Why you should be using stem water potential as an irrigation management tool
Phoebe Gordon, Madera and Merced Counties
Water management is one of the most important farming practices you or your clients should be practicing, full stop. And, with the coming of SGMA, whether your local groundwater management coalition decides to do a slow reduction on water use or a hard cut toward the end, knowing that you’re applying just enough water when the trees need it is going to be critical for maximizing your orchard’s water use efficiency. The tools that have proven to have the best success are using evapotranspiration (ET) to know how much water to apply, soil moisture sensors to let you know how much water is in your soils, and using stem water potential to measure tree stress. I most commonly encounter growers who exclusively use ET or soil sensors or rarely both, to manage irrigation. While they are great tools, they both have a significant flaw, which is they don’t tell you what your trees are actually experiencing. Luckily there is a tool, the pressure chamber, that allows us to monitor tree stress.
Soil moisture sensors are simple to use and directly measure soil moisture content, and are great for timing irrigations and assessing and adjusting the length of your irrigation sets. They can be electrical conductivity sensors, tensiometers, or neutron probes. Permanently installed ones can also take measures constantly, allowing you to passively collect large data sets and learn your soil’s wetting and drying patterns. In order to ensure you’re getting good information, they require correct installation and if they are to remain permanently in the field, good contact between the soil and sensor. In fields that have soil textures with different water holding capacities, they need to be installed in multiple spots, and you should install enough of them so that you can monitor the soil moisture at multiple depths in the soil profile. One of their most significant weak spots, particularly for those sensors that are relegated to the top several feet of soil, is that they often do not allow you to monitor the entire rooting depth of your orchard. Trees can extend their roots several meters into the soil depending on species and available rooting depth of your orchard site. If your site has a shallow water table or a particularly deep rooting profile, you could be severely underestimating the soil’s plant available water. Additionally, if your site has several very different soil types, the usefulness of your sensors is limited by how many you are willing to install.
Evapotranspiration has similar flaws to soil sensors: while it is a good measure of how much water your orchard has lost on a weekly basis (provided you are correctly assessing its canopy cover) you will have no way of assessing whether the applied water is meeting or exceeding your orchard’s water demands (calculated ET can exceed your orchard’s water demand if you have overestimated canopy cover, if you have significant amounts of stored soil water from winter rains, or if you have a shallow water table that your orchard is accessing).
Plant water stress measures are an essential check against both of these irrigation methods. Right now, the simplest, cheapest, and most accurate tool to use is still the pressure chamber, nicknamed the pressure ‘bomb.’ You cannot use this method by itself, as all it tells you is whether or not you need to irrigate. Additionally, its usefulness is limited by how often you take measures. If you use it in conjunction with ET, and better yet soil sensors, you can have a great understanding of what is going on in your orchard water-wise. ET tells you how much to add, soil moisture tells you what’s in the soil and can help inform the timing and length of your irrigation sets, and pressure bombing can be used either to time irrigations or as a periodic check to ensure your current practices are adequate. It is also the best way to correctly manage reduced irrigation during almond hull split to regulate hull rot.
Image 1: Soil-plant-atmosphere continuum of water potentials
In order to understand why stem water potential works so well, it’s helpful to have a quick overview of how water moves from soil through plants into the air. Water is lost through stomata in leaves to the atmosphere through diffusion. This generates tension on the water held in xylem in the plant, as water wants to move up to take the place of water lost in the leaf. The tension draws water up through the plant from the roots. Tension is the opposite of pressure, so it’s measured with the same units: bars in agriculture, and megaPascals (mPa) in scientific literature. One bar is equal to 0.1 MPa.
The degree of tension in a plant is dependent on air temperature and humidity as well as the plant water status. Water will evaporate from a wet surface (in the case of plants, leaves) more quickly when temperatures are hot and when the humidity is lower. This generates more tension on the water in plants. Similarly, as the soil dries out, plants are able to remove less water from the soil, preventing them from replenishing what was lost. As the trees continue to lose water, more tension is generated in the xylem as what was lost hasn’t been fully replaced, and plant water potential measures will drop.
The pressure chamber was developed as a way to measure the water tension in plants. The basic process is to cut leaf or twig from a tree, put it in a chamber and increase the pressure in the chamber until it equals the tension of water in the leaf. If you continue to increase the pressure, water will start leaving through cut surfaces, which with careful leaf removal will only be the cut end of a petiole. This is typically the point where measurements are taken, though skilled pressure chamber operators are able to see water wet the base of the petiole just before it starts to bead on the surface.
There is quite a bit of evidence that supports the use of stem water potential as a measure of plant stress. It has been shown in many studies that withholding irrigation reduces (makes the value more negative) stem water potential in fruit and nut trees. Stem water potential has been found to correlate with stomatal conductance, which is a measure of how many stomata are open and taking up carbon dioxide and in turn losing water. It has also been found to correlate with the vapor pressure deficit, or the difference in the maximum amount of water air can hold at a given temperature, and the actual amount of moisture it is holding (a large vapor pressure deficit means water from wet surfaces such as leaves evaporates more quickly). Research into pressure chamber operations has found that covering a shaded leaf near the trunk for at least ten minutes (which gives you stem water potential) results in reliable measures that reflect changes in atmospheric temperature and relative humidity better than simply cutting off a leaf and measuring its stress (called leaf water potential).
Just like soil moisture measurements, stem water potential can vary in an orchard. Anything that increases the difficulty in root uptake of water from the soil will decrease stem water potential, which happens as soils dry out. Trees under the same irrigation regime but in very different soil textures, such as a clay loam vs sandy loam, will have different stem water potentials as the soil dries out; the stem water potential of trees in the sandy loam will drop more rapidly than those in the clay loam.
Stem water potentials vary over time as well. Provided your orchard soils have enough water to support your trees, stem water potential will be at its highest (least negative) in the early morning hours before the light is intense enough for photosynthesis to start. This is because the trees have spent the entire evening taking up water from the soil. The amount they take up is dependent on the soil available water; if soil moisture levels are low enough your trees will not be able to adequately replace the water they lost. As the day continues, temperatures rise and relative humidity drops, and the stem water potential will begin to decrease (become more negative) until solar noon, approximately 1-3 PM (if we didn’t have daylight savings, this window would be 12-2 PM). After this time period, tree water status will slowly recover through the late afternoon and night.
Image 2: Bagging leaves to obtain midday stem water potential
So now, how do you actually take stem water potential, and what do you do with the values? Assuming you have a pressure chamber (pressure bomb) and know how to use it, bag leaves close to the trunk of the tree as low in the canopy as possible. If you’re taking measures on walnuts, bag the terminal leaflet; the entire leaf is too big for a pressure chamber. Wait at least ten minutes (but it’s okay to wait for longer – if I have a lot of field work I will bag leaves first thing in the morning and return to them later in the day). Excise the leaf, leaving the entire petiole intact, and immediately insert into your pressure chamber and take the measure. You can then compare the value to general stress ranges (see Table 1), however these values typically do not take weather conditions into account.
Image 3: Measuring stem water potential
Alternatively, by taking note of the temperature and relative humidity, you can calculate what a tree’s water potential may be if water uptake from the soil was not limited and nothing prevented transpiration, which is called the baseline water potential (Table 2). You should not be attempting to keep your orchard at the baseline, as it is a reference point that takes climactic conditions into account. High soil moisture levels can lead to increased disease susceptibility. Wet soil conditions are more conducive to root diseases, and continuously wet soils can increase orchard humidity and lead to more foliar diseases. However, the baseline can be used to assess whether your irrigation program is effectively addressing crop water needs. Does an irrigation increase an orchard’s water potential to or near the baseline within a day after irrigation? If not, you may be underirrigating. Does your orchard remain close to the baseline by the time your next irrigation rolls around? You can hold off on irrigating by a few days (if possible) or reduce the amount of applied water in your next round. Are your stem water potential values above the baseline? Your soils are probably too wet. I’ve seen this in an orchard during the spring of 2019 after our extremely wet winter, and during the summer in a flooded part of the field.
In almonds and prunes you should start an irrigation when the block’s average stem water potential is 2-4 bars below the baseline, depending on whether you are trying to induce mild stress; in walnuts and young orchards you can begin irrigating at 2 bars below the baseline. While we have some ideas as to what generalized stress levels in pistachios are, we currently do not have baseline values.
| Stress level | Almond2 | Pistachio1 | Prune2 | Walnut1 |
|---|---|---|---|---|
| No stress | -2 to -6 bars | -2 to -4 bars | -2 to -6 bars | -2 to -4 bars |
| Mild stress | -6 to -8 bars | -6 to -8 bars | -6 to -8 bars | -4 to -6 bars |
| Medium stress | -14 to -18 bars | -8 to -10 bars | -14 to -18 bars | -6 to -8 bars |
| Severe stress | -18 to -20 bars | -18 to -20 bars | -18 to -20 bars | -10 to -12 bars |
| defoliation | Less than -20 bars | -20 to -30 bars | Less than -30 bars | -12 to -14 bars |
Almonds and Prunes:
| Temperature | Relative Humidity | |||
|---|---|---|---|---|
| 20 | 30 | 40 | ||
| 80 | -7.5 | -7 | -6.6 | |
| 90 | -8.7 | -8.1 | -7.6 | |
| 100 | -10.4 | -9.6 | -8.8 | |
Walnuts:
| Temperature | Relative Humidity | ||
|---|---|---|---|
| 20 | 30 | 40 | |
| 80 | -4.6 | -4.3 | -4.1 |
| 90 | -5.2 | -4.9 | -4.6 |
| 100 | -6.1 | -5.7 | -5.3 |
Table 2: Adapted from Fulton, A. 2018. “Using baseline SWP for precise interpretation). Published in Sacramento Valley Orchard Source.
More resources:
Baseline stem water potential calculator: http://informatics.plantsciences.ucdavis.edu/Brooke_Jacobs/index.php
Using Baseline SWP for Precise Interpretation by Alan Fulton