The Effect of Water Deficits on Anthocyanin and Tannin Concentrations of Merlot Grapes

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Flavonoids in grapes, particularly anthocyanins and tannins, have great impact on the quality of wines, specifically in the areas of color and astringency.  The composition of these compounds in grapes depends on many factors, including grape variety, water status, and other environmental/climatic variability.  This variability in grapes leads to a large variability in flavonoid concentration in finished wine as well, which can further be manipulated by different wine making procedures and techniques.   In regards to water status during the growing season, it is known that changes in water availability alters the concentrations of these major flavonoids, however, it is unclear precisely how and to what extent these changes are reflected in the grape.

Berry ripening may be either accelerated or decelerated depending upon the timing and duration/severity of the drought.  Skin growth itself could be inhibited during droughts, which could alter the proportion of skins and seeds to total berry weight, which is an important indicator of grape quality.  In terms of flavonoid synthesis, tannins are synthesized earlier in the season, while anthocyanins are synthesized later.  Therefore, the timing of the water deficit could be critical in the development of one or both of these important quality components of grapes and wine.  Specifically, it is thought that since flavonoids are primarily located in the skins of grapes, when berry growth is inhibited by some mechanism, the resulting concentrations of flavonoids in the grape will be increased due to an increased surface:volume ratio.

The goal of the current study, therefore, was to examine the influence of water status on grape berry growth, skin tannins, and anthocyanins in Merlot grapes in order to determine the extent that which water deficits induce changes in grape berry composition.  To date, results of similar studies have been inconsistent.  By understanding how water deficits alter flavonoid composition (specifically tannins and anthocyanins), one may be able to develop specific vineyard management strategies in order to maximize the quality of the grapes, and ultimately quality of the wine produced from those grapes.

Methods

Experiments were performed in 2004, 2005, 2007, and 2008 in a vineyard of Merlot (Vitis vinifera) which was planted in 1993.  The vineyard was located at an experimental farm at the University of Udine in northeast Italy on soil (49% sand, 31.5% silt, and 19.5% clay) with 12% gravel, 0% slope, 29.3% field capacity, and a permanent wilting point of 19.3%.  Orientations of the rows were north-south, with spacing at 1m between plants, and 2.5m between rows, and about 4000 vines per hectare.  Vines were trained on a spur cordon system.

Water control to the vines was achieved by sheltering the rows under a tunnel covered by a polyethylene film.  The tunnel was placed over the whole experimental block that included four rows of 60m in length (240 vines).  Experimental rows were the two center rows of the four, since rain water could possibly seep into the edges of the tunnel and uncontrollably change the water status of the two rows closest to the edge.  The first and last 8 plants of each experimental row were also excluded due to the same reasons as mentioned just previously.  Water was supplied to the vines by a sub-surface drip irrigation system with emitters at 2.5L per square meter per hectare.  Each emitter was 0.6m apart and there was 2.5m between each irrigation line.

Plant water status was measured by midday measurements of stem water potential.  To measure this, two leaves per plot (on each side of the row) were covered with aluminum foil coated plastic bags for one hour, in order for the stem and leaf water potential to equilibrate.  After one hour, the leaves were removed and stem water potential was measured by a pressure chamber.

Two water/irrigation treatments were established: a control were vines were irrigated once a week in order to keep the stem water potential between -0.2 and -0.6MPa, and a water deficit (WD) treatment were vines were irrigated to maintain a stem water potential of -0.8 and -1.4MPa during the ripening period.  To maintain this level, irrigation was cut off on the WD vines 43, 34, 45, and 47 days after anthesis in 2004, 2005, 2007, and 2008, respectively.  With the exception of 2007, one more irrigation was applied to WD vines between veraison and harvest.

Each irrigation treatment was replicated on four plots of 12 vines each.  Both control and water deficit treatments were performed under the polyethylene tunnels, to account for any microclimatic variation caused by being under the tunnel.

Grape berries were sampled every 7-14 days, from 21-40 days after anthesis to harvest.  For each sampling day, two sets of 30 berry samples were collected from each plot.  One set was collected to measure juice soluble solids (^oBrix) and titratable acidity, while the other set was collected for anthocyanin and tannin analysis.

Results

•       Midday stem water potentials were significantly lower in water deficit treatments than in control treatments from 55 days after anthesis until harvest.
•       Stem water potential decreased progressively in water deficit treatment vines throughout the ripening period, while stem water potential for the control vines remained consistently higher than -0.65MPa.
•        There were differences in the severity and the timing when the deficit became very severe.
•       Across all four seasons, water deficit significantly reduced the final berry weight and pH, and had no effect on soluble solids, titratable acidity, skin weight, or skin/berry weight.

o   There was a significant effect of season on all size and chemistry parameters, as well as a significant season x irrigation treatment interaction for berry weight, soluble solids, and titratable acidity (parameters were significantly different in some years but not others).

•       Anthocyanin concentrations significantly increased in the water deficit treatment but had no effect on skin tannin concentrations.

o   There were significant season effects on anthocyanin and tannin concentrations, and significant season x irrigation treatment interactions for tannin concentrations (significant changes in some years but not others).

o   Tannin concentrations were not affected by irrigation treatment.

•       Water deficit inhibited berry growth, though it did not alter the berry growth pattern during the season.
•       Maximum berry weight was reached at the same time for both treatments.
•        At harvest, water deficit berries were 7.6% to 20.1% smaller than control berries.
•       Skin tissue was 7-15% of berry weight, though skin and berry growth was relatively inconsistent from season to season.
•       Berry soluble solids were not consistently affected by irrigation treatment.
•       Titratable acidity was not affected by irrigation treatment.
•        Anthocyanin concentrations increased faster in water deficit berries than control berries, and were significantly higher in water deficit berries after 30 days into the treatment.

o   Water deficit increased the overall mean anthocyanin concentration at harvest by 50% compared to controls.

•       Overall mean tannin concentrations at harvest were not significantly different between treatments.

o   There was a significant treatment x year interaction (the treatment was significant in some years, but not others), and water deficit significantly increased tannins in 3 out of the 4 years.

Conclusions

The results of this study showed that anthocyanin concentrations in Merlot grapes significantly increased with the water deficit treatment; whereas only in certain years did this deficit alter skin tannin concentrations.  These results indicate that the relationship between fruit ripening and water status is complex.  Other studies have shown that tannin synthesis occurs earlier on in the ripening period, whereas anthocyanin synthesis occurs later on in the season.  Therefore if most or all of the tannins are synthesized before the water deficit occurs, it is likely that the drought will not significantly affect the concentrations of the compound.  However, if the water deficit occurs before synthesis is complete, as it happened with the anthocyanin concentrations in this experiment, final concentrations will likely be much more variable and affected by the drought.

The authors continued to elaborate on this relationship between flavonoid concentrations and water deficits in that the increase is due to an upregulation of anthocyanin biosynthesis but no corresponding increase in tannin biosynthesis.  Based on the results from other studies, the authors could not rule out a possible inhibition of anthocyanin degradation instead of the upregulation theory described earlier.  More research that focused on this type of physiology would need to be performed in order to get a more accurate understanding of the mechanisms involved with the increase in anthocyanin concentration when under a water deficit.

Generally speaking, the results of this study give some insight into how water deficits may alter the quality of grapes which would ultimately lead to changes in the quality of the wine produced from those grapes.  More work needs to be done to further understand the mechanisms behind the changes observed in the study, and should include many more parameters, as chemical composition of grapes can be very complex with each part playing a different role in different environmental and climatic situations. 

I’d also be curious to see a study that combined not only the types of experiments performed in this study, but to also take it one step further and create wines from the grapes under the different irrigation treatment, in order to determine how water deficits actually affect overall wine quality, instead of simply making assumptions.  Also, a study incorporating several different varieties of grapes may be important as well, as studies have shown that different grape varieties are affected by environmental or climatic conditions differently.

I’d love to hear what you all think of this study.  Can you think of ways to improve upon this study?

Source: Bucchetti, B., Matthews, M.A., Falginella, L., Peterlunger, E., and Castellarin, S. 2011. Effect of water deficit on Merlot grape tannins and anthocyanins across four seasons. Scientia Horticulturae 128: 297-305.

DOI: 10.1016/j.scienta.2011.02.003
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