Session
Session Nine
Nine
Fruit
Fruit size
size and
and production
production
New
New Zealand
Zealand and
and Australia
Australia Avocado
Avocado
Grower
’s Conference
’05
Grower’s
Conference’05
20
-22 September
20-22
September 2005
2005
Tauranga
Tauranga,, New
New Zealand
Zealand
Photo-oxidation in Avocado leaves
Andrew Mandemaker, AIC
Jonathan Cutting, AIC
Prof. Allan Green,
University of Waikato
78
Photo-oxidation
• Occurs when there is excess light
during periods of low temperature
• Photo-oxidation is damage caused
by ‘free radical’ oxygen
• Avocado leaves become golden
yellow
• Photosynthetic performance is
possibly reduced, therefore
the productivity of the tree
79
80
Photo-oxidation
• Photo-oxidation may cause
leaves to drop early
• Mature leaves are thought to be
important because they provide
support for fruit set and spring
growth
81
Masters thesis
research
•
The research will consist of two
parts:
1. Regular monitoring of photosynthetic
performance 2005/06
2. Experiment during 2006 winter to
evaluate effectiveness of the current
industry treatment in restoring
photosynthetic performance
82
Research questions
1. What photosynthetic performance
achieved during summer and
months in the Bay of Plenty?
2. What is the relationship between
leaf yellowing and photosynthetic
performance?
3. Is there an effective treatment to
return photosynthetic performance
to normal?
83
Plant physiology
measurements
•
•
•
•
•
CO2 assimilation
Stomatal conductance
Chlorophyll fluorescence
Xylem tension
Chlorophyll content
and environmental parameters
84
CO2 assimilation and
stomatal conductance
• CO2 assimilation is a direct measure
of photosynthetic performance in the
leaves. Data is collected in two
ways:
– with an automated leaf gas exchange
chamber, under ambient conditions on a
single leaf for up to 3 days
– spot measurement of 50 labeled leaves
under saturating light intensities at
85
86
87
Photosynthesis
20
Leaf 1
Leaf 2
15
A(µmol m-2 s-1)
10
5
0
-5
26th
27th
28th
29th
-10
Date (March)
88
30th
31st
Chlorophyll fluorescence
• Excess light energy arriving at the leaf
can be re-emitted as fluorescent light
• The ‘quantum
efficiency’ of the
leaf can be
determined,
providing a
measure of plant
stress
• A portable Mini-PAM meter collects data
from dark adapted leaves
89
Chlorophyll fluorescence
Quantum efficiency of PSII
0.84
0.82
0.80
North facing
South facing
0.78
0.76
6
8
10
12
14
Time of day
90
16
18
20
Xylem Tension
• Xylem tension is the negative
pressure within the xylem vessels
that is created by the evaporation of
water from the leaf
• Measured with a ‘pressure bomb’
device
• Xylem tension provides a measure
plant water stress
91
Xylem tension
Willis orchard 26 and 27 March 2005
0.0
Tree
Tree
Tree
Tree
Xylem tension ( MPa)
-0.4
A
B
A
B
-
day 1
day 1
day 2
day 2
-0.8
-1.2
-1.6
-2.0
6
8
10
12
14
Time of day
92
16
18
20
Leaf chlorophyll content
• Photosynthtic performance is
on chlorophyll content
• Measured with a
SPAD, instant
chlorophyll meter,
calibrated against
chlorophyll
extraction
• Chlorophyll is a proxy for leaf colour, leaf
yellowing can be monitored with changes in
chlorophyll content
93
Future Work
• Evaluate the use of low biuret
urea and magnesium sulphate
a restorative treatment for
photo-oxidation yellowing
• Trial to be completed during a
yellowing period during winter
2006, or created artificially with
chilling chambers
94
In Summary
• Photo-oxidation causes avocado
leaves become yellow in high light,
low temperature conditions
• Research is currently underway to
investigate the relationship between
photo-oxidation and photosynthetic
performance
• An experiment will be conducted to
evaluate a treatment for photooxidation
95
Acknowledgements
• Bright Futures
Scheme, Tertiary
Education
Commission
• Mr. Fred Willis,
Katikati
96