Table grape quality from vineyard to shelf – Maties researcher investigates

Filling knowledge gaps

Currently, there are many knowledge gaps regarding the morphological, chemical and pathological status of table grapes at different points along the value chain, running from vineyard to shelf. Also, in terms of technological advancement, this particular value chain is lagging behind others, Strydom points out.

Limited available scientific information on vine growth balances, physiology (plant processes) and grape quality parameters hinder this type of advancement.

“This (lack of information) needs to be addressed to improve the competitiveness of the industry, given the increasing pressure on available water resources,” she says.

Strydom hopes her research will help fill some of these knowledge gaps to improve water management practices, in particular. “Producing the quality needed to enter new markets requires technological advancement,” she says.

Table grapes are intensively irrigated to control vine growth, yield (bunch and berry size), ripening conditions and grape quality at harvest and during storage.

By manipulating the mesoclimate, plant behaviour can be affected. It could provide a means to buffer vineyards, or even stabilise vineyard conditions during erratic seasonal weather, Strydom points out.

“On a larger scale, this could also protect a vineyard against the potential effects of climate-related weather variability,” she says.

“Sustainable production in the table grape industry requires proper water supply (timing and amount) during the berry growth phase to ensure sufficient yields.”

Spotlight on quality indicators

Cultivation practices, water deficits and storage affect the quality indicators of table grapes, cautions Strydom. The levels of sugar and acidity, as well as phenolics and volatiles provide information on grape quality. These compounds further indicate if the vines are being stressed by abiotic factors.

Increased knowledge of these secondary quality indicators would add significant value to the description of grape quality, Strydom points out.

“The physical characteristics of the berries, such as their size and firmness, are important indicators of eating quality and storage potential. We also know that storage affects berry firmness and split, phenolic content, antioxidant activity and flavour”.

Strydom further explains that this is why the measurement and analysis of multiple variables in this study will improve our understanding of vine reaction to abiotic factors. Affirms Strydom: “It necessitates an investigation along the whole value chain.”

Manipulating the mesoclimate

Strydom is conducting a trial in a Crimson Seedless vineyard in the Robertson region. As part of her research, she is considering how changes in water regimes and in mesoclimatic conditions affect the vines.

More specifically, she is comparing open field conditions to conditions in which table grapes are covered with plastic. She is also considering the effect of bunch manipulation techniques (including thinning and sizing) on vegetative and reproduction growth.

Although Crimson Seedless – one of the top six cultivars exported from SA – is the most studied cultivar globally, there is still much to learn about it in terms of grapevine responses, she says.

Growers can manipulate the climatic conditions in which table grapes are grown through their choice of row orientation, trellis systems, row and vine spacing, irrigation, netting and plastic covering.

Due to its effect of advancing bud break and time of ripening, protection, as well
as improving grape quality, simulating alternative (mesoclimatic) conditions with overhead plastic covering is, according to Strydom, a technique “worth exploring”.

Technological advancement is hindered by limited available scientific information on vine growth balances, physiology and grape quality parameters.

The study

Strydom is focusing her investigation on the effect of different water treatments
on Crimson Seedless vines. Her study comprises a total of three trials and will run until March 2023.

Two trials (one under open field conditions and one under overhead plastic covers) are being conducted in the same Crimson Seedless vineyard.

The experimental design of the study includes a randomised block strip plot with different water regime and mesoclimate treatments.

Different water treatments are being applied in this vineyard, ranging from 100% water (standard farm practice) to 55% of the standard farm irrigation practice.

The plastic covers used to change mesoclimatic conditions are installed after bud break to help protect the vines. Soil water content and mesoclimate are monitored at three grape ripeness levels.

A third similar trial is being conducted on the cultivar Italia in the Apulia region of Italy.

This part of the study includes an additional focus on the sensory analysis of this aromatic cultivar. (Crimson Seedless has a relatively neutral aroma when compared to the musky aroma of Italia.)

The scientists involved in this trial are based at the University of Foggia, the University of Turin and the University of Palermo. Results obtained from studies on Italia will provide pointers on how other aromatic cultivars may react to similar treatments under local conditions, Strydom says.

She believes the information gained from the three trials will, collectively, provide the industry with guidelines on the application of alternative technologies, such as overhead plastic covering.

“We also expect to gain more knowledge about eating variables that will enable us to determine flavour variability (as a result of different mesoclimatic conditions) for specific grape cultivars,” she says.

Crimson Seedless is the most studied cultivar globally, but there is still much to learn about it in terms of grapevine responses.

Early results

“We know changes in environmental and mesoclimatic conditions affect grapevine phenology,” Strydom says. “We also know that growing table grapes under (a plastic) cover affects bud break and grape quality. Simulating alternative mesoclimatic conditions is thus well worth exploring in our local context to provide growers with more management tools.”

So far, preliminary results show that the different treatments affect the grapes’ quality, storage ability and physiological variables.

The potential of the treatments to alter growth and quality variables will have to be verified by repeating the trial over at least two more seasons.

Strydom hopes her research will provide new information on how table grape vines respond to different water treatments, especially in terms of grape quality. She also aims to develop advanced grape quality parameters and tools to prevent over-irrigation and under-watering on farms.

PHOTOS BY JANENE STRYDOM.
Main image: 1: Climate data retrieval from data logger.