Mike Miller, ANL contact (rmmiller@anl.gov)

Predicting effects of environmental change on whole plants, especially changes in multiple factors, will require improved understanding of how mycorrhizae respond to alterations in host partitioning of assimilated carbon and soil nutrients and how fungal responses feed back to the host plant. This project is determining whether a fundamental mechanism controlling the mycorrhizal association is the balance between photosynthate partitioning to belowground structures and the host's need for symbiont-supplied nutrients. Specifically, the issue of how the allocation of photosynthate between shoot and root affects the amount, growth rate, and allocation of the mycorrhizal fungus is being studied.

The project is taking advantage of two ongoing FACE experiments involving additions of both CO2 and O3; the DOE Rhinelander, Wisconsin, site located in a simulated regenerating hardwood forest, and the University of Illinois soybean site (SoyFACE) located in a typical Midwestern agroecosystem. Because the plant species used in the two experiments represent different growth strategies, mycorrhizal types, and sensitivities to CO2 and O3, a range of responses in partitioning of biomass in both the host and mycorrhizal fungi is anticipated.

Recently identified signature fatty acids offer the potential to identify stress to arbuscular mycorrhizal fungi (AMF) that biomass measurements may not demonstrate. The signature 16:1w 5c phospholipid fatty acid (PLFA) and associated 16:1w 5c neutral lipid fatty acid (NLFA) offer great promise as indicators of AMF biovolume and energy reserve, respectively. Hence, the ratio of 16:1w 5c NLFA to PLFA should be a good measure of fungal stress, and it may be a good predictor of potential host decline in an altered environment. A change in this ratio might reflect direct effects of the treatments (e.g., alterations of photoassimilate availability to the mycorrhizal fungus) or indirect effects, such as competitive interactions between the mycorrhizal fungus and root pathogens.

To date, a greater concentration of 16:1w 5c PLFA is being produced in aspen roots with O3 fumigation, but elevated CO2 has little influence on this marker. The 16:1w 5c NLFA also increases with elevated O3, indicating that AMF partitions a greater portion of the photosynthate they receive to storage reserves in response to elevated O3. These responses are best expressed as the ratio of marker NLFA-to-PLFA, i.e., the amount of AMF storage lipid per unit of PLFA, where an increase in this ratio is evident for elevated O3 and CO2 x O3 treatments.

Links:

DOE-BER Program for Ecosystem Research: http://www.sc.doe.gov/ober/CCRD/per.html
Aspen Face site: http://aspenface.mtu.edu/index.html
Soy Face site: http://www.soyface.uiuc.edu/index.html