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Pittwater: current research being undertaken

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.