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May 2018 WINES&VINES 85 WINE EAST GRAPEGROWING autumn acclimation or spring de-acclimation, may also lead to damage. Regardless of when they occur, freeze events make the assessment of vines for cold damage an inevitable and com- mon practice in cool-cold climate viticulture. Grapevine cold hardiness is comprised of anatomical, cellular and biochemical charac- teristics that are controlled by the grapevine genome. These intricate factors associated with cold hardiness can lead to elastic and plastic responses to cold stress. The responses vary relative to the vine's annual cycle, the fre- quency of freeze episodes during the dormant season, and with the degree to which adaptive genetic processes can be fully expressed. It is not surprising that attempts to define cold hardiness, whether within the vine, as a response to variation and timing of weather conditions, or as a relation between the two are very complex. A large variation in cold hardiness is seen between different cultivars, as well as within a single vine. 6 On the same date, differ- ences as large as 15° C have been measured from the same vine. Thus, methods of evaluating hardiness of vine tissues must be precise. This is yet another reason why there can be wide differences in the hardiness level attributed to any specific cultivar (see the table on page 86). The cold hardiness of dormant buds is criti- cal to the development of more viable seeds. Each dormant bud belonging to any species of grapevine has three potential shoots. The pri- mary bud is the largest and most anatomically advanced of the three. However, it is also the least hardy and most susceptible to winter conditions. A healthy primary bud has the greatest capacity to produce berries with seeds (see the photos A to C below). Its loss, whether by winter cold, spring frost or insect predation, shifts production possibilities to the secondary shoot, which bursts later in most sea- sons and has a r e d u c e d capacity for f r u i t a n d seed forma- tion. Should the second- ary be lost, the tertiary shoot, while typically fruitless, can still produce a canopy and allow the perennial vine to sur- vive another year. Carbon assimilation and distribution and vine cold hardiness: source- sink interaction In nature, the key process for perennial plant survival is carbon fixation via photosynthesis, and the distribution of carbon-based molecules to important locations within the vine. This process is called "carbon partitioning," and the specific locations, termed "sources" and "sinks," control carbon partitioning. M. Keller provides an excellent explanation of the kinet- ics of assimilate partitioning: they are dynamic, sensitive and hierarchical. 7 The influencing factors are: a) Proximity: it is advantageous to be close to the source (leaf); b) Connection: it is advantageous to be directly connected to the source; c) Interference: a loss of vegetative sink (through shoot tipping and hedging) can positively influence "cross transfers" advantageous to other sinks; d) Communication: vines; e) Competition: vines; and f) Development: vines. As Keller states succinctly: "For an organ or tissue to be a strong sink, it pays to be large, close to the source and to have good connections." Mature leaves and storage tissues such as roots and trunks are sources, which allocate carbon to developing tissues, termed sinks. These include young leaves, shoot and root tips, and fruit-bear- ing clusters, which utilize this incoming carbon for growth and seed development. These sinks are found in various locations throughout the vine, and the strength of a given sink may change during the annual cycle, and by season. 1 With this in mind, genes con- trolling vine hardiness can only be fully ex- pressed after the metabolites and building blocks necessary for continuing the life cycle have been produced at adequate levels. Con- sequently, a goal of viticulture practices in cool climates must be to meet the needs of the weakest sink in order to achieve maximum gene expression in vines. Previous work at Michigan State University has demonstrated that cold hardiness of primary buds was impaired the following winter more by varying levels of leaf removal at veraison than Brix accumulation in the current season or bud fruitfulness the following season. This adds to the concept that bud cold hardiness is a rela- tively weak sink, and any management practice Crop insurance for your vineyard! No one wants to have a loss, but they do happen. We work with growers in New York, California, Pennsylvania, Oregon and Washington State. Call or email for an estimate. Coverage may cost less than you think. Off: (239) 789-4742 Fax: (239) 789-4743 Email: info@agriskmgmt.com www.agriskmgmt.com Cold injury to grape buds. A) Healthy compound bud; B) Discolored tissues indicating injury to primary bud; C) Compound bud with cold injury to primary, secondary and tertiary buds.