Crop coefficient (4) : how does heat wave effect water use?

How much does vine transpiration increase during a heat wave?

To address that question, a team of Australian researchers led by Edwards reported the following observations in 2011.While working in a greenhouse with Cabernet Sauvignon, researchers generated  a 2-day high-temperature event by imposing maximum daytime temperatures near 41°C.  Before imposing the heat wave event contrasted deficit irrigation treatments had also been imposed. Authors observed that the combination of heat wave and soil moisture deficit created leaf loss and wilting compared to well-watered vines. Physiological measurements (like vine transpiration and photosynthesis) gave an insight to understand why damages occur.

First, researchers  measured vine transpiration before the heat event. They found  that a vine experiencing limited soil moisture supply used less water than a well watered vine. This is exactly what we would expect, based on our previous discussion (See article #3 of this serie). : Vine Transpiration ratio is lower than its maximal value (called Kcb)  when  soil moisture is limiting.  Figure 1 shows a reduction in transpiration ratio when the vine is under deficit irrigation, while climatic demand is the same for both treatments.

Figure 1 : Leaf water use under saturating light ; no heat wave (adapted from Edwards et al, 2011)

Second, authors repeated the same  measurements during the heat wave and they observed very different responses. The transpiration  ratio increases more than 3 fold under well water conditions, while no increase is observed under deficit irrigation, as illustrated in Figure 2.

Figure 2:  Leaf water use under saturating light ; under heat wave  (adapted from Edwards et al, 2011)

Figure 2  illustrates  that if soil moisture supply is non limiting during a heat event, then vine Transpiration ratio increases. However, under limiting soil moisture supply during a heat event, the Transpiration ratio does not increase. 

Third, they compare photosynthetic activity variations for different transpiration levels. Researchers found that  by increasing its transpiration, the vine maintains a similar level of Carbon assimilation even during heat wave. However, if transpiration does not increase during heat, not only leaf damages and wilting symptoms are observed, but a major collapse in leaf photosynthetic activity is also measured. In turn, the drop in photosynthetic activity can jeopardize fruit sugar accumulation.

Conclusions

  • When a heat event happens, you should be ready to observe an increased Transpiration ratio, especially  during the early season stages.  If you do not observe an increase, you may assume that heat will increase leaf temperature while carbon assimilation collapses.
  • Winemakers and vineyard managers need to carefully manage irrigation scheduling, to ensure that root systems have access to water and vines are actively transpiring prior to a heat event.

What is  vine transpiration response to heat in field conditions?

Extrapolating from a pot experiment to the field is difficult because of various  factors such as a greater root volume, a more important fruit load or a much higher vapor pressure deficit. However, we can use the concepts higlighted by Edwards and his team to improve irrigation practices in response to Transpiration ratio variations measured with sap flow sensors. We will discuss this in our next blog, stay tuned….

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