Back to July 2018

No Holding Back, Good ‘Ole H20

As your trees begin work to fill their kernels, now is the time to make sure that they're getting enough water.


Model 600 from PMS Instrument is a Scholander Pressure Chamber used to check plant's stress. (Photo courtesy of PMS Instrument Company)

Let’s face it. Fruiting is a high-stress event for any tree. If there are other stresses compounding this event, either this year’s fruit quality or next year’s performance will be impacted significantly. As a perennial crop, ultimately what you do this year sets that crop up for success or failure next year. However, there are some stresses you can mitigate now to ensure a good quality fruit this year.

As I peruse the database of pecan articles, and with my understanding of the development of the pecan fruit, there is an agreement about what factor is of most importance from this time of the season to harvest. It’s water!

This time of year it is critical for the pecan trees to get the right amount of water. This can mean too much, as can be sometimes the case for our friends in the eastern growing regions, but it can also be too little for the western growing regions.

I appreciate Dr. McEachern’s comments in the March issue of Pecan South on the month-by-month calendar of events for 2018. For the month of July, he stated, “Tree and crop stress must be prevented for the next 60 days. Irrigation is now critical; apply weekly as per soil holding capacity and tree use without saturation.” (This means knowing your soil texture.)

Actually, water stress any time of the active growing season can have detrimental effects. Richard Heerema, Ph.D., with New Mexico State University (NMSU) typically outlines these well in the Pecan Production Short Course in New Mexico. Here’s the breakdown: water stress reduces photosynthesis, increases sticktights, increases pre-germination (vivipary), increases fruit abortion, decreases yield, decreases shoot growth, and decreases trunk growth.

And if you think about water stress in terms of too much water, the effects can be first costly and wasteful. Second, too much water creates an anaerobic condition for the roots (especially true in higher clay soils); reduces water and nutrient uptake; increases root suffocation and death; and causes iron chlorosis, stomatal closure, nitrogen leaching, and denitrification. In this way, too much water can affect the whole development process, or life cycle, of the pecan tree.

If you think about the development stages of the pecan fruit alone, there are two stages where water is most critical. The first stage—the pecan sizing stage—is already done by now. This is where your pecan trees, given enough water, have expanded their shells to the maximum allowed size, determined by the cultivar and genetic characteristics.

Right now though, we are entering the second stage of pecan fruit development: the kernel filling stage. In terms of water stress and its impact, this stage is critical. We are talking about the fruit that goes to market (quality, kernel size, health benefits, plus flavor)!

No holding back now!

I was taught long ago that pecan leaves do not wilt. Unlike other fruiting species, there is no visual symptom of water stress when it comes to pecans. So, how do we know how thirsty the trees are, or how much water is available in the soil?

Some farmers use a shovel and dig below the soil surface crust to see and feel for themselves. I have seen others walk the orchard with a 4- to 5-foot rebar welded with a handle to push into the soil. If it stops at 2 feet, they add more water. If it pushes down below 3 feet, they hold back.

There are of course more sophisticated devices available too. Soil moisture monitoring tensiometers have been used for a long time and by many. One reason is that they are affordable and of simple design. The disadvantage is that they rely on a suction to measure tension or pull on soil water potential; the suction can break, thus making the tensiometer inaccurate and decreasing efficiency.

There are also electrical resistance blocks that are a little more pricey than the tensiometers but are also simple, easy to use, can be left in the field, and will not have to maintain a suction or tension with the soil. The disadvantages are that it does not take into account the soil texture thus irrigation threshold is affected, and it is also inaccurate at saturation.

The more expensive but highly accurate soil moisture monitoring devices are those that use dielectric methods. These devices can also be connected to dataloggers to easily keep track of historical data and can be connected via Bluetooth, internet, or phone to make accessible from a smartphone or computer. The disadvantages are the price and the need for soil specific calibration for some models.

Although these devices are valuable for telling how much moisture is in the soil, they still are limited to the specific area where they are placed. When the root system of an orchard is so huge and variable, this limitation can affect the accuracy of your monitoring. Some orchards can have more than three or even four different soil types in a given field. Soil texture and conditions greatly influence these readings. In this sense, it is difficult to know or recommend where the measurements should be made; thus, a grower may probably need more than one device, which in turn, creates an even bigger investment.

Plus, there are situations where the water is there and the soil is wet, but the trees are still not getting it. This is typically true where there are heavy salt issues in the soil and water. So, you have to ask the trees where they stand in terms of water stress.

One of the best, proven methods is by a device called the Scholander Pressure Chamber, (aka “pressure bomb,” not to be repeated out loud in an airport). You can find the different models and prices ($1,000 and up) on www.pmsinstrument.com. I am unaware of other manufacturers. This one device has the ability to give a novice user so many valuable measurements, such as leaf water potential, predawn leaf water potential, shaded leaf water potential, and midday stem water potential. The measurement of choice for me, especially when taking gas exchange measurements, is the midday stem water potential.

According to a publication in HortScience by Sanjit Deb, Manoj Shukla, and John Mexal with New Mexico State University, this measurement has been demonstrated to be most useful and stable in pecan when it is taken between noon and 4 p.m. When taking midday stem water potential measurements, the entire leaf must be placed into a foil bag, or between foil sheets, to be darkened for at least 20 to 30 minutes prior to measurements, because you want the leaf to equilibrate to the same potential as the stem (thus, “stem” water potential).

This graphic shows how to use a Scholander Pressure Chamber to measure a leaf’s stem water potential. (Graphic Courtesy of PMS Instrument Co.)

Choosing the leaves for measurements is a selective process. Undamaged leaves in the shaded portion of the canopy and close to the trunk are perfect. When the leaf has had time to equilibrate, the petiole or leaf stem sticking out of the foil bag is cut as close to the branch as possible and placed—still bagged—into the chamber, which is screwed down until a rubber gasket completely surrounds the petiole and closes off the chamber from the outside environment.

Pressure is then slowly increased in the chamber with an attached pressurized nitrogen tank until the water starts to flow over the cut petiole. Why nitrogen? Industrial nitrogen has been cleansed of all humidity, so relative humidity never changes in the chamber. The second the water begins to flow across the cut surface of the petiole the pressure switch is stopped and the measurement reading recorded. The units are usually given in megapascals (MPa) and easily converted to bars or psi if one wanted.

What does the reading mean though? Remember, water moves down a water potential gradient. From positive, or close to positive in the soil, into the roots and up the vascular tissue, to more negative in the leaf. Basically, there is no water stress in pecans at -0.6 MPa and higher (it will never be positive). In research, photosynthesis in pecan trees begins to be negatively impacted at -0.9 MPa and lower. Trees are considered to be in high water stress at readings of -1.0 MPa and lower. I have seen readings in pecan at -15.0 MPa—major water stress! But these were neglected trees in the desert.

Therefore, if using a pressure chamber to “ask” the trees of their water stress, maintaining a water potential of -0.8 to -0.9 MPa during the active growing and critical development stages will prevent significant reductions in gas exchange. Thus, allowing the tree to use carbon assimilation and sugars for quality development of pecan fruit.

It may take several tries, and it will take many, many, many measurements to fine tune your irrigation scheduling and frequency to accomplish this, but it will prove to be a sound method for telling you if the trees are water stressed. After all, you’re measuring the source of what drives that fruit quality and yield!

Author Photo

Joshua Sherman

Joshua Sherman is an Area Extension Horticulture Agent with the University of Arizona, Willcox, Arizona. jdsherman@email.arizona.edu