Effect of temperature and exposure time on cold hardiness of primary buds during the dormant season in `Concord', `Norton', `Vignoles' and `St. Vincent' grapevines

Summary Primary bud cold hardiness in `Concord', `Norton', `Vignoles', and `St. Vincent' grapevines was studied at Southwest Missouri State University Research Campus in Mountain Grove, Missouri, USA. Canes from mature vines were exposed to 210, 25, 0, 10 or 20°C for 2, 4, 6 or 8.d in early December, late January, and early March during the 1998–99 winter season. Cold hardiness of primary buds was in¯uenced by temperature, exposure time, and dormant period. Temperatures of 10 or 20°C had similar deacclimating effects on primary bud cold hardiness of all cultivars during the entire winter. Cold hardiness of primary buds exposed to 0¼ was greater than that of buds exposed to 10 or 20°C only in early March for all cultivars and late January for `Concord'. Exposure to 0°C was as deacclimating as that to 10 or 20°C otherwise. Cold hardiness of primary buds becomes more sensitive to temperature as dormancy progresses. Longer exposure was advantageous for cold hardiness of primary buds only at 25 or 210°C. `Concord' and `Norton' were generally more sensitive to temperature than were `Vignoles' and `St. Vincent'. The greater cold hardiness of the tested non-vinifera cultivars is due to both their greater cold hardiness at high temperature and their sensitivity to temperature change, acclimating faster and deeper at lower temperature. During the early and middle dormant season, exposure to 10 to 20°C for 2.d was sufficient to deacclimate primary buds to their minimum cold hardiness. However, exposure to 210°C even for 8.d was not always sufficient to acclimate them to their maximum cold hardiness.