Drivers of apoplastic freezing in gymnosperm and angiosperm branches

Abstract

It is not well understood what determines the degree of supercooling of apoplastic sap in trees, although it determines the number and duration of annual freeze–thaw cycles in a given environment. We studied the linkage between apoplastic ice nucleation temperature, tree water status, and conduit size. We used branches of 10 gymnosperms and 16 angiosperms collected from an arboretum in Helsinki (Finland) in winter and spring. Branches with lower relative water content froze at lower temperatures, and branch water content was lower in winter than in spring. A bench drying experiment with Picea abies confirmed that decreasing branch water potential decreases apoplastic ice nucleation temperature. The studied angiosperms froze on average 2.0 and 1.8°C closer to zero Celsius than the studied gymnosperms during winter and spring, respectively. This was caused by higher relative water content in angiosperms; when branches were saturated with water, apoplastic ice nucleation temperature of gymnosperms increased to slightly higher temperature than that of angiosperms. Apoplastic ice nucleation temperature in sampled branches was positively correlated with xylem conduit diameter as shown before, but saturating the branches removed the correlation. Decrease in ice nucleation temperature decreased the duration of freezing, which could have an effect on winter embolism formation via the time available for gas escape during ice propagation. The apoplastic ice nucleation temperature varied not only between branches but also within a branch between consecutive freeze–thaw cycles demonstrating the stochastic nature of ice nucleation.

The Possible Role of Non-Structural Carbohydrates in the Regulation of Tree Hydraulics

“…In late winter, branch-and trunk-soluble sugars of Pinus koraiensis positively correlated with PLC and were overall more abundant than in summer [49]. This indicates that sugars mobilization over winter might represent a cold hardening strategy [48], but could be also putatively related to the need of maintaining and/or restoring hydraulic function [50,51] (see below).…”

Die hard: timberline conifers survive annual winter embolism

“…Based on the presented study, several knowledge gaps with respect to winter embolism can be identified: first, the process of ice formation in plant xylem and how it leads to embolism is still not sufficiently understood (e.g. Lintunen et al , , ; Charrier et al , ). Considering the relevance of high altitude and latitude forest ecosystems, substantial knowledge of the eco‐physiological limits of trees is important, and information on freezing‐related mechanisms a prerequisite.…”

Propagating ice front induces gas bursts and ultrasonic acoustic emissions from freezing xylem

“…1) at the sample base. Freezing was induced when the xylem temperature at the stem base (outside the cuvette) was around −1.5°C in the saturated samples and around -3.0°C in the dehydrated samples to mimic their natural freezing temperatures (Lintunen et al 2018). In experiment 3, artificial freezing was induced at the stem base at −1.5°C in the first two cycles, while freezing occurred spontaneously in the third cycle (due to the dehydrated surface of the debarked section, induction of freezing was not possible).…”