PROCEEDINGS: Index of Abstracts

ROOT GROWTH AND RESPIRATION OF ASPEN IN RESPONSE TO OZONE AND ELEVATED CARBON DIOXIDE

M. D. Coleman, R. E. Dickson, and J. G. Isebrands-1

1-USDA Forest Service, Forestry Sciences Lab, Rhinelander WI 54501.

The response of tree root systems to interacting environmental stress is poorly understood in comparison to knowledge of above-ground organs. This research investigates the effect of ozone (O₃), elevated carbon dioxide (CO₂) and their combination on root system growth and respiration. Adventitiously rooted cuttings of three aspen clones (271, O₃ tolerant; 216, intermediate; 259, O₃ sensitive) were grown in 7 pots containing peat:sand:vermiculite (2:1:1). Plants were placed in treatment chambers and exposed to either ambient air (control), ambient + 350 ppm CO₂, 160 PPM O₃ (8 h daily), elevated CO₂ and O₃. Root respiration was measured by sealing the entire pot into the measurement cuvette of an open-flow gas exchange system. Roots and soil were then separated; roots were dried and weighed, and soil was returned to pots and remeasured to estimate heterotrophic soil respiration. After 12 weeks, O₃ treatment caused up to a 48 percent decrease in root dry weight with a corresponding 53 percent decrease in root system respiration rate (umol plant^-1 s^-1) compared with control treatments. Changes in root dry weight and respiration rate generally followed the O₃ sensitivity rankings of the clones. In contrast, elevated CO₂ vs. control caused up to a 63 percent increase in root dry weight and a 47 percent increase in root system respiration; again there were big clonal differences in responses. Compared to the control treatment, there was no net change in root weight due to the combined O₃ and elevated CO₂ treatment yet root system respiration declined slightly. When specific root respiration rate (umol g^-1 s^-1) was calculated, there were no consistent treatment effects; however specific root respiration declined as plants aged, and interesting clonal differences were also observed. For clones 216 and 259, specific root respiration generally increased with either O₃ or elevated CO₂ in 12-week-old plants, but for clone 271 specific root respiration decreased with treatment. These results show that aboveground environmental stress affects the growth and physiology of aspen roots in complex ways, and the particular response obtained has a strong genetic component.