THE dry and windy conditions ahead of the 2011 harvest are proving the merits of Distell’s recent investment in technology to far more accurately measure vine water status, according to Drikus Heyns, a senior specialist viticulturist with the company.

He says the small portable pressure chambers or ‘pressure bombs’ as they are also termed, gauge the leaf water potential in the vines as a basis for determining optimal irrigation strategies. Developed initially for research purposes, they are now used in southern France, Spain and to a limited extent elsewhere. Distell, as South Africa’s largest producer of wines is piloting the technology not only to further enhance grape quality but to employ scarce water resources more strategically.

Heyns says although it is important to observe as many variables as possible in the vine’s environment, the most sensitive indicator of stress is 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 says 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 is enough, it is critical to the health of the vines and to the quality of the grapes they produce. When there is too much water stress, the stomatae (pores) of the leaves close, 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 stomatae. 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.

According to Heyns, 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.”