Crop coefficient (3): monitoring transpiration ratio variations during growth

In our previous blogs we discussed how sap flow sensors can be used to characterize vine water needs under site- specific conditions. Various authors have shown that  the use of sap flow sensors can reduce error in estimating vine water needs thus offering an opportunity to save water and manage water deficit more accurately. Other works have highlighted the benefits of imposing different levels of water deficit at key stages of plant development to improve the quality of fruit and wine composition. For those reasons, the sap flow  approach is attractive, particularly when  irrigation is needed.

How to compare transpiration ratio with plant based crop coefficient (Kcb) ? 

The sap flow approach uses the concept of plant based crop coefficient, Kcb  discussed in our previous blogs here  and here . The mathematical computation of Kcb can be derived from the transpiration ratio.  The  question is then ” how do you compute a transpiration ratio?”

 The transpiration ratio is simply the result you get by dividing vine water use by water demand as described in formula 1. Formula 1 shows that – for a given climatic demand (ETref), Transpiration ratio is high when vine transpiration is high. ..and vine transpiration is high when there is plenty of water in the root system.

Formula 1: computation for transpiration ratio

In conclusion, under “standard” conditions, meaning :

  • no heat wave
  • soil moisture supply not limiting

the Tr ratio reaches a maximum value, noted Tr. Ratiomax. Simply put, the concept of Tr. Ratiomax reflects that vine transpiration can not be higher than its upper limit, set by plant size and climatic conditions. In theory, actual Tr. Ratio can only be as high as Tr. Ratiomax. As soil moisture declines, Tr. Ratio gets gradually lower than its maximal value.

Tr. Ratiomax  is also called plant based crop coefficient, Kcb , as described in formula 2.

Formula 2: relationship between transpiration ratio and Kcb when soil moisture supply is not limiting and in absence of heat wave

(Note: other factors besides plant size may  affect Kcb , such as Nitrogen concentration and photosynthetic activity, but this is beyond the scope of this blog.)

What are the practical applications of Tr Ratio variations monitoring ?

#1: Tr. ratio directly tracks vine water deficit variations

A direct application from formula 1 and 2  is that, if actual Tr. Ratio gets lower than Kcb value, it is because soil moisture supply gets limiting and down regulates vine transpiration. (This is true regardless of climatic variations since ETref variations  are also considered by formula 1).

In conclusion: The larger is the gap between actual Tr. Ratio and Kcb, the more severe is vine water deficit induced by limiting soil moisture supply. Thus, Tr. Ratio variations directly reflect vine water deficit variations …and this can be monitored continuously.

#2: Tr. ratio variations tell you if irrigation is needed, even during growth

When leaf area grows, Kcb also grows as discussed here.Thus, when leaf area is growing, Kcb is a moving target. Consequently, while leaf area is expanding,  it is not possible to assess the “gap” between actual Tr. Ratio and Kcb. You may ask: “how to assess vine water needs when leaf area (thus Kcb) is growing?”. There is an easy fix to deal with that situation.

During leaf area growth,  instead of monitoring the “gap”  you need to monitor variations of  transpiration ratio with  time. Below are 2 tips to help you assess whether a vine needs water during leaf area development.

First, if you are wondering if an irrigation is needed, remember that the first thing the vine does, even when soil moisture is barely limiting, is to stop its leaf area from growing. This was elegantly demonstrated by numerous studies such as Lebon et al, 2006. As such, if the apexes are alive and growing, you do not need to worry for soil moisture deficit. Apex activity is screaming :”There is no vine water deficit due to a lack of soil moisture supply!”. Irrigation is not justified and any water addition on a growing vine is a waste. If you see leaf burning symptoms while leaf area is actively growing, they are not caused by a lack of soil moisture.

Second, if leaf area growth is slowing down, it may be difficult to assess whether apexes are still growing. That is precisely the situation when tracking Tr. Ratio variations with time becomes useful. If you are near the end of shoot growth cycle, Tr ratio can only behave in 3 different ways.

  • Actual Tr. Ratio  declines with time: for sure, leaf area growth has stopped. The vine has reached its final size. A declining Tr. Ratio reflects that vine transpiration can not keep up with climatic demand. At this stage, you may see no visual symptoms. This is not surprising,  all the leaves are still young and the vine is  “programmed” to resist severe level of vine water deficit (to know more, read blog on cavitation 1 to 5 here). It may be a good idea to irrigate, unless you want to impose a moderate water deficit pre-veraison (which could be justified with red varietals before veraison)
  • Actual Tr. Ratio increases with time: the amount of vine water use today exceeds the amount of water the vine used yesterday, even considering the effects of climatic demand variations. Obviously, if the vine can increase its water use, water supply is not limiting. You do not need to irrigate.
  • Actual Tr. Ratio maintains its value with time: the amount of vine water use today is proportional to the amount of water demand imposed by the climate. The vine only “listens” to climatic variations and vine water use matches them. There is no down regulation of vine water use caused by soil moisture supply limitation. You do not need to irrigate

In our next blog we will discuss how to diagnose vine water use response to a heat wave.

Stay tuned.

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