According to Drikus Heyns, as South Africa's largest producer of wines, Distell is piloting the technology not only to further enhance grape quality but to more strategically employ scarce water resources.
A senior specialist viticulturist with the company, he has just returned from two years in France, where he completed an OIV-accredited MSc in Viticulture and Oenology at Montpellier University.
He said although it was important to observe as many variables as possible in the vine's environment, the most sensitive indicator of stress was the vine itself. "Until now, we have had to rely mostly on soil water measurements to assess water stress in the vines. However, the portable pressure chambers we are currently using in a range of vineyards across the winelands allow our viticulturists to measure water stress in the vines themselves and to fine-tune irrigation schedules far more accurately than was possible in the past."
He said vines actually benefited from some water deficit as too much water had a negative effect on grape flavour, but to gauge exactly how much stress was enough, was critical to the health of the vines and to the quality of the grapes they produced. When there was too much water stress, the stomata (pores) of the leaves closed, inhibiting the daytime intake of CO2, slowing down photosynthesis and limiting growth and the production of the grapes.
Focusing at this stage exclusively on red wine vineyards in the Durbanville, Philadelphia, Paarl and Stellenbosch areas, Heyns and his team visit the vineyards weekly, testing the same vines each time to assess vine health and water needs.
In each instance, leaves are cut and bagged in plastic bags to limit evapotranspiration and are first covered with an aluminium sheet, which stops the impact of the light in opening the stomata. The petiole of the vine leaf (the stalk attaching the leaf to its stem) is cut and the leaf quickly placed into the chamber with the cut end of the petiole protruding from the chamber. As the pressure in the chamber equals the pressure or tension in the leaf, the sap is forced out of the cut petiole surface. A pressure gauge indicates the level of stress experienced by the vine with the degree of stress indicated by the vine, called stem water potential (SWP). The pressure reading enables the user to determine the irrigation required.
Allowing for the rooting depth of the vines, soil texture, soil moisture content, vine canopy size as well as the type of trellising and row spacing, vine water requirements are determined by the extent of evapotranspiration that occurs in the vine and how far the vine is in the ripening of its grapes. In addition, each wine grape varietal has its own particular water demands.
"With the current conditions in the lead-up to the harvest proving slightly warmer than over the past few years, the pressure chambers place us in a far better position to manage the health of the vines and ensure optimal quality fruit to the cellars," he adds.
Distell is working very closely on this project with the French viticulturist Prof Alain Deloire, presently based at the University of Stellenbosch, who has extensive international experience in strategies to promote plant health and optimal harvesting times.
