1. Limitations to carbon uptake
Questions: What limits carbon uptake in stressed and unstressed
plants? Does abiotic stress influence a plant’s capacity to
respond photosynthetically to defoliation?
Methods:Measure light-saturated photosynthesis pre-defoliation, and
over time post-defoliation. Determine photosynthetic responses to
varying CO2 concentrations to help identify biochemical
limitations to photosynthesis. Quantify chlorophyll, rubisco and
total N concentrations in the leaves. Determine photosynthetic
responses to varying light levels. Measure chlorophyll fluorescence
of stressed and unstressed plants once stresses become
obvious.
1a.
Stem photosyntesis
Questions:Does stem photosynthesis occur? If so, how important is it
to the carbon budget? Does it upregulate in response to
defoliation?
Methods:select a stem section on defoliated and undefoliated trees,
in the upper crown zone. Tie back branches/leaves on the
undefoliated trees to allow light to reach the stem and acclimation
to occur prior to measurements, because it is known that stem
photosynthesis is sensitive to light environment. Take measurements
under high light (use a lamp) and no light. In order to develop a
carbon budget for leaves versus stems, it will be necessary to look
at photosynthetic responses to varying light in both leaves and
stem (already done once for leaves), and link in to actual light
conditions in the crown (data already collected for Autumn).
Quantify chlorophyll concentrations in the
bark/chlorenchyma.
Measurements will initially be done in a single
irrigated and fertilised plot. If results are interesting, the work
may be extended to stressed trees.
2. Carbohydrate dynamics
Questions: Does defoliation influence carbohydrate dynamics in the
plant? If so, what is the effect on stored versus soluble
carbohydrates, and what are the implications for recovery from
future events such as pest attack?
Methods: Measure soluble sugars and starch in leaves, stems and
roots of defoliated and undefoliated plants experiencing varying
degrees of N and water stress. Samples will be taken from 3
leaves per tree. Fine and coarse root samples will be taken from a
single rooting zone per tree, with the aim of standardising the
zone between trees. A small quantity of root will be harvested and
dried, and starch/sugars will be extracted from a ~0.1 g sub-sample
using the ?perchloric acid? method and then assayed using the
‘phenol? method. When a subsample of trees are harvested in
August 2007, more detailed analysis will be performed of sugar and
starch concentrations in defoliated and undefoliated plants.
This sampling will be used to test
methods. The methods developed will be used to investigate
pre-and post-defoliation carbohydrate allocation patterns in
spring. Other methods, such as use of stable isotopes, also will be
investigated for the second harvest, in conjunction with Professor
Mark Adams.
3. Nitrogen dynamics
Questions: Does defoliation influence nitrogen uptake and allocation
in the plant? What are the effects of the interaction of
defoliation and abiotic stress on uptake and tissue N
concentrations? What are the implications of this for carbon
uptake?
Methods:Soil
N mineralisation and N concentration will be monitored 3-monthly to
determine N availability. Foliar N concentrations will be
determined at at least 3-monthly intervals, and chlorophyll
concentrations will be quantified coincident with each
photosynthetic measurement. Partitioning of N within the leaf
between chlorophyll, rubisco and storage? proteins will be
determined at 3-monthly intervals. When trees are harvested in
August, N concentrations in root, stem, leaves and bark will be
determined.
4. Influence of defoliation and stress on foliar defence
mechanisms
Questions: does defoliation, or the interaction of defoliation and
abiotic stress such as low N or water availability, alter foliar
chemistry or morphology to increase the potential for future pest
attack? If so, is there a seasonal component to
responses?
Methods: compare the chemical profile of foliage from stressed and
unstressed trees experiencing defoliation or no defoliation, and
determine seasonal changes. Identify potential antifeedants
(both phenolics and terpenes). Conduct in vitro
bioassays
5. Modeling productivity in response to the interaction of
defoliation and abiotic stress
Questions: what are the implications of physiological responses to the
interaction of defoliation and abiotic stress for tree
productivity?
Methods: The
light interception model in the forest health module of Cabala will
be used to estimate the productivity of defoliated and undefoliated
trees growing with and without water or nitrogen stress. The
Pittwater data set will be used to validate the model.
