Plant-based Irrigation Scheduling: Principles and

Applications

Mae Culumber, UC Cooperative Extension, Fresno County, and Bruce Lampinen, UC Davis, Dept. of Plant

Sciences

Why measure stress?

0

5

10

15

20

25

30

0 10 20 30 40 50 60

Stom

atal

Con

duct

ance

Plant Stress (SWP)

Inverse relationshipIncreasing plant stress

decreases stomatal conductance and CO2 assimilation

Location (bar)

Air above tree -95

Air near leaf -70

Air in leaf -12

Xylem in leaf -10

Xylem in scaffold -8

Xylem in trunk- -7.5

Xylem in root -0.6

Soil -0.3

Water potential in the soil-plant-atmosphere continuum

-0.3 bars equals -30 cbars

Midday stem water potential?•A plant pressure chamber is used to measure the tension in a leaf

•Midday stem water potential is normally expressed as a negative value since you are measuring a tension in the xylem

•Water potential readings done on a bagged leaf at midday are known as midday stem water potential (MSWP)

The baseline midday stem water potential varies with temperature and humidity

Normal range for California’s central valley in summerare shown in blue box

A = MSWP sampling time period ~1 to 3 pm

No stress(-9 to -12 bars)Some stress

(-12 to -14 bars)Severe stress

( less than -15 bars)

Pistachio diurnal stem water potential

A

Time (PSDST)

6 8 10 12 14 16 18 20

Mid

day

stem

wat

er p

oten

tial (

bars

)

-20

-15

-10

-5

0

Fully wateredmild to moderate stresssevere stress

Where should your measure MSWP?

Bagging a low, shaded leaf on the northeast quadrant of the tree at least 15 minutes before sampling at midday-allows equilibration with water potential of the trunk

Pistachio leaves are compound and it is difficult to fit the whole leaf in a mylarbag. Rather than use the whole leaf you can use the terminal or terminal plus next two adjacent leaflets

leaf

terminalleaflet

leaflet

Bagging 3 terminal leaflets (this is done on the tree 15 minutes before sampling)

Air Pressure

(PressureGauge)

Magnifying Glass

Water Coming Out(Artists conception)

(Mylar bag)

(Seal)

Phloem and xylem fluid

Where’s the end point?

phloem fluid only

Real SWP after dabbing

Phloem and xylem fluid blowing

Interpret the readings

-9 to -12 bars = non-stressed

-12 to -14 bars = OK, some stress

More negative than -15 bars = severe stress

Salinity, drainage, and tree vigor influence stress levels

SWP Irrigation decisions• Mature trees- allow SWP to drop 2-4 bars below

baseline before irrigating• Do not irrigate in spring until SWP is below baseline

(3-4 bars)• Young trees should be kept near baseline to promote

growth• -15 bars is considered moderately stressed, -18 bars

is considered severely stressed

Plant Based Monitoring: Dendrometers

track small changes in water stress and trunk growth

Plant Based Monitoring: Dendrometers

Annual trunk growth for a well irrigated walnut or almond

tree might be about 8000 microns in a

growing season (1/3 inch increased radius)

Tree trunk growth curve

Maximum Daily Shrinking (MDS)

• MDS is the difference between daily maximum and minimum trunk diameter

• Less water in soil or more demand from weather or crop causes the trunk to shrink more each day

Temp

IrrigationStress Stem response

Heavy and salty soilJuly 12 2016

PHYTECH ON HEAVY LETHENT COMPLEX SOILS, CORCORAN RD 4 MILES NORTH OF

GARCES HWY, 3RD LEAF PISTACHIOS.

Courtesy of Ismail Siddiqi

Canopy Temperature: Continuous Infrared Thermometer

canopy (Tc) deviates from air temperature (Ta), dependent on the rate of transpirationTesti et al. 2007

Research focused on developing a non-water stressed baseline for irrigation scheduling

Estimate ET from surface temperature, reflectance, and vegetation indices to evaluate irrigation system and scheduling across the orchard

Conclusions

• Stem water potential provides information not available from soil based or remotely sensed data

• Stem water potential is measured by bagging a lower canopy leaf at least 15 before sampling

• The sample is cut off the tree and immediately placed in the pressure chamber

• Leaf is pressurized until the xylem sap emerges• Latex oozing causes confusion in pistachio

Conclusions

• General range for midday stem water potential in pistachio

• -9 to -12 bars- probably do not need to irrigate• -12 to -14- mild to moderate stress• initiate irrigation to get trees back near but not at baseline• Values more negative than -15 bars- severe stress likely

impacting photosynthesis and vegetative growth• Remotely sensed information can give good indication

of variation across orchard (as can a Google Earth image)

• More work is need to correlate various remotely and proximally sensed methods of assessing stress