2. Water stress during flowering?
• What are the causes of alternate bearing?
• Does water stress contribute to poor fruit set?
• “Flowers use 80% of the water”
• Pollen tube growth is sensitive to water potential
2
3. Previous studies
Whiley et al. (1988)
• Inflorescences increase canopy surface area of ‘Fuerte’ by 90%
• Estimated that flowers add 13% to tree transpiration
Blanke and Lovatt (1993)
• Compared the surface morphology and transpiration of leaves
and flowers
• No estimates of total water use
Our hypothesis:
• Inflorescences add significantly to tree transpiration, leading to
soil moisture deficits and plant water stress
3
4. Methods
• Hass Avocado in the Bay of Plenty, New Zealand
• Flower and leaf removal from whole trees and branches
• Measured - inflorescence and leaf area
- inflorescence and leaf stomatal conductance
- tree and branch transpiration using sap flow
- soil moisture, water potentials, photosynthesis
• Compared transpiration estimated from surface area and
conductance with transpiration measured using sap flow
4
5. Measuring inflorescence surface
area
• Fluidized bed of ‘Ballotini beads’
• Dip in glue – weigh – dip in beads – reweigh
• Area = change in weight added by coating of glass beads
5
6. Inflorescence surface area correlated
with fresh weight
35
Inflorescence surface area (cm2)
30
25
20
15
10
5
y = 18.84x
R² = 0.94
0
0 0.5 1 1.5 2
Inflorescence fresh weight (g)
6
7. Inflorescence area as a proportion of total
surface area
0.6
0.5
Inflorescence area / Total area
0.4
0.3
0.2
0.1
0
Total One-sided
inflorescence area inflorescence area
7
8. Measuring stomatal conductance
• Stomatal conductance was measured using a gas exchange
system
• Response to humidity was measured as an indicator of the
level of stomatal control over transpiration
8
9. Stomatal response to humidity
0.35
Stomatal conductance (molm-2s-1)
0.3 leaves
0.25
0.2
0.15
inflorescences
0.1
0.05
0
0.5 1.5 2.5 3.5
Vapour Pressure Deficit (kPa)
High humidity Low humidity
Low temperature High temperature
9
10. Transpiration response to humidity
5
4.5
Transpiration (molm-2s-1)
4
leaves
3.5
3
2.5
2
1.5 inflorescences
1
0.5
0
0.5 1.5 2.5 3.5
Vapour Pressure Deficit (kPa)
High humidity Low humidity
Low temperature High temperature
10
11. Measuring transpiration in the field
Heat pulse sap flow probes
Paired branches, deflowering and deleafing
11
12. Transpiration (L hour-1) Transpiration from sap flow
10 Tree 2 deflowered tree 1
tree 2
8
6 Tree 1 deleafed
4
2
0
300 305 310 315 320
Day of the year
12
13. Transpiration (L hour-1)
10 Tree 2 deflowered tree 1
tree 2
8
6 Tree 1 deleafed
4
2
0
300 305 310 315 320
2
Ratio (Tree 1 / Tree 2)
Day of the year
1.5 De-flowering = 8% reduction in transpiration
1
0.5 De-leafing = 78% reduction in transpiration
0
300 305 310 315 320
Day of the year 13
14. Comparison – Area vs Transpiration
Infl. Transp. / Total Transp.
0.2
0.16
0.12
0.08
0.04
y = 0.53x
0
0.1 0.2 0.3 0.4
Infloresecence area / Total area
14
15. Soil moisture and plant water
potentials
• Experiment conducted under mild conditions with adequate
rainfall
• No significant decline in soil water content over flowering
• Plant water potentials and stomatal conductance were
unaffected by flower removal, even with heavy flowering
15
16. Conclusions
• Inflorescences contribute up to 35% of the total transpirational
surface area during flowering
• Inflorescence surface conductance is half or less that of
leaves
• Inflorescences contribute up to 15% of the total transpirational
water loss during flowering
• The flowers do not use 80% of the water, but soil moisture
and irrigation management still matter
• Soil moisture deficits and tree water stress are unlikely to
contribute to alternate bearing if carefully managed
• Flower water relations may still be a factor in fruit set
16
17. Acknowledgements
NZ Avocado Industry Council
Jonathan Dixon
NZ Foundation for Research, Science and Technology
Mike Clearwater
m.clearwater@waikato.ac.nz
17
