Where is the best place to grow wine grapes,” I am often asked. The answer I give surprises many people. I say, “A cool desert, that is where! Deserts are typically sunny, but should be cool, and with sustainable supplies of irrigation water.” A desert is preferred because rainfall can be a problem for quality wine production.
There are two significant problems associated with rainfall, both relating to how difficult it is to control, in terms of timing and quantity. Firstly, rainfall induces many fungal diseases on leaves, shoots and fruit, which may have direct or indirect effects on fruit ripening and wine quality. Secondly, and often less appreciated, is that water supply is a principal means of regulating vine growth and physiology to maximize fruit ripening and potential wine quality.
In brief, we prefer to have slight moisture stress during the period of active shoot growth after flowering to inhibit lateral shoot growth and to limit leaf expansion and size. In association with an appropriate training system, this will help maintain a light, porous canopy—essential for wine quality. Secondly, and critically, we can use moisture stress to stop shoot tip growth in the period just before veraison. This is essential to avoid carbohydrate competition between the active growing shoot tip and the ripening berries.
If grapes are grown in a desert, of course, we need to irrigate. This gives us a chance to manipulate vine water stress at our will, in the absence of rainfall. The rest of this article discusses how to manage the desired level of water stress.
Irrigation research was one of my first projects when I started a viticulture career in the mid-1960s in Australia. Then, drip irrigation was very new, and I published one of the first studies on the method with wine grapes, comparing drip to flood irrigation.
This was also a time of new technology for measuring plant and soil moisture. Gypsum block and tensiometers were common then, and soon soil capacitance meters were to be introduced to measure soil moisture.
Evaluating the Pressure Bomb
In the late 1960s, pressure bombs used to measure leaf and stem water potential were introduced. The pressure bomb was a powerful tool to directly measure plant water stress, and help understand how grapevines respond to soil moisture conditions and the daily pattern of weather conditions.
After sunset, grapevines recover gradually from the water stress of the day before. Then, at sunrise, the plants begin to experience mild water stress. As air temperature increases, and as humidity decreases, so water stress experienced by the plant increases, being at a maximum in early afternoon. As sunlight levels decrease towards late afternoon, the water stress experienced by the grapevine recovers somewhat, again declining substantially after sunset.
Our published studies determined a major impact of current weather conditions on grapevine water stress. Grapevines experience the most water stress with bright sunlight, high temperatures, low humidity and high wind speed. These are all conditions that cause the most rapid water loss from the vines.
As soils dry out, the level of plant water stress is higher during the morning and in the afternoon. However, one must be careful to distinguish the effects of soil moisture from those of higher sunlight, temperature, wind speed and lower humidity. It is challenging to take spot measurements with the pressure bomb during the day to predict soil moisture conditions. Direct measurement of soil moisture profiles is preferred, which are much less variable over the day.
Use of Plant Appearance
I had almost side-by-side vines with different soil moisture conditions in an irrigation trial I conducted, and I soon learned how the appearance of vines change as they develop water stress. One of the most obvious symptoms is that shoot tips stop rapid growth, and eventually, they stop growth altogether. This symptom relates to moisture stress over several week’s duration.
Another of the visual effects of water stress is on leaf inclination. Initially, the petioles droop a little, and as stress continues and becomes worse, the leaves first hang vertically and then begin to cup by folding inwards along the main vein. When very stressed, you will see the backs of several leaves if you look along the row. I was working with the Shiraz (Syrah) variety, and the leaf backs are hairier than the front, so they are easy to distinguish. These symptoms may take several days or a week to develop.
Leaf Temperature Assessment
This assessment relates to present vine water stress. When vines are water-stressed, stomata (leaf pores controlling water loss on the leaf underside) partially or fully close, and so the loss of water from the leaf ceases. Transpiration (like evaporation) acts to cool leaves. So a sunlit leaf will have a temperature not so different from that of the air, perhaps a little warmer or cooler. However, when the vine is water-stressed, sun-exposed leaves are noticeably hotter than air temperature because the stomata close, and shade leaves are around air temperature.
I proposed a leaf temperature-based water stress index, which is in Table 1 (Below). Measurement is suggested in the early afternoon, or when it is sunny and air temperature reaches its maximum. Mid-shoot leaves well exposed to the sun are tested. I suggest pressing the leaf blade between fingertips and palm and quickly sensing leaf temperature on the palm. One must take an instantaneous impression of leaf temperature, as holding a leaf will quickly bring its temperature to that of your hand!
The reader might be thinking, “Why not use an infrared thermometer to measure air temperature as we have seen used recently to indicate forehead temperature with Covid-19 virus detection?” Indeed, such devices are now quite cheap, portable and accurate, but be careful always to measure leaves with the same angle to the sun.
Leaf temperature will give an instantaneous measure of vine water stress. In contrast, leaf inclination and shoot tip growth assessment will indicate water stress over the previous two weeks or longer. Therefore, leaf temperature can give a better indication of water stress, and so, irrigation needs, while shoot growth will advise how effective the irrigation has been.
The clever vine irrigator might believe these visual guides more than those of randomly taken pressure bomb tests to manage vineyards to optimize wine quality. Modern irrigation monitoring is developing systems based on thermal images, either close up or remote, related to my simple system of using one’s hand!