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Title.jpgBy, Cecilia Parsons | Associate Editor

 

Value of red table grape varieties is dependent upon berry color. Achieving the desired red hues at harvest is the goal for growers seeking premium prices for their crop.

Numerous Factors Affect Coloration

University of California Cooperative Extension viticulture advisor Ashraf El-Kereamy in Kern County said there are many factors that can affect coloration and are used on red seedless table grape varieties.

Red table grape varieties such as Crimson Seedless and Flame Seedless, under certain conditions, may require help in coloring while some new varieties of red table grapes develop color without assistance. Flame and Crimson are popular varieties, El-Kereamy said, and still in favor with growers due to their harvesting time—the varieties need a little more attention to bring out the color.

The red, purple and black colors in table grapes are due to the plant pigments, anthocyanins. These pigments are derived from the basic products of photosynthesis and are converted by enzymes to flavonoids and coupled to sugar molecules by other enzymes yield the final anthocyanin pigments.

Disruption in any of the enzyme mechanisms by genetic, environmental or cultural practices could alter anthocyanin production and affect berry coloration.

Lack of Color at Harvest

What causes disruption and subsequent lack of grape color at harvest is complicated. El-Kereamy said red coloration is under hormonal control that is influenced by several factors. Some can contribute to optimal berry coloration, other factors, which may be out of the control of the grower can contribute to lack of color.

Use of nutrients or supplements as a part of a vineyard management plan can effect color due to composition or mode of action if they are applied at the critical stage for anthocyanin induction and development.

Nitrogen and Potassium

Nitrogen (N) and potassium can influence grape color and must be managed. Moderate nitrogen supply before bloom and moderate potassium during veraison can help in optimizing anthocyanins. However, excessive N can have a negative effect on color. Fruit ripening and coloration can be delayed by too much applied nitrogen. Determining the optimal amount of N for vine growth without over application is essential. Foliar potassium application can boost grape anthocyanin accumulation and coloration, but it can reduce berry size in some cases.
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Deficit Irrigation

El-Kereamy said deficit irrigation at the proper time can assist in bringing on berry color.

A study funded by the California Table Grape Commission found total berry skin anthocyanin contents and individual pigment compounds increased with deficit irrigation at two experimental sites in Coachella and San Joaquin valleys. Deficit irrigation induced expression of several genes involved in anthocyanin accumulation.

Other Cultural Practices

There are some other cultural practices growers can use to improve berry color. Large, dense canopies that prevent light from reaching the developing fruit can stall berry coloration. The cultural practice of shoot and leaf removal to allow more light to reach the fruit can help with coloring. Vine nutrition and rootstock selection also effect canopy size, contributing to color determination.

Plastic covers on grapevines are used to protect them from rain events and extend harvest. Transparent or green plastic covers are used with red varieties to let in light and assist with coloration.

Red table grapes grown on sandier soils will color more than the same varieties grown on heavier soils, El-Kereamy said.

Temperatures have a significant effect on anthocyanin biosynthesis and accumulation. Anthocyanin biosynthesis increases with temperature until the maximum of 95 degrees F. Temperatures above 95 reduces anthocyanin biosynthesis and degradation is increased causing poor red coloration. Water management and building a good early season canopy can help overcome the negative effect of high temperatures. Grape anthocyanin biosynthesis and accumulation are best when nighttime temperatures are below 73 degrees F. This presents a problem for growers in the Coachella Valley and some southern parts of the San Joaquin Valley.

Ethylene

El-Kereamy’s studies on anthocyanin in grapes demonstrated that grapes produce a small amount of the ethylene at veraison which induces expression of genes and starts anthocyanin accumulation in red grapes. Internal ethylene concentration in grapes affects anthocyanin and color. An external source of ethylene releasing compounds applied to grapes causes an increase in internal ethylene and also activates anthocyanin. According to El-Kereamy the high temperatures inhibit the coloration due to hormonal changes that act against activation of anthocyanin biosynthesis genes.

Abscisic acid

Another plant hormone known for its role in anthocyanin biosynthesis is abscisic acid or ABA. An increase in the ABA content of berries coincides with veraison and red color initiation in grapes. The application of ABA at veraison
also stimulates anthocyanin biosynthesis. Commercial products that contain an ethylene releasing compound or ABA as the active ingredient are commonly used in vineyards to improve color. El-Kereamy said that attention should be given to varietal differences, timing of the application and other cultural practices during the application. Practices or conditions that suppress the internal concentration of ethylene will result in poor coloration.

IMG_6058.jpgEthephon

The commercial product Ethephon is standard practice for table grape growers who need help with color. Ethephon is a plant growth regulator used to promote fruit ripening, abscission, flower induction, and other responses. It is applied as a tank spray. It moves inside the berries and releases ethylene and activates anthocyanins biosynthesis.

It does not have an effect on berry size, El-Kereamy said. ProTone is the commercial product with ABA and it can be mixed with Ethephon and used as a spray application for color. Both of these products are plant growth regulators. Other plant growth regulators gibberellic acid and cytokinins are known for their effects on ethylene and ABA and have a negative role in coloring grapes.

Ethephon is listed as a pesticide, El-Kereamy said and must be used according to the label. There is a re-entry period and Pre-harvest interval period. That requirement makes timing an application tricky. The spray is applied at color break stage, not before. You want to be ‘pushing the color,” El-Kereamy said. Quality of the fruit can be affected if applied too close to harvest.