1. The Target Seedling: Plant Water Relations
John G. Mexal
Plant & Environmental Sciences
New Mexico State University
Las Cruces, NM

2. Outline Define the importance of water relations in seedlings,
Describe how it’s measured,
Explain how to interpret the data,
Give an overview regarding new knowledge/culturing regimes/target expectations over the past 20 years
Use water to achieve and maintain Target

3. Importance of Water– Seedling after Colombo et al. 2001
Seedling Attributes
Morphological
Diameter
Height
Dry Weight
Root System
Balance
Bud Morphology
Physiological
Photosynthesis
Moisture Stress
Electrical Impedance
Root Growth
Cold Hardiness
Dormancy
Chemical
Nutrients
Carbohydrates
DNA
Others

4. Importance of Water-- Manager
Too little → Fail to achieve Target
Too much → Increased utility costs
Increased fertilizer costs
Increased pest pressure
Increased runoff hazard

5. How is water status measured?
Water Potential (ψ)= Chemical free energy of water; basically the ability of water to do work.
Pure, free water: ψ = 0 [zero ability to do work]
Pascal (Pa) = SI derived unit of pressure, stress, Young's modulus and tensile strength. Measures force per unit area.
-0.03 Mpa = kPa = ,000 Pa = bars ≈ -⅓ atm = “FC”
-1.5 Mpa = -1,500 kPa = -1,500,000 Pa = bars ≈ -15 atm = “PWP”
Rules of Thumb:
Water flows from high concentration (high ψ) to low concentration (low ψ)
The steeper the gradient, the faster the water movement
High humidity to low humidity
Low salt concentration (soil) to high salt concentration (plant cell)

6. Putting Water Relations in Context Relative Humidity Comparison
R.H.
(%) ψ
(MPa)
Maximum Pore Size (μ)
20.00
-222.0 --
50.00
-50.0
0.03
92.7
-10.0
0.3
99.27
-1.00 3
99.93
-0.10
99.99
-0.01 30
99.999
-0.001
300
100.00
n.a.

7. How do nurseries schedule irrigation?
Technique
Visual/Tactile
Container Weight
Measurement
PMS
Medium
Water Budget
Nothing/Wilting

8. Measuring the water status of seedlings A few ‘caveats’
What are you measuring?
Balance of water remaining in a container?
Balance of water remaining in a block of containers?
A seedling after a night of recovery?

9. Container weight vs xylem water potential Khadhuri, unpubl.
Xylem Water Potential (MPa)
Weak correlation between medium moisture content and seedling water potential
Essentially no correlation if moisture content > 50%
Moisture Content (% w/w)

10. Utility of soil mositure sensors Picea glauca Lamhamedi et al. 2005
Variability changes through growing season
Weak correlation to seedling growth
Crop Tracking Recos:
11-19 sensors/2 sprinklers OR
Measure 4 seedlings
N.B. 15% cull; cited low profit margin as a concern. So, would not recommend covered production or travelling boom, but did not recommend better coverage.

11. Preferred irrigation scheduling tool!!

12. Measuring the water status of seedlings A few ‘caveats’
What are you measuring?
Balance of water remaining in a container?
Balance of water remaining in a block of containers?
A seedling after a night of recovery?
These are ‘response variables’
Shouldn’t you measure the independent variables?
Radiation
VPD T

13. Measuring the water status of seedlings Pan Evaporation
ETo (mm/d)
ETo = 0.75 Ep
Advantages:
Integrates drivers of ET
Radiation
VPD
Temperature
Estimates real-time soil moisture loss (overestimates)
Use with data recorder
Wikipedia
Calif. Agric.

14. Measuring the water status of seedlings Atmometer -- Etgage™
Advantages:
Integrates drivers of ET
Radiation
VPD
Temperature
Predicts soil moisture loss
Can be used to convert ETo estimates to ETg
Use with data recorder
ETgage (mm/d)
Penman-Monteith (mm/d)

15. Current or historic climate
State Climatologists:
Archive historic weather data
Develop climate-based tools
Use both historic and real time data
Good examples:
California:
Northwest Region:
Canada:

16. Irrigation Scheduling-Canada Scagel, pers. comm.

17. Definitions: PET vs ETo
PET = evaporation + transpiration from a crop under non-stressed conditions
ETo = evaporation + transpiration from a ‘reference crop’ (grass) under non-stressed conditions; also referred to as ETr
Crop coefficient (kC) = ETC / ETO
Penman-Monteith Equation (FAO):
ETo = 0.408Δ(Rn – G) + γ(900/[T + 273]) μ2(es – ea)
Δ + γ (1 – 0.34μ2)
where
ETo = reference evapotranspiration [mm day-1], Rn = net radiation at the crop surface [MJ m-2 day-1], G = soil heat flux density [MJ m-2 day-1], T = air temperature at 2 m height [°C], μ2 = wind speed at 2 m height [m s-1], es = saturation vapor pressure [kPa], ea = actual vapor pressure [kPa], es - ea = saturation vapor pressure deficit [kPa], Δ = slope vapor pressure curve [kPa °C-1], γ = psychrometric constant [kPa °C-1].

18. Available historic and real time ETo data http://www. usbr
ETo (mm/d)
ETo (mm/d)
Reference ET (cm/yr)
Coastal
Interior
Brookings 91
Lakeview
134
Forest Grove
104
Echo
146
San Diego
118
Calexico
182

19. Real time data or Long-term average for scheduling. http://www. usbr
Long-term average of nearby station = good first approximation
Does not replace management responsibilities– see June 2009
ETo (mm/d)

20. Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al
Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al. & Bawazir et al.)
ET or ETo (mm/d)

21. Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al
Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al. & Bawazir et al.)
ET or ETo (mm/d)
Phase 1
E only

22. Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al
Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al. & Bawazir et al.)
ET/ ETo (mm/d)
Phase 2
Leaf expansion

23. Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al
Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al. & Bawazir et al.)
ET or ETo (mm/d)
Phase 3
ET = ETo

24. Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al
Daily pecan ET compared to ETo in the Mesilla Valley, NM (Sammis et al. & Bawazir et al.)
ET or ETo (mm/d)
Phase 4
Senescence

25. Wonderful! But I don’t grow pecans. What does this mean for me?
ET or ETo (mm/d)
Phase 1 =
Phase 3 =
Phase 4 =
Phase 2 =

26. Wonderful! But I don’t grow pecans. What does this mean for me?
Phase 1 = emergence (E only)
Phase 3 = ET = Eto
Pecans = Fescue = Pistachio = Mesquite = Creosote = Pine = Bermuda grass = Kentucky blue grass
Phase 4 = applies only to deciduous crops
What about Phase 2?
D 1-7
D 9-14
D 14-21
D 7-8
Zone of Absorption

27. Wonderful! But I don’t grow pecans. What does this mean for me?
ET or ETo (mm/d)
Phase 1 = emergence (E only)
Phase 3 = ET = Eto
Pecans = Fescue = Pistachio = Mesquite = Creosote = Pine = Bermuda grass = Kentucky blue grass
Phase 4 = applies only to deciduous crops
What about Phase 2?

28. Wonderful! But I don’t grow pecans. What does this mean for me?
ET or ETo (mm/d)
Phase 1 = emergence (E only)
Phase 3 = ET = Eto
Pecans = Fescue = Pistachio = Mesquite = Creosote = Pine = Bermuda grass = Kentucky blue grass
Phase 4 = applies only to deciduous crops
What about Phase 2?

29. What about Phase 2?
Linear relationship between budbreak and ‘full’ canopy closure
‘Full’ effective coverage occurs at 65% to 70% of canopy coverage.
100% ?
Effective Crop Canopy

30. What about Phase 2?
Linear relationship between budbreak and ‘full’ canopy closure
‘Full’ effective coverage occurs at 65% to 70% of canopy coverage.
100%
30%
Effective Crop Canopy

31. Recommendations Use all the tools at your disposal
Rely on the drivers of ET
Radiation
Vapor Pressure Deficit
Temperature
Eto vs ETg
Historical data covers >90% of environmental situations
Maintain comfort zone with physical tools
Pressure chambers
Scales
Moisture sensors
ETo (mm/d)

32. Physiological effects of moisture stress conditioning
Parameter
Effect
References
Photosynthesis/Biomass accumulation ↓
Cleary ‘71; Cregg ‘94; Nzokou & Cregg ‘10; McMillan & Wagner ‘95
Transpiration
Seiler & Johnson ‘85, ’88; Villar-Salvador et al. ’99
Carbohydrates ↑
Villar-Salvador et al. ’99
Osmotic adjustment ↔
Seiler & Johnson ‘85, ‘88
Seiler & Cazell ‘90; Villar-Salvador et al. ’99
R:S
McMillan & Wagner ‘95
Root Growth Potential
Vallas Cuesta et al. ‘99; Villar-Salvador et al. ’99
Cold-hardiness/Dormancy ↑↔
Almeida et al. ‘94; Timmis & Tanaka ‘76;
Zaerr et al. ’81
Blake et al. 1979
Survival
Vallas Cuesta et al. ’99
van den Driessche ‘91

33. How fast can we fall off the cliff? Landis, et al. 1989
Target Values
Weight Loss (kg)
Etg
Guidelines
Ψ (MPa)
Content (%)
(mm)
0.00
482 --
Saturated, too wet
~390
~1.60
~8.7
‘Field Capacity’ (drainage)
-0.01
235
4.04
~13.3
Upper limit for rapid growth
-0.05
130
5.77
9.6
Lower limit for rapid growth
-0.10 98
6.26
2.7
Hardening phase
>10.0
7.89
9.0
Ovendry medium
Eto (mm/d)

34. Xylem Water Potential (MPa)
Low soil moisture determines seedling water potential (after Dinger and Rose 2009)
Xylem Water Potential (MPa)
Management Implications
High soil moisture is weakly correlated with seedling water status
Low soil moisture is highly correlated
Slope = steep
0.05 m3/m3 = 1.25 MPa
Difficult to regulate???
Regression includes only soil moisture <0.3 m3/m3.
Soil Moisture (m3/m3)

35. Xylem Water Potential (MPa)
Low soil moisture determines seedling water potential (after Dinger and Rose 2009)
Xylem Water Potential (MPa)
Management Implications
High soil moisture is weakly correlated with seedling water status
Low soil moisture is highly correlated
Slope = steep
0.05 m3/m3 = 1.25 MPa
Difficult to regulate???
Regression includes only soil moisture <0.3 m3/m3.
Soil Moisture (m3/m3)

36. Scheduling irrigation to harden ‘Target’
Avoid incomplete wetting
Monitor uniformity
1-3 days can mean difference between hardening and death
Root pruning alters soil moisture volume
Dumroese