Hardiness is not a simple matter. Much of what passes for "winterkill" really results from plants that are still growing when the first frosts of winter hit, or when some cultivars are too anxious to start growing rapidly at the first sign of spring.
The same sort of problems are encountered by grape breeders who are trying to combine the good qualities of European strains with the hardiness of American species.
This article gives some idea of the complexity of the matter, and maybe some guidance in sorting out the photoperiod and temperature components of dormancy induction.
https://journals.ashs.org/jashs/view/jo ... e-p261.xml
Jour Am Soc Hort Sci 134(2): 261-274 (Mar 2009)
Mapping of Photoperiod-induced Growth Cessation in the Wild Grape Vitis riparia
Amanda Garris, Lindsay Clark, Chris Owens, Steven McKay, James Luby, Kathy Mathiason and Anne Fennell
In grapevines (Vitis spp.), the timing of growth cessation in the fall is an important aspect of adaptation and a key objective in breeding new grape cultivars suitable for continental climates. Growth cessation is a complex biological process that is initiated by environmental cues such as daylength and temperature, as well as water and nutrient availability. The genetic control of growth cessation in grapevines was studied by mapping quantitative trait loci (QTL) in a hybrid grape population. An F2 mapping population was developed by selfing a single F1 plant derived from a cross between an accession of the North American species Vitis riparia and the Vitis hybrid wine cultivar Seyval (Seyve-Villard 5–276). A linkage map was constructed using 115 simple sequence repeat (SSR) markers and six candidate genes in a population of 119 F2 progeny. The markers provided coverage of the 19 Vitis linkage groups with an average distance between markers of 8.4 cM. The critical photoperiod for growth cessation in lateral buds for the parents and F2 progeny was determined in a replicated field trial in 2001 and 2002 and under controlled photoperiod treatments in a greenhouse in 2002, 2003, and 2004. QTL analysis using composite interval mapping identified a single major QTL in the field and greenhouse trials. However, the field and greenhouse QTL mapped to different linkage groups in the two different environments, suggesting the presence of additional, nonphotoperiodic cues for induction of growth cessation in the field. In the greenhouse, where noninducing temperatures were maintained, a QTL on linkage group (LG) 13 explained 80.0% to 96.6% of the phenotypic variance of critical photoperiod for growth cessation. In the field, where vines experienced natural fluctuations in temperature and rainfall in addition to the naturally decreasing photoperiod, a QTL on LG 11 explained 85.4% to 94.3% of phenotypic variance.
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