Sammendrag
Freezing damage is a crucial factor in the cultivation of perennial crops. Overwintering plants acclimate to decreasing temperatures in their environment and thus, prevent freezing damage of plant tissue. To assess effects of photoperiod (short day SD: 10 h and long day LD: 16 h) on metabolism and freezing tolerance of octoploid strawberry (Fragaria × ananassa Duch.), acclimation experiments were carried out at above-zero temperature (4°C) using 6 cultivars with contrasting cold tolerance: Florence, Frida, Korona, Sonata, and the breeding lines GN1189.3 and GN1196.15. Sampling of crown (meristem) and leaf tissue was carried out at time points 0days, 7d, and 14d. Freezing tolerance was determined using electrolyte leakage testing with a decreasing temperature gradient ranging from 0°C to -20°C (12 freezing points) and subsequent calculation of LT50 values. GC-qMS metabolite profiling was carried on polar extracts from samples of the same plants using a derivatization technique.
A decrease in LT50 temperature after 7d and towards 14d revealed that all varieties were adapting to lower temperatures with much stronger effects in leaf tissue. Both Florence and Frida were least frost tolerant after 7d (leaf and crown samples), while varieties Frida and GN1189.3 (leaf), and Frida and GN1196.15 (crown) showed highest frost resistance after full cold acclimation of plants. More than 500 distinct metabolite peaks were detected in either leaf or crown samples. Varietal differences could be clearly explained by principal component analyses (PCA), which were more pronounced in leaf tissue with Florence segregating from all other genotypes. Correlation analysis of metabolite levels with freezing tolerance (LT50 values) was carried out across 7d and 14d time points, SD and LD treatments, and varieties. Much stronger effects were found in leaves compared to crown tissue. Increased frost tolerance showed strong connectivity to components of the Krebs-cycle, ascorbic acid, certain amino acids and aromatic compounds (decreased levels). In contrast, enhanced concentrations of monosaccharides (fructose, glucose), disaccharides (sucrose), raffinose and catechin (only in crowns) were clearly correlated with increased frost tolerance. A comparison of SD and LD treatments did not reveal distinct compounds responsible for photoperiod variation; however, Florence was the most responsive variety with 6 significantly different metabolites (e.g. α,α-trehalose, β-D-1-O-methylgalactopyranoside, succinic acid). Based on field trials (2012/13), the same varieties can be ordered according to winter survival observations: Korona > Frida > GN1196.15 > Sonata > GN1189.3 > Florence. These results are discussed on the background of experimental data from metabolite profiling and freezing tests.
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