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Elevated CO2 and Plant Defense
Reference
Gleadow, R.M., Foley, W.J. and Woodrow, I.E.  1998.  Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic Eucalyptus cladocalyx F. Muell.  Plant, Cell and Environment 21: 12-22.

What was done
Eucalyptus seedlings were grown in glasshouses for six months at atmospheric CO2 concentrations of approximately 400 or 800 ppm and fertilized twice daily with low or high nitrogen solutions to investigate the interactive effects of elevated CO2 and nitrogen availability on growth and nitrogen allocation between photosynthetic and plant defense components.  The main defense component studied was prunasin, a sugar-based compound that produces cyanide in response to tissue damage caused by foraging herbivores or mammals.

What was learned
Elevated CO2 increased total plant biomass by 98 and 134% relative to plants grown at ambient CO2 in high and low nitrogen treatments, respectively.  In addition, at the low nitrogen treatment, elevated CO2 stimulated greater root growth, as indicated by a 33% higher root:shoot ratio.  Hence, proportionately more carbon from photosynthesis was allocated to roots, as opposed to shoots, in this treatment, possibly to allow for greater volumetric soil exploration and the eventual acquisition of more nitrogen. 

Elevated CO2 caused no significant change in leaf prunasin content.  However, because the average leaf nitrogen content on a dry weight basis decreased under high CO2 (by 18% and 36% for the high and low nitrogen treatments, respectively), the proportion of nitrogen allocated to prunasin increased by approximately 20% relative to that allocated at ambient CO2 concentration.  Also, as atmospheric CO2 enrichment led to partial photosynthetic down regulation, the mobilization of nitrogen away from rubisco and other photosynthetic proteins could possibly account for the additional nitrogen allocated to prunasin.  And once again, as reported in many other studies, the reductions in rubisco and other photosynthetic proteins due to elevated CO2 did not significantly impact the carbon gains being made, as plants exposed to elevated CO2 had approximately twice the biomass of plants grown at ambient CO2.

What it means
As the atmospheric CO2 concentration continues to rise, this Eucalyptus species will probably experience photosynthetic down regulation without significantly affecting the growth stimulation brought about by elevated CO2.  This phenomenon should thus lead to larger Eucalyptus trees with better-developed root systems, particularly in regions where soil nitrogen fertility may be low.  In addition, increasing levels of CO2 should allow nitrogen to be mobilized away from photosynthesis and into leaf defence components so that this species can maintain a stable degree of protection from herbivores as the CO2 content of the air rises ever higher.

Reviewed 15 October 1998