Understanding how vines cope with periods of hot weather and extended drought conditions
Introduction
This Fact Sheet aims to give growers a more advanced understanding of how grapevines function during periods of hot weather and under prolonged drought conditions.
Research could never examine every variety in every situation, but it is hoped that by improving and applying their knowledge of grapevine functions and water use, growers will be in a better position to cope with current and future environmental conditions.
1 Vine physiology
As every grower is intimately aware, water enters the soil, is taken up by the roots, moves through the vine trunk and is then out of the vine through the leaves. A waxy cuticle covers both surfaces of the leaf to minimise water loss, but throughout the underside are small pores called stomata.
The stomata are where the leaf interacts with the air around it. Carbon dioxide moves in and is available for photosynthesis, whereas water vapour moves out and is lost to the plant. The vine is able to regulate water loss (transpiration) by adjusting the aperture of the stomata, but by reducing stomatal conductance in this way the vine also reduces the rate at which CO2 can enter the leaf, which in turn reduces the rate of photosynthesis. Consequently, water loss and carbon gain are directly linked, so reducing water loss reduces the sugars available for fruit production, growth and building of reserves.
2 Air dryness
The rate of transpiration is also driven by the vapour pressure deficit (VPD) of the air, the higher the VPD the quicker water evaporates from the leaf. Stomata respond to increasing VPD, but the extent of this is variety dependent, for instance during periods of high VPD Grenache vines close their stomata much more than Shiraz or Chardonnay vines and thus use less water and are at less risk of wilting.
As transpiration occurs during the day the loss of water from the leaf reduces leaf water potential, whereas at night there is very little loss of water from the leaf so water potential rises until it is only slightly lower than that of the soil. Leaf water potential is basically a balance between water loss from the leaf and water coming into the leaf and this balance is controlled by the extent of stomatal opening.
In times of water stress the rate of transpiration will exceed the influx of water into the leaf and the stomata will close to prevent the leaf reaching wilting point. However, as mentioned above, this reduces photosynthesis so the drought affected vine will fix less carbon from the air and be less able to grow, bear a large crop etc. As leaf water potential is also reduced by water stress it can be used to estimate the level of stress a vine is under.
3 Measuring vine stress
Unfortunately for us the ability of stomata to regulate water loss also regulates water potential, so it is difficult to use daytime measurements in this way. However, pre-dawn measurements are not usually affected by stomatal conductance and so are a much better estimate of stress.
Actual values will be dependent on variety, soil type and growing conditions, but a rule of thumb guide is that a pre-dawn leaf water potential below -0.2 MPa indicates stress and a value below -0.4 MPa indicates moderate to severe stress. A wellwatered vine would normally have a value above -0.2 MPa.
Finally, another side effect of stomatal closure is the effect on leaf temperature. Transpiration cools a leaf in the same way that an evaporative cooling system cools a house, so the lower the rate of transpiration the higher the leaf temperature, and vice versa. This means that water stressed leaves also tend to be hotter than those of well-watered vines.
It is important that we have ways of assessing vine stress and that we can do it with relatively simple technology. For example, growers in the US are being encouraged to measure vine water potential and use it for irrigation scheduling. Also, there is currently work on the use of simple infrared thermometers to estimate whole canopy stomatal conductance.
4 Irrigation Techniques in Extended Drought
In times of limited water supply growers can manage by deficit irrigation, ie providing less water than the vine will potentially use. For the purpose of this Fact Sheet, we will define extended drought as the effects of a water deficit over multiple seasons.
5 Sustained Deficit Irrigation
This is where there is a reduction in irrigation but this reduced amount is applied across the entire season.
When using a sustained deficit strategy, we are increasing the water stress on the vine and would expect to see the consequences of that, as discussed above. Indeed, this does happen and stomatal conductance appears to be reduced throughout the season. This in turn reduces photosynthesis and consequently there is a decrease in yield. However, the drop in yield is less than the drop in water use, so the water use efficiency of the vine has been improved. This is true even at very low water applications, for example, 0.6 ML per hectare in the Riverland.
On the other hand, there are risks. Firstly, very low rates of irrigation may lead to unacceptably high levels of salt in the berries and secondly, we do not know how many years this is sustainable for. Shiraz has been grown in the Murray Valley for three seasons on approximately 34% of normal irrigation (~2.0ML/ha/annum) without a collapse in vine capacity, but there were indications of a drop off in yield over that time (average of approximately 25%).
Using a sustained deficit strategy (that is reducing irrigation for the entire season) generally improves the yield per ML of irrigation. However, reducing irrigation too far may increase salt in the berries and there is some indication that using it for several consecutive years may result in reduced yield capacity.
6 Regulated Deficit or ProlongedDeficit Irrigation
This is where there is a reduction in irrigation but the reduction is applied for part of a season. This could be as a result of natural causes or strategic irrigation control, for example, reducing early season irrigation year on year, or not providing post-harvest irrigation for multiple years. Extending current deficit practices, either in time or in severity, is certainly another means of lowering water use in tim es of drought. In an experiment run in Sunraysia, a standard Regulated Deficit Irrigation (RDI) regime was extended in both time and severity, with a two to three week period of no irrigation immediately following the end of the RDI period and repeated for six seasons.
During the prolonged deficit (PD) drought period, itself a reduction in conductance, transpiration and photosynthesis were also observed, again as might be expected. However, those effects continued for some time after soil moisture was restored and it is only once senescence has started, post-harvest, that the effect is lost. So, it appears that a late deficit period, just before veraison, runs the risk of having a much greater effect on carbon gain than might be surmised from accounting only for the period without water.
The result of this was that growth in subsequent seasons was reduced, even before the PD period was applied for that season, as was yield. In fact, unlike with the sustained deficit strategy, the yield was reduced by almost the same proportion as the water saved. So there was little or no effect of PD on water use efficiency in terms of yield.
Simply extending current deficit irrigation practises may not improve yield per ML and it also carries the risk of reducing vine capacity over time.
7 The Effects of High Temperature Events
A high temperature event is obviously a relative term and the effects of one are also likely to be relative. The Australian Wine and Brandy Corporation’s definition of warm climate regions encompasse s those with a mean January temperature in the region of 23°C or higher, but within that definition there can still be fairly different climates. For example, although the Riverland, Murray Valley and the Riverina all have a similar number of days over 35°C each season, the Riverina typically has fewer days over 40°C. Furthermore, Langhorne Creek has a similar number of days over 40°C as the Riverina, but much fewer over 35°C.
Along with varietal differences, it is consequently difficult to define temperature ranges that may be treated as an extreme event. However, the effects of heat stress will be similar in all vines. Interes tingly, there is no evidence of a stomatal response to temperature, but as we’ve seen, VPD increases as temperature increases and stomata do respond to VPD. Stomatal closure then leads to even higher leaf temperatures and can further increase the risk of lethal temperatures being reached, although before that there can be permanent damage to the photosynthetic system.
Clearly, the best way to survive high temperatures is to transpire rapidly and thus keep the leaf relatively cool, but when the vine is water stressed this isn’t possible and the change in VPD alone could be fatal if stomata did not close to minimise the increase in transpiration that occurs as VPD rises. In fact, stomatal conductance of water stressed vines subjected to heat shock will reduce relatively more than well-watered vines, even though their conductance is less to start with.
The result is that leaf loss after a high temperature event is much higher in water stressed than well-watered vines. As an example, in an experiment where vines at different irrigation levels were exposed to a heat shock of 45°C for two days, none of the well watered vines lost more than 25% of their leaves, on the other hand, one third of the vines receiving the least irrigation lost over 50% of their leaves.
The effect of leaf loss on whole season carbon gain may be much higher than the actual proportion of leaves lost, due to the cumulative loss of the photosynthesis that they would have provided over the season and potential reduction in growth as a result. Leaves that survive a short period of heat shock tend to recover quickly and even highly water stressed vines can return to pre-heat rates of stomatal conductance within two to five days.
Higher temperatures during the growing season are likely to become increasingly common, but when well-watered vines are exposed to even very severe, heat stress they can recover rapidly and suffer minimal damage. Even drought affected vines recover rapidly from heat stress, showing how resilient the grapevine is. However, the leaf loss during a high temperature event when the vine is water stressed may be unacceptably high.
Glossary of Terms
Transpiration: the movement of water out of the leaf, the higher the measurement the faster water is lost to the atmosphere.
Conductance: the ease with which air moves through the stomata, the higher the measurement the more easily water or CO2 can exit or enter the leaf. This is largely determined by how open the stomata are and can be illustrated by how much easier it is for a person to blow through a wide tube than a very narrow tube.
Photosynthesis: the fixation of CO2 from the air into sugars which are then used to construct the vine itself. The higher the measurement the faster the vine incorporates carbon.
Water potential: the ‘suction force’ by which water moves into and through the vine. The more negative the measurement the stronger the ‘pull’. Water moves from high (less negative) to low (more negative) regions of the vine.
Vapour pressure deficit: the difference between the actual amount of water in the air and how much water would be in the air if it was saturated (at 100% relative humidity).
Author
Mark Krstic
Grape and Wine Research and Development Corporation
