Partitioning photoperiod- and chilling-driven cold hardiness in a Vitis riparia × V. vinifera F1 population.

Grapevine cold hardiness governs cultivar distribution and winter survival, and is a key constraint on viticulture in cold regions. We evaluated bud cold hardiness in a Vitis riparia × Vitis vinifera F1 population by differential thermal analysis across three environments (a California vineyard, an NDSU field, and an NDSU greenhouse), sampled nine times through the 2023-2024 dormant season. Mean low-temperature exotherms (LTEs) differed significantly among sampling times, and hierarchical clustering separated the genotypes into five hardiness classes defined by mean LTE, from very hardy (≤ -21.4 °C) to very susceptible (≥ -13.4 °C). Field vines reached a midwinter maximum of -22.7 °C, whereas greenhouse vines were most susceptible (-14.6 °C), and nine genotypes were hardier than the tender parent. Genotype accounted for only 7% of the phenotypic variance, giving a low single-observation heritability (H²i = 0.07) but a moderate entry-mean heritability (H²c = 0.49), which are distinct statistics reported separately. The seasonal trajectory was well described by logistic and response-surface models (R² = 0.99) and, on an accumulated-chilling axis, by a Weibull model (half-hardening at 184 chilling degree-days). A state-space model estimated the chilling contribution at ≈6.7 °C and identified three genotypes (536-078, 539-127, 536-142) that attained most of their hardiness under the short-day, moderate-cooling greenhouse regime without sub-zero exposure; because that regime combined shortening photoperiod with a 4 °C decline, these are described as short-day/low-temperature responsive rather than photoperiod-responsive. Mean hardiness and plasticity were uncorrelated, providing no evidence of a hardiness-stability trade-off. The December field and greenhouse samplings exposed the greatest among-genotype variance and are the most informative conditions for future QTL mapping; the nine genotypes hardier than the tender parent, and a hardy, stable subset, are identified as parents for cold-hardiness introgression.