Endogenous Physiological Mechanisms of Frost Hardiness in Flower Buds of Fruit Trees | Natürliche Frostschutzmechanismen bei Obstgehölzen – von Supercooling bis Anti-Freeze Proteinen

The devastating spring frost on 19–20 April 2017 throughout Europe sparked off the present study. The objective of this work, as part of a master thesis, was to investigate and review current knowledge on the process of ice crystallization and endogenous plant protection mechanisms. The process of ice formation or crystallization depends on the availability of antifreeze proteins (AFP) and ice nucleation active substances (INAS), which anchor on the membrane of INAS bacteria. INAS such as small sand and dirt particles, tiny mineral crystals and INAS bacteria such as Pseudomonas and Erwinia can serve as nucleotides for ice nucleation and crystallization. Without ice nucleotides, water exhibits the capacity of supercooling; in the 0 to −5 °C temperature range, supercooling is prevented by the presence of INAS bacteria, which become ultimately responsible for the tissue damage. Flowers of fruit trees with bacteria have been reported to freeze at −2 °C compared with −5 °C without Pseudomonas as INAS bacteria. When the ice crystals penetrate the plant via the cells, intra-cellular ice formation causes the freezing or frost damage. Small, irregularly shaped intra-cellular ice crystals are supposedly harmless to the cell, particularly if they melt before reaching (and damaging) the cell wall. Rapid freezing prevents the water to diffuse out of the cells to extra-cellular sites, results in a large number of small harmless ice crystals, whereas slow freezing ends up in a small number of large harmful ice crystals. Cold tolerance can include the following physiological mechanisms for frost hardiness, which can prevent or reduce intra-cellular ice formation (freezing)1) Reduction of (free and unbound) water content including free water bound to dedicated proteins (dehydrins) in the cold period2) Accumulation of osmotically active substances such as sugar (glucose, fructose, sucrose, sorbitol), minerals (potassium etc.) and amino acids (proline etc.).3) Availability of antifreeze proteins (AFP), which bind to the surface of the ice crystals and positively influence their size, shape and growth, thereby preventing them from penetrating the plant cell4) Supercooling, a process, which prevents intra-cellular ice formation (frost damage) by preventing the formation of ice nucleotides5) Structural ice barrier—no connection to the xylem tissues6) Delayed flowering and time difference between flowering on one or two-year-old woodEffective functioning of these natural endogenous plant protection mechanisms in winter requires slow cold acclimation in autumn and their maintenance in spring. Overall, the challenge for future fruit breeding, selection and growing in a situation with late spring frosts due to climate change is a) to encourage these above endogenous physiological cold hardiness mechanisms in the orchard and b) to maintain them, as they are otherwise lost with increasing temperatures and concomitant bud break and bud scale unfolding during de-acclimation in spring.