July 2015 Issue of Wines & Vines
 
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Search for Zinfandel genetic diversity

 
by Michael Penn, Michael Anderson, Jim Wolpert and Andrew Walker
 
 

A Zinfandel Heritage Vineyard Project was initiated by Dr. Jim Wolpert of the Department, of Viticulture & Enology at the University of California, Davis, in the early 1990s with the goal of finding greater genetic diversity in Zinfandel.

practical winery vineyard
 

In 1990, there were 34,000 acres of Zinfandel growing in the state, and only four registered clonal selections available from Foundation Plant Services. The registered selections were considered to make low colored wines from very large berries and compact clusters prone to bunch rot. Clearly, more genetic options were needed.

Wolpert began a search for Zinfandel selections in vineyards scattered across California that were 60 years and older, and assembled a collection team consisting of UC Davis emeritus specialist Amand Kasimatis, and farm advisors Glenn McGourty, Rhonda Smith, Paul Verdegaal, Ed Weber, Jack Foott, Janet Caprile and Donna Hirschfelt. The team collected more than 90 cuttings from 13 counties in the hope of improving the diversity of available Zinfandel material. The main characteristics they searched for were small berries, loose clusters and a lack of red leaf virus symptoms.

Heritage Zinfandel
 
Heritage Zinfandel selection 89 was selected from an El Dorado County vineyard.

History of Zinfandel in California
The history of Zinfandel in California, as described by historian Charles Sullivan, began in 1851 with the importation of various grapevine cuttings including Zinfandel material by Capt. Frederick Macondray and Samuel Perkins, his associate in the Massachusetts Horticultural Society.

Zinfandel vineyards throughout California were planted during a grape acreage expansion in the 1880s and 1890s. Some of these vineyards still exist today and have been registered and catalogued by the Historic Vineyard Society (historicvineyardsociety.org). These pioneering vineyards were the source of Zinfandel material that would be collected in the 1990s.

Before coming to California, the original Zinfandel material was most likely imported to the East Coast of the United States by George Gibbs, a nurseryman and horticulturist in Long Island, N.Y. His first shipment of grapevines most likely arrived in 1829 from a Royal Botanical collection of grapevine cultivars that had been gathered from across the Hapsburg Austro-Hungarian Empire and maintained at the Schoenbroen Palace in Vienna, Austria.

No further historical records of the origin of the cultivar could be found by Sullivan in his research. Other clues to its origin would be left to ampelographic and genetic research done at UC Davis and the University of Zagreb in Croatia.

The quest for greater genetic diversity in Zinfandel first began when Austin Goheen, then director of Foundation Plant Services, was at the University of Bari in Apulia, Italy, visiting Giovanni Martelli. While dining, Goheen remarked that their wine was remarkably similar to California Zinfandel. He was told it was the Primitivo variety.

Researchers at UC Davis first imported Primitivo cuttings into the United States in the 1960s, and isozyme tests were performed in 1976. DNA fingerprint analysis by Carole Meredith in 1994 confirmed that Primitivo was genetically identical to Zinfandel.

Explorations in 2004-05 by Meredith, assisted by Mike Grgich and Croatian colleagues, discovered a few vines along the Croatian coast (former territory of the Austro-Hungarian Empire). DNA testing found that these vines, identified as Crljenak Kastelanski, were identical to California Zinfandel.

Further collections and genetic analysis by an Italian and Montenegran team found vines in Southern Croatia and Montenegro named Tribidrag and Kratosija to also be genetically identical to Zinfandel.1 These European cultivars represent clues to the center of origin of our once quintessential Californian variety and possible sources of useful genetic diversity.

California Heritage Zinfandel block
Selections from the Heritage Zinfandel explorations by Wolpert and his team were propagated, field-grafted to St. George rootstock and planted at UC Davis’ Oakville Station in Napa Valley, in a replicated and randomized 2.9-acre block in 1995. The vines were trained in the traditional head-trained, spur-pruned manner and planted on a uniform, 180 cm deep, gravelly Bale Loam.

After five years of screening for viticultural characteristics and the presence of disease by visual examination and PCR testing, 20 Heritage Zinfandel selections were selected and moved to the next phase of the program. These selections were field-grafted onto St. George rootstock (8-foot x 6-foot vine spacing).

Funds to plant, maintain and take data from this California Heritage Zinfandel block were provided by the Zinfandel Advocates and Producers (ZAP).

The vineyard was divided into five blocks, with each block containing all 20 selections, plus two Primitivo selections and the original FPS Zinfandel selection. Each replicate block contained 18 vines (six vines x three rows) of each selection.

The vines were pruned to 12 two-bud spurs per vine and were crown-suckered to ensure each vine had the same number of shoots. Cluster-thinning was performed so that all shoots had two clusters. Occasional winged-clusters were removed to decrease the potential for bunch rot. Weed control, fertilization and fungicide treatments were the same across the entire block.

In 2013, a group of California winemakers from ZAP and researchers gathered to taste and rank wines made from the 20 selections. Six selections were chosen as distinctive, and these were advanced to intensive viticultural and fruit analysis to determine if these features correlated with the observed wine differences.

In addition to the six selections, one of the original FPS Zinfandel selections that had been available for growers for many years was included, along with a Primitivo selection from Italy, both of which had been planted alongside the Heritage selections at the Oakville Station.

This study evaluated five data vines in the middle of each six-vine x three-row replicate. All samples were collected on Sept. 25, 2013. Clusters and berries were randomly selected from across the vine from both interior and exterior positions. Two clusters were harvested from each of the five vines, one on the north and one on the west side of each vine.

The 100 berry samples were processed and analyzed at the UC Davis Teaching Winery Laboratory the following day for the following viticultural and enological properties: fresh berry weight, Brix at harvest, titratable acidity, pH, yeast assimilable nitrogen (YAN) and malic acid.

Ten clusters were weighed to generate an average cluster weight for each Zinfandel selection. An additional 100 berries were randomly gathered from the same data vines and put in the freezer for seed count, phenolic extraction and HPLC analysis at a later time. In March 2014, pruning weights were collected from the same data vines to generate an average pruning weight per selection. On March 21, 2014, the buds were examined to determine if any of the clonal selections had early or late bud break.

The study was complicated by the movement of Grapevine Red Blotch Associated Virus (GRBaV) into one block, which resulted in removal of that data from the analysis. Disease symptoms were generally not found in the other four blocks, and if they were, care was taken not to use the vine as a data vine. The fruit data taken from infected vines were consistent with other studies that found GRBaV reduces sugar accumulation.

Genetic variation
Grapevines are typically propagated vegetatively from cuttings, leading to identical copies of the grapevine variety through each successive generation.

The DNA in cells of grapevine buds, and those of all living organisms, are constantly mutating at a very slow rate. Most of these mutations are repaired, others damage the buds, and occasionally these mutations result in a noticeable and beneficial change such as a shoot with looser clusters or berries that ripen earlier or later.

Cuttings from these altered shoots can be propagated into vines and vineyards with the new characteristic. Genetic variation is most commonly seen as changes in berry color or flavor, disease susceptibility/resistance, shoot growth behavior or sugar accumulation.

2013 harvest results
One goal of this study was to identify such genetic variation that had occurred in selections of California Heritage Zinfandel. The results of the 2013 data were consistent with previous observations and analysis collected by Mike Anderson and Wolpert in the years 2005–12.

Across the enological and viticultural traits of titratable acidity, malic acid, YAN, bud break scores, pruning weights and fresh berry weight, there were no significant differences found among the selections of Heritage Zinfandel or Primitivo.

Significant differences were observed in five parameters in 2013. The Primitivo selection had a lower cluster weight (344.0 g) than all of the Zinfandel selections (374.3–479.5g), and there were fewer berries per cluster. One heritage selection had a significantly larger cluster weight and therefore more berries per cluster. Differences for these traits among the traditional FPS selection and the other five heritage selections were not significant.

The same Primitivo selection had a significantly higher Brix value (24.8°) on the same day of harvest than all of the other Zinfandel selections (22.7°–23.6°), and a higher pH (3.45) compared to the Zinfandel selections (3.31–3.42).

Conversely, the heritage selection 89 had a lower Brix (22.7°) and lower pH (3.31). However, when compared to previous years’ data, the heritage selection 89 did not trend differently in any other year, while the Primitivo consistently showed a repeating trend.

In 2013, the Primitivo had fewer seeds per berry (1.5) compared to the Zinfandel selections (1.8). The data suggests that Primitivo has looser clusters due to fewer berries per cluster and fewer seeds per berry that results in a faster accumulation of sugars and loss of acidity during ripening.

Summary
The goal of the Heritage Zinfandel project was to investigate whether genetic variation could be identified and gathered from diverse old-vine Zinfandel vineyards.

The results of the 2013 data suggest that the heritage selections are phenotypically identical in their expression when examined in a single vineyard. Further analysis of phenolic, aromatic and genetic profiles can be undertaken.

Given that genetic changes and mutations are typically slow to accumulate, and infrequent in their propagation, we did not expect to see large numbers of differences among selections in this experiment.

One interpretation might be an affirmation of the idea of terroir, as this study would seem to confirm that Zinfandel vines and wines from historic vineyards behave and taste differently from each other because of the unique soil and climatic differences that they are cultivated throughout California.

The authors wish to thank the Zinfandel Advocates and Producers for their continued commitment to Zinfandel research, which has made this study possible.

Bibliography
1. Calo, A., A. Costacurta, V. Maras, S. Meneghetti, and M. Crespan. 2008 “Molecular correlation of Zinfandel with Austrian, Croatian, and Hungarian cultivars and Kratosija, an additional synonym.” Am. J. Enol. Vitic. 59: 205–209.
 
2. Maletic, E., I. Pejic, J.K. Kontic, J. Piljac, G.S. Dangl, A. Vokurka, T. Lacombe, N. Mirosevic, and C.P. Meredith. 2004 "Zinfandel, Dobricic, and Plavac Mali: The genetic relationship among three cultivars of the Dalmatian coast of Croatia." Am. J. Enol. Vitic. 55: 174--180.
 
3. Piljac, J. 2004 Zinfandel: A Croatian-American Wine Story. Zrinski d.d. Cakovec, Croatia.
 
4. Sullivan, C.L. 2003 Zinfandel: A History of a Grape and Its Wine. University of California Press.
 
5. Wolpert, J.A. 1996 “Performance of Zinfandel and Primitivo clones in a warm climate.” Am. J. Enol. Vitic. 47: 124–126.

 
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