Scheduling Irrigation with a Pressure Chamber Part 1

Hello, I'm Dr. Alexander Levin, viticulturist on faculty

with the Department of Horticulture

and core faculty member of the Oregon Wine Research Institute

at Oregon State University.

Today we're here in Southern Oregon

to demonstrate how to use the pressure

chamber to schedule irrigations in wine grape vineyards.

Why use a pressure chamber

The two most important questions you

need to answer for irrigation scheduling are how much

and when.

The pressure chamber is an important tool

that can be used to determine when you should irrigate.

Precisely controlling the application of water

and managing plant stress in wine grape vineyards

is critical for optimizing fruit yield and quality.

Depending on your production goals,

you may want to manage irrigation

so that your vines are not stressed for water at all

during the growing season, or you

may want to moderate water stress at key times

to manage vine vigor and/or improve fruit quality.

Either way, it's hard to manage plant stress

if you don't measure it.

Many growers use soil-based sensors

to monitor soil moisture conditions and schedule

irrigation events.

However, due to the variables of distribution

of water in the soil profile and uneven soil drying,

soil-based measurements may not be

representative of the overall condition of the vines.

By using the pressure chamber, you directly

measure the level of water stress

your vines are experiencing because it measures

the plant and not the soil.

Think of it as measuring the plant's blood pressure.

Before we dive into the instrument and the measurement

technique, let's first review what it is we

are actually measuring.

Water moves through plants from the soil

to the atmosphere along what is called a Soil-Plant Atmosphere

Continuum, or SPAC.

Water is transported in the vine through a network

of tiny pipes called the xylem.

These pipes extend uninterrupted from the root tips

all the way along a chute to the leaves.

Water is carried through the xylem

from the soil to the atmosphere, much like a straw in a drink.

In this way, the plant is connected

to both above and below-ground environments

and dynamically responds to both.

Because plants are the perfect integrators

of soil and atmosphere, plant-based measurements

are generally the most effective compared

to soil-based measurements.

When the sun rises, the light stimulates

the opening of tiny pores on the leaves, called stomates.

Water vapor is lost through these pores.

That water must be replaced.

So water is drawn into the leaf from xylem

in the petiole, which draws water

from the stem, which draws water from the trunk

and then from the root, which draws water from the soil.

There is resistance to water flow at each step of the way.

This chain of resistance creates tension in the xylem from root

to leaf.

The flow of water through the vine,

and the resulting tension in the xylem,

increase until the daily maximum is reached at midday,

between noon and 2:00 PM.

The tension in the xylem increases

because it becomes more difficult for the plant

to extract water from the drying soil as the day progresses.

After 2:00 PM or so, the flow of water through the vine

begins to decrease along with the tension in the xylem.

We can measure the tension in the xylem of the leaf

by using the pressure chamber.

And this tells us something about how much

water stress the plant feels.

This is done by enclosing a single leaf

inside a plastic sandwich bag, excising the leaf from the vine

by cutting the petiole with a sharp razor blade,

and putting the bagged leaf in a sealed chamber

with the cut petiole end remaining

outside of the chamber.

It's important to leave enough petiole tissue attached

to the leaf blade to allow it to fit into the chamber

and still protrude through the sealed grommet.

Then we slowly apply external pressure

to the leaf in the chamber and watch for sap

to rise up to the cut end of the petiole.

The amount of pressure needed to make the sap just

appear at the cut surface is related to the amount of water

tension that leaf was under.

The higher the pressure, the greater

the original tension, and the greater

the water stress of the plant.

The most commonly used units of pressure

are bars and megapascals.

The pressure we apply to the leaf and read on the gauge

is a positive number because we must

balance the tension in the xylem, which

is a negative number.

So the number that describes the tension

is the negative value of whatever we see on the gauge.

For example, if we measure 10 bars of balancing pressure,

then the tension is minus 10 bars, or minus 1.0 megapascals.

We can think of the negative number as a water deficit.

The lower the negative number, the greater

the water deficit and the greater the stress.

For example, a value of minus 12 bars, or minus 1.2 megapascals,

would indicate a greater degree of water stress

than a value of minus 8 bars, or minus 0.8 megapascals.

The scientific name we give to this negative number

is the water potential.

There are two main factors that affect the measurement of water

potential--

first, the position of the leaf on the plant, and second,

the environmental conditions that affect the whole plant.

Outer canopy leaves exposed to full sun

are losing water much more rapidly

than leaves that are in the shade.

So the water potential of the outer canopy leaves

is reduced compared to the inner canopy leaves.

Weather, soil dryness, and root health

also affect water potential values.

Weather conditions are the primary factor.

For example, cool, humid weather and wet soil

will give you a higher, less negative water potential

value, such as minus 6 or minus 7 bars.

As the soil dries and the plant has more difficulty extracting

water, you'll see a strong effect

on the water potential reading.

As tension in the xylem increases,

you'll read a lower water potential value,

such as minus 10 or minus 11 bars.

In fact, any condition that negatively

affects root health, such as disease, physical damage,

poor aeration, or compaction may negatively impact water uptake

and gives you lower water potential values.

Outro

Thanks for watching Part 1.

Now, to see the pressure chamber in use,

please join us for Part 2 of Scheduling Irrigation

with a Pressure Chamber.