The Effect of Irrigation on the Chemical Composition of Grapes

Every chemical compound in grapes and in wine play some role in the life of the grape/wine, be it during physiological processes during the growth stage, or in the finished wine itself, where it may contribute to the taste and flavor of the wine or the stability of the beverage over time.  For example, anthocyanins are responsible for the color of the grape berries, and ultimately for the finished wine.  Also, flavonols, while they are colorless in the skins of grapes, they are thought to act as a sort of shield against UV radiation.  The exact composition of these compounds in grapes and wine depend on a variety of factors, including grape variety/genetics, environmental factors, and viticulture and winemaking practices.  Studies have also suggested that anthocyanin and flavonol composition is a function of grape growth and skin characteristics.

http://www.environment.gov.au/water/ topics/images/drip-irrigation.jpg

Most research to date has focused on the phenolic composition of grapes and wine, with very little focus on the many remaining chemical compounds in the fruit and finished beverage.  Phenolics should not be the only thing considered during these research studies, as there is likely a synergistic effect between multiple compounds in the system.  Understanding of the full chemical composition of grapes and wines are important not only from a purely scientific standpoint, but also for the grape grower and winemaker due to the direct effects on fruit and wine quality.

The goals of the study presented today were to determine the effect of irrigation management (a viticultural factor that may possibly alter the chemical composition of grapes/wine) on plant yield and physiology, as well as grape berry morphological characteristics, polyphenol and metal composition.  The study also sought to determine the absence of irrigation all together could have an effect on grape quality.

Methods

The experiment was performed in 2008 in a 5 year old vineyard in Montegiordano Marina, Southern Italy.  The climatic conditions there are considered “very hot” (climatic region 5).

The experimental vineyard plot was 0.3ha, with 10 rows of spur-pruned vines trained to a permanent horizontal unilateral cordon.  Distance between vines was 2.5m with 1m between rows.  Final plant density was 4000 vines per hectare.  Rows were planted in a north-south orientation.

Half of the plants were subject to irrigation from the early stages of fruit set to veraison using water amounts equal to 100% of cultural evapotranspiration.  Specifically, this equaled 24L per plant per each irrigation event (10 total) at 5 day intervals.  The other half of the plants were not subject to irrigation.

Meteorological variables that were measured or calculated were: temperature, rainfall, and photosynthetic photon flux density.  Physiological characteristics measured or calculated were leaf-to-air vapor pressure deficit, stem water potential (in order to determine plant water status), leaf gas exchange, chlorophyll florescence, basal florescence yield in dark-adapted leaves, maximal florescence yield in dark and light conditions, maximum quantum yield of PSII photochemistry in dark-adapted leaves, and finally the effective quantum yield of PSII in light-adapted leaves.

At harvest, 30 plants per treatment were randomly selected and the following were measured/calculated: number of clusters and yield per plant, cluster weight, number of berries per cluster, total berry weight per cluster, and the number of leaves per shoot.  For each plant, 3 clusters were randomly selected.

Berries from each cluster were separated into different weight categories: 1) less than 0.60g; 2) between 0.60 and 0.90g; 3) between 0.90 and 1.25g; and 4) greater than 1.25g.  For each plant, 20 grapes per weight class were randomly selected to measure/calculate berry fresh weight, berry diameter at the “equator”, and berry diameter at the “poles”.  The following characteristics were calculated for the berries: surface, volume, surface/volume ratio, the ratio of berry surface/berry weight, and the ratio of skin weight/berry weight.  Skin thickness and soluble solid content of berries was also measured.

For anthocyanin and flavonol extraction and analysis, three clusters per plant were randomly selected and berries separated into the aforementioned weight categories.  Anthocyanins and flavonols were measured, as well as levels of iron, copper, zinc, and calcium.

Results

(Note: I’m leaving out many exact details about values due to space limitations, but if you need to know exact numbers/values of any item presented in the results, just ask and I’ll see if those details are available and will let you know).

• The growing season was marked with high temperatures and low rainfall.

o   Max temperatures ranged between 15.3 and 38.5^oC.

o   Min temperatures ranged from 12.3 and 29.1^oC.

o   Rainfall during the experimental growing season was a very low 21.9mm.

•  There were no significant differences between irrigated and not irrigated plants in regards to net photosynthesis.
• There were no significant differences in transpiration values between either of the treatments.
• There were no significant differences in stomatal conductance between either of the treatments.
• Maximum quantum yield of photosystem II and actual quantum yield of PSII reaction centers in leaves were not affected by irrigation treatment.
• Mean numbers of clusters per plant were not different between treatment groups.
• Yield per plant, cluster weight, and total berry weight were significantly different between treatment groups, with higher values occurring in the irrigation group.

o   Irrigation significantly increased the frequency of grapes with greater than 1.25g mass and reduced the frequency of grapes with less than 0.6g mass.

• Irrigation treatment significantly affected berry fresh weight and skin fresh weight.

o   Irrigation significantly affected berry surface/volume ratios, and were significantly higher in irrigated plants.

o   Skin fresh weights were higher in non-irrigated plants, which resulted in a decrease in skin specific surface and increased in skin specific weight.

o   For the two intermediate weight categories, there were significant differences between the two treatment groups were noted for seed weight per berry as a result in the differences between seed number per berry.

§  There were more seeds in non-irrigated plants than in the irrigated treatment group.

•  Soluble solid content was significantly higher in the non-irrigated group than the irrigated group.
• Total anthocyanins were significantly higher in the non-irrigation group than the irrigation group.

o   This result was positively correlated with berry weight.

• Significant differences were found in the concentrations of petunidin-3-O-acetylglucoside, peonidin-3-O-acteylglucoside, and petunidin-(6-O-caffeoyl)glucoside.

o   Levels were higher in non-irrigated plants (9x, 18x, and 10x, respectively).

• Levels of single anthocyanins increased with decreasing berry weight.
• Berries from irrigated plants had significantly lower ratios of acetylated anthocyanins/coumaroylated anthocyanins.
• Total flavonols were not significantly different between the two treatment groups.

o   Levels of single flavonols were significantly higher in heavier berries.

• Iron, copper, and zinc levels were significantly higher in berries from irrigated plants than from non-irrigated plants.
• Calcium levels were not significantly different between the two treatment groups.
• Metal levels significantly decreased in increasing berry weight.
• There were no differences in berry skin thickness between either treatment group.
•  No significant differences were found in the number of skin layers and thickness of the berries between either treatment group.

Conclusions

One undesired outcome of this experiment was the near drought-like conditions of the weather during the experiment.  This resulted in plants being subject to moderate-severe water stress, which caused some leaf necrosis and can influence the micro-climate at the cluster.  Specifically, it has been shown that this type of stress may affect berry size and chemical composition, thereby potentially changing the outcomes of some of the tests, and making it generally more difficult to tease out cause and effect.

The results of this study also showed that total anthocyanins were higher in grapes from non-irrigated plants than in irrigated plants.  This results in a positive influence on the long-term color stability of wines, as these compounds working in concert with tannins and flavonols to strengthen color stability in the aging beverage.  Additionally, increases in these compounds and well as the observed increases in petunidin-3-O-acetylglucoside and peonidin-3-O-acteylglucoside, can have positive sensory benefits to the finished wine as well.

Another interesting result from this study is that metal levels significantly decreased with increasing berry weight.  Excess metal concentrations in wine are known to cause negative sensory characteristics, delay the fermentation process, and increase instability.  Fe, Cu, and Zn were all found to be significantly lower in grapes from non-irrigated plants than in irrigated plants.

Overall, the results of this study suggest that less irrigation increased the quality of the finished wine.  Specifically, little to no irrigation results in lower berry yield and a reduction in berry size without negatively affecting grape quality in terms of the chemical composition of the grapes.  This study confirms what many in the wine industry in that grapes grown under water stress conditions can result in higher quality wine (provided there are no set-backs during the winemaking process).  Even though many already knew less water is better, this study paints a good picture of exactly how the chemical composition of the grapes changes when subject to these drier conditions.

There are many more results to this study that I did not cover due to time and space considerations, but I’d love to hear your thoughts or questions on them, even if I didn’t specifically cover it.  What do you all think of the study?  What would you like to have seen done differently (if anything).  I, for one, would have liked to see them create experimental wines from these two treatment groups and measure the same compounds to see how irrigation actually alters the chemical composition of the finished wine and not just the starting point grapes.  Do these differences carry through the winemaking process?  Are different winemaking techniques better suited to maintaining the original/similar chemical composition of the grapes?

I’d love to hear what you think! Please feel free to comment below!

Source: Sofo, A., Nuzzo, V., Tataranni, G., Manfra, M., De Nisco, M., and Scopa, A. 2012. Berry morphology and composition in irrigated and non-irrigated grapevine (Vitis vinifera L.). Journal of Plant Physiology 169: 1023-1031.

DOI: 10.1016/j.plph.2012.03.007
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!