPa), supercooling capacity significantly increased <−19.5 °C, however, the correlation between actual water potential and supercooling capacity was poor. Frost-hardening treatments increased the supercooling capacity of bud primordia (−0.6 K day−1) and lowered Ψtact (−0.2 MPa day−1). Frost-dehardening treatments reduced supercooling capacity (+1.1 K day−1), and at the same time, increased Ψtact (+0.3 MPa day−1). In contrast, artificial drying of bud primordia in the range observed seasonally (−2.0 MPa) had no effect on supercooling capacity. These results suggest that there is no causal relationship between desiccation and the supercooling capacity of bud primordia in P. abies, but rather it involves other compounds within the cells of the bud primordium that reduce the water potential.

Deep supercooling enabled by surface impregnation with lipophilic substances explains the survival of overwintering buds at extreme freezing

“…Strikingly, the extent of freeze dehydration appeared not much different to that found in P. abies where buds finally get frost damaged by intracellular freezing when they cannot retain their deeply supercooled state (Kuprian et al, 2017). However, only recently it was shown that low midwinter MPa values of P. abies bud tissues have no effect on their supercooling capacity (Kuprian, Munkler, Resnyak, & Neuner, 2018). From these findings, the functional involvement of freeze dehydration or at least a dose-effect relationship between freeze dehydration and freezing resistance of buds is not so clear.…”

“…alnobetula are suggested to either belong to Type I or II supercooling buds. Type III can be excluded, as these buds after supercooling would show intracellular freezing upon frost damage (Kuprian et al, ; Kuprian et al, ; Rajashekar & Burke, ), which, using DTA and IDTA, could be demonstrated is not the case for buds of A . alnobetula .…”

“…In contrast, when P . abies buds were artificially desiccated, their supercooling capacity did not increase (Kuprian et al, ). From this, other processes rather than freeze dehydration such as the accumulation of cryoprotective substances in cells have been suggested to be involved in the stabilization of the supercooled state (Kuprian et al, ).…”

“…This was seen in buds of P . abies (Kuprian et al, ) that are of Type III (Kuprian et al, ). When spruce buds were infiltrated with water, or the gap‐less coverage by bud scales was broken by incision, the buds were unable to supercool.…”

“…However, only recently it was shown that low midwinter MPa values of P . abies bud tissues have no effect on their supercooling capacity (Kuprian, Munkler, Resnyak, & Neuner, ). From these findings, the functional involvement of freeze dehydration or at least a dose‐effect relationship between freeze dehydration and freezing resistance of buds is not so clear.…”