Journal Article

Freezing pattern and frost killing temperature of apple (<i>Malus domestica</i>) wood under controlled conditions and in nature

Manuel Pramsohler, Jürgen Hacker and Gilbert Neuner

Edited by Marilyn Ball

in Tree Physiology

Volume 32, issue 7, pages 819-828
Published in print July 2012 | ISSN: 0829-318X
Published online May 2012 | e-ISSN: 1758-4469 | DOI: http://dx.doi.org/10.1093/treephys/tps046
Freezing pattern and frost killing temperature of apple (Malus domestica) wood under controlled conditions and in nature

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The freezing pattern and frost killing temperatures of apple (Malus domestica Borkh.) xylem were determined by differential thermal analysis and infrared differential thermal analysis (IDTA). Results from detached or attached twigs in controlled freezing experiments and during natural field freezing of trees were compared. Non-lethal freezing of apoplastic water in apple xylem as monitored during natural winter frosts in the field occurred at −1.9 ± 0.4 °C and did not change seasonally. The pattern of whole tree freezing was variable and specific to the environmental conditions. On detached twigs high-temperature freezing exotherms (HTEs) occurred 2.8 K below the temperature observed under natural frosts in the field with a seasonal mean of −4.7 ± 0.5 °C. Microporous apple xylem showed freezing without a specific pattern within a few seconds in IDTA images during HTEs, which is in contrast to macroporous xylem where a 2D freezing pattern mirrors anatomical structures. The pith tissue always remained unfrozen. Increasing twig length increased ice nucleation temperature; for increased twig diameter the effect was not significant. In attached twigs frozen in field portable freezing chambers, HTEs were recorded at a similar mean temperature (−4.6 ± 1.0 °C) to those for detached twigs. Upon lethal intracellular freezing of apple xylem parenchyma cells (XPCs) low-temperature freezing exotherms (LTEs) can be recorded. Low-temperature freezing exotherms determined on detached twigs varied significantly between a winter minimum of −36.9 °C and a summer maximum −12.7 °C. Within the temperature range wherein LTEs were recorded by IDTA in summer (−12.7 ± 0.5 to −20.3 ± 1.1 °C) various tiny clearly separated discontinuous freezing events could be detected similar to that in other species with contrasting XPC anatomy. These freezing events appeared to be initially located in the primary and only later in the secondary xylem. During the LTE no freezing events in the bark and central pith tissue were recorded. Attached twigs were exposed to various freezing temperatures at which LTEs occur. Even if 60% of XPCs were frost-damaged twigs were able to recuperate and showed full re-growth indicating a high regeneration capacity even after severe frost damage to XPCs.

Keywords: blackheart injury; deep supercooling; differential thermal analysis; freezing exotherm; freezing injury; ­ice nucleation; ice propagation; infrared thermography; xylem ray parenchyma cells

Journal Article.  6623 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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