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70 WINES&VINES February 2017 GRAPEGROWING PRACTICAL WINERY & VINEYARD P henological development in grapevines has been described as being asynchronous (not occurring at the same time). 2,3 Begin- ning with bud break, there are differences in the date when individual buds on a grapevine begin to swell and open. Likewise, at bloom, cap fall in flower clusters does not always occur at the same time, which, in turn, is related to asynchronous softening and color development during véraison. 8 At any time during the growing season, some variability in phenological stage between shoots on the same vine is to be expected and observed (see photo below). 2 Due to the asynchronous nature of pheno- logical development in grapevines, there is a concern that differences in ripeness levels of berries within a cluster or between clusters on the same vine can have a negative impact on wine quality. Several viticultural and enological practices have been developed to address non-uniform ripeness levels. Berry-to-berry variation in ripeness is usually addressed at grape recep- tion at the winery with practices such as hand sorting, optical sorting or flotation. Cluster-to- cluster variation in ripeness level can be ad- dressed at grape reception or through cluster removal in the vineyard prior to harvest. Assumption of phenological autonomy Green thinning is commonly performed at véraison to remove clusters with delayed maturity (green clusters) in order to improve uniform ripeness between clusters at harvest. Since green cluster-thinning is performed several weeks before harvest, there is an as- sumption that differences in ripeness ob- served at véraison will persist until harvest. This is based on an underlying assumption that shoots on the same grapevine are pheno- logically autonomous, meaning that the phe- nological development of a shoot (or cluster) on a grapevine is not influenced by the phe- nology status of other shoots. Another way to describe this is to say there is no synchroniza- tion of phenology between shoots or clusters. There is no empirical evidence to support the assumption of phenological autonomy in grapevine shoots, yet the practice of green thinning is commonly adopted in some viti- cultural areas. The concept of branch or shoot autonomy is not new. 7 A good amount of research has been done on several aspects of branch (branch is a general term for trees, whereas shoot refers to grapevines) autonomy in pe- rennial species. Most of the research has fo- cused on one aspect of branch autonomy (namely carbon economy), which posits that, at some point in branch development, all of the carbohydrate requirements for growth, respiration and fruit ripening are supplied by the leaves on that branch. Recent research on this source-sink aspect of shoot autonomy in Concord grapevines indicates that shoot au- tonomy may not be valid for grapevines. 5 Some research has explored whether branches are autonomous with respect to their water relations or nutrient status. 7 Phenological autonomy has received little or no attention, as it was not considered part of the branch (shoots in grapevines) autonomy theory. The objective of this study was to test the underlying assumption inherent in green thinning, which is that shoots on grapevines are phenologically autonomous. Challenging the assumption A field trial in a mature Merlot vineyard was established in the Oak Knoll AVA of Napa Val- ley. The Merlot vines (clone 181) were planted in 1999 on 3309C rootstock. The vines were trained to a bilateral cordon and spur-pruned. Vine spacing is 8 feet by 6 feet, and the trellis is vertical shoot positioning (VSP). All cultural practices (trunk suckering, shoot positioning and cluster-zone leaf removal, etc.) were per- formed on the same date. The treatments compared early pruning (Jan. 23, 2015) and late-pruning (March 23, 2015) applied to separate vines, opposite cor- dons on the same vine and alternating spurs on the same vine. The trial was laid out as a three-by-two factorial with shoot location treatments, two pruning date treatments and six replications of each treatment combination. These were arranged in a complete random- ized block design. The phenology of shoots appearing from spurs was evaluated weekly beginning at bud break using a modified Eichhorn-Lorenz (EL) scale until fruit set, 2 then biweekly until the onset of véraison, and then weekly through the end of véraison. The phenological stage of shoots from count spurs was monitored. Evidence of Phenological Shoot Autonomy in Grapevines Understanding an important cause of variable ripeness levels between clusters By Michael Sipiora and Amanda Cihlar Asynchronous phenological development is evident in Chardonnay grapevines. The berries at left are peppercorn size, while 100% cap fall is seen at right.