Water Movement in Plants
|Natural systems move matter across gradients:
- Solutes (Concentration)
Plants move water across a water potential gradient (Ψ)
What is Water Potential (Ψ)?
Water potential is an integrated measurement of plant response to the environment.
Consider it to be equivalent to the blood pressure of a plant.
Total plant water potential is defined as:
Ψ = Ψp+ Ψs+ Ψm+ Ψg
Ψs= Osmotic Potential
Ψm= Matric Potential
Ψg= Gravity Potential
To Measure Plant Water Potential…
|Pressure Chamber (Bomb)
||In situ Psychrometers
In Situ Psychrometer vs. Pressure Chamber?
Strong relationship between the two instruments:
Leaf Psychrometer Applications
Leaf Psychrometers have been tested on:
Leaf Psychrometer Installation
1. Select flat leaf surface for leaf psychrometer
2. Position leaf into the slot of the clamp
3. Select sandpaper grit based on thickness of leaf cuticle
4. Apply water on cuticle remover for lubrication
5. Abrade surface of leaf in a circular motion
6. Apply silicon grease around psychrometer surface
7. Spread silicon evenly on psychrometer surface perimeter (Approx. 0.5mm deep)
8. Insert psychrometer chamber into clamp, secure by twisting the chamber on the leaf
Why do you need to abrade leaf surface?
- Water vapour on the leaf diffuses through the substomatal cavity
- The cuticle layer varies across different plant species:
- Cuticle resistance effects the water potential measurement
- Leaf abrasion reduces cuticle resistance and improves water vapour diffusion in leaf
- Leaf abrasion reduces equilibration time of leaf and psychrometer chamber
Level of Abrasion:
- Wheat plants (4-weeks old) were grown in soil media
- Leaf psychrometers were installed on five leaves:
- Wheat leaf 1/leaf 2 were adjacent leaves
- Wheat leaf 3/leaf 4 were adjacent leaves
- Control leaf had no abrasion treatment
- All wheat plants were irrigated together
Uninstallation of Leaf Psychrometer
- Leaf 1 – Installation showed signs of failure after 4.5 days
- Leaf 2 – Measurements continued to show reliable leaf water status after a week
- Leaf 3 – Installation showed signs of failure after 4.5 days
- Leaf 4 – Measurements continued to show reliable leaf water status after a week
- Three potted corn plants grown in a growth chamber for 4-weeks
- Each leaf used 600-grit sandpaper to abrade the cuticle layer
- Two leaf psychrometers were installed per plant at different leaf heights (Top and Bottom)
- Reliable Measurements from leaf psychrometer installations varied between species of plant:
1. Wheat leaves provided up to a week.
2. Corn leaves may provide up to two weeks.
- Abrasion of leaf is required to provide reliable leaf water status measurements
- Installation duration is dependent on a systematic abrasion technique
- Reliable measurements are determined based on response to night time recovery and/or irrigation
Rice Genotyping with Leaf Water Potential (LWP)
Study conducted by Sibounheuang et al. (2006) demonstrated variations in LWP in 6 different rice genotypes using pressure chambers.
Objectives of experiment:
A. Determine genotype variations by measuring LWP at different leaf positions and plant sizes.
B. Examine whether genotype variations of different canopy size and water conductance are associated with LWP.
- Glasshouse experiment with automated temperature control system
- Six rice lines were tested and have known differences in LWP, osmotic potential, and osmotic adjustment from Jongdee (1998)
- Midday water potential was measured with pressure chamber at 4 positions: Tip, Sheath, Base and Stem during a 10 day stress period
- Plant sizes were determined by xylem anatomy (vascular bundles and stem cross section area)
Experiment A Results
- Genotype differences in midday LWP could be based on hydraulic conductance
- Leaf water potential was different among the 6 genotypes and demonstrated the same trend
- Leaf water potential and xylem area relationship showed genotypes with higher LWP showed larger xylem area
B. Canopy Size of Genotypes Associated with LWP
- Field experiment with rainout shelters to induce stress period of four rice lines
- Midday LWP was measured under 3 irrigation treatments (Irrigated, 14 and 18 days of stress) and 4 canopy sizes (control, 1/3 and 2/3 leaf removal and six tillers remaining)
- Midday water potential was measured at 4, 10 and 14 days after imposed stress
- Canopy size was measured by the number of vascular bundles
Experiment B Results
- Genotype variations of reduced canopy sizes (removing leaves/tillers) were not significantly reflected in LWP
- Differences in LWP among genotype variations were were not due to canopy size
- Leaf water potential expressed in different genotypes maintained the same trends, however differences were not due to canopy size or leaf area
Sibounheuang et al. (2006) Conclusions
Genotype variation expressed in LWP and change in water stress were largely seen at leaf tip.
Larger xylem size were associated with high LWP demonstrating a higher internal water conductance.
Hydraulic conductance of vascular bundles could have caused the genotype variation seen in rice.
Leaf Psychrometer Conclusions
- PSY1 data logging systems provide wireless, continuous and automated measurements with a wide range of applications
- Leaf psychrometers demonstrated reliable plant water status for wheat and corn crops up to a week of continuous measurements
- Plant water status of rice crops have previously been monitored by the pressure chamber technique for genotyping. Leaf psychrometers provide the opportunity to continuously monitor leaf water status of rice in real time