Dry and windy conditions prior to the 2011 wine grape harvest are proving the merits of wine and spirits pro- ducer Distell’s recent investment in technology to more accurately measure vine water status, reports Distell senior specialist viticulturist Drikus Heyns.

He explains that the measuring technology comprises small portable pressure chambers, or pressure bombs, which gauge the leaf water potential in the vines as a basis for determining optimal irrigation strategies.

The technology was initially developed for research purposes, but it is now used in southern France, Spain and, to a limited extent, elsewhere.
Distell is piloting the technology not only to further enhance grape quality but, also 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 using in a range of vineyards across the Cape Winelands enable our viticulturists to measure water stress in the vines and to refine irrigation schedules more accurately than was possible in the past,” he adds.

Further, Heyns explains that vines actually benefit from some water deficit as too much water has a negative effect on grape flavour.

To gauge exactly how much water stress is enough 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, or pores of the leaves, close, inhibiting the daytime intake of carbon dioxide, slowing down photosynthesis and limiting growth and the production of the grapes,” he says.

Distell is, at this stage, focusing exclusively on testing the technology at red wine vineyards in the Durbanville, Philadelphia, Paarl and Stellenbosch areas in the Western Cape. 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 to limit evapotranspiration. The leaves are also covered with an aluminium sheet, which prevents light from opening the stomatae.
The petiole of the vine leaf, which is the stalk attaching the leaf to its stem, is cut and the leaf is quickly placed in 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. The degree of stress indicated by the vine is called stem water potential. The pressure reading enables the user to determine the irrigation required.

Allowing for the rooting depth of the vines, soil texture, soil moisture content and 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.

“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,” says Heyns.

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