Introduction
With consistently low flows into the Murray Darling Basin over the last few years and unprecedented restrictions on irrigation water allocations, winegrape growers need to consider alternative vineyard management options to maintain profitable businesses.
In the absence of additional water allocations growers may need to consider re-planting their vineyards with varieties and rootstock materials which respond better to low irrigation regimes and salinity.
Vineyards and Water Use
In what was once a normal year, about 6% or 580GL, of Australia’s irrigation water was applied to vines. The efficiency with which this water is used in a vineyard is affected by the way that the water is applied. With flood irrigation only about 20% of the applied water is actually used by the vines, with the rest lost to evaporation. The effective proportion of water use is greatly improved with drip irrigation and can be better still if soil evaporation can be reduced.
How Vines Use Water
Nearly all of the water used by the vine passes through the leaves. Scientists have spent considerable time trying to understand how this water loss is controlled at the leaf level and whether we may be able to influence this process to make vines more water-use-efficient. We know that vines in drying soil transpire (or lose water) less than vines in wet soil because the roots produce chemical signals to slow the rate of water loss. However, this slow down also reduces photosynthesis and grape quality. There is always a trade-off between these counterpoised priorities – to reduce water loss but maximize photosynthesis. Deficit irrigation technologies such as Reduced/Sustained Deficit Irrigation and Partial Rootzone Drying seek to strike the best balance.
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Using an assumed relationship between expected weather conditions and vine transpiration it is possible to predict annual irrigation requirements. In the past this has varied from about 2ML/ha for the cooler districts (Barossa, McLaren Vale, Clare) to 6 or 7 ML/ha for the hotter districts (Riverland, Sunraysia). However, this does not take into account that some varieties may have different requirements to others.
Heat Tolerant Varieties
Grape varieties vary in their response to heat and water stress and therefore require different management techniques . Sometimes varietal differences reflect the geographical origin of the grape. For example, Shiraz is thought to have originated from the relatively mild environment of the northern Rhône Valley in France whereas Grenache may have developed in the more arid environment of the Mediterranean basin in northeast Spain. These differences in evolutionary background mean that Grenache has a more acute response to heat stress than Shiraz. For the most part, in Australia we are growing vines in, what is for them, a stressful environment. This emphasizes the need for irrigation and increases the likelihood of stress responses.
Varieties and Water Use
Varieties can differ in terms of their root sensitivity to water stress, their leaf response to evaporative demand (VPD), the difference in canopy size and vine properties and their different hydraulic conductivity (ie the physical movement of water within the plant). Using the Grenache/Shiraz comparison, on a mild day in the Barossa Valley, there was little difference in transpiration between the varieties but on a hot and dry day Grenache stops transpiring and saves water (isohydric or pessimistic behaviour) while Shiraz continues transpiring and uses more stored soil water (anisohydric or optimistic behaviour). All vines respond in some degree to changes in VPD by partially closing their stomata so that hot dry conditions do not cause excessive transpiration but some are better at this than others. Using knowledge of varietal response to VPD allows us to rank common varieties for tolerance.
Rootstocks and Stress Management
Rootstocks can also influence vine performance by improving vigour which exploits available spoil volume, the ability to transport water ie its hydraulic conductance, its influence on varietal scion and its ability exclude or tolerate salt.
Salt Tolerance
The references in this tool suggest considerable variation in performance of any one rootstock and this may be due to differing local conditions, especially the presence of salinity. Rootstocks vary considerably in their ability to tolerate salt but most are superior to own-roots.
- Sensitive/moderately sensitive rootstocks o Own roots, 1202C, Kober5BB, Teleki5C, SO4
- Moderatly tolerant to tolerant rootstocks o Ramsey, 1103 Paulsen, Ruggeri 140, Schwartzmann, 101-14, Rupestris St George
Rootstock Case Study
The following case study shows how Cabernet Sauvignon on Ramsey rootstocks cope with severely reduced irrigation.
An experiment in the SA Riverland applied water ranging from as high as 6.7ML/ha to 0.6ML/ha then tested the effects of these varying water regimes on yield, transpiration and wine quality.
The study found that canopy vigour was reduced as water amount fell, but surprisingly, even at the lowest irrigation level end of season canopies looked reasonable. Transpiration was reduced by factor of 4-6 between the high and low water treatments. Yield was also reduced, but only by around 50% even in the lowest water treatment.
High levels of salt were measured in leaves, petioles and juice/wine, suggesting that these vines were surviving by accessing salty (6-8dS/m) water present at depth under the vines. Soil solution samplers showed that there was a layer of good (<1dS/m) water above the salty water present throughout the season despite nil irrigation from beginning of January and little rain.
It is possible that due to the rootstocks this salty water may have sustained these vines.
Survival
The ability of Ramsey rootstock to explore a large soil depth and to exclude salt probably helped these vines survive and produce a crop.
