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

Edith Kuprian,Anna Resnyak,Caspar Munkler,Marian Oberhammer,Gilbert Neuner,Sabrina Koch,Othmar Buchner

doi:10.1093/treephys/tpx142

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.

Highlights

Norway spruce (Picea abies) bud primordia survive freezing by supercooling and extra-organ ice formation (Pukacki 1987, Buchner and Neuner 2009)
During freezing with decreasing freezing temperatures, bud primordia become successively freeze dehydrated as temperatures decrease and water migrates across the ice barrier from the bud primordium to a cavity formed inside the pith of the shoot where large extracellular masses of ice develop, a process referred to as extra-organ freezing (Kuprian et al 2017)
Without the application of ice nucleation active (INA) bacteria the low-temperature exotherm (LTE) in summer months occurred at temperatures very close to the high-temperature exotherm (HTE) (Figure 1B), which often made the distinction between the two events problematic or impossible (Figure 1C)

Summary

Introduction

Norway spruce (Picea abies) bud primordia survive freezing by supercooling and extra-organ ice formation (Pukacki 1987, Buchner and Neuner 2009). These processes occur in the vegetative buds of many other cold-hardy conifers, with the exception of pines (Sakai 1978, 1979, Sakai and Eiga 1985, Ide et al 1998), and the reproductive buds of angiosperms (Quamme et al 1995). During freezing with decreasing freezing temperatures, bud primordia become successively freeze dehydrated as temperatures decrease and water migrates across the ice barrier from the bud primordium to a cavity formed inside the pith of the shoot where large extracellular masses of ice develop, a process referred to as extra-organ freezing (Kuprian et al 2017)

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