The effect of ozone on below-ground carbon allocation in wheat

Short-term (CO2)-C-14 pulse and chase experiments were conducted in order t o investigate the effect of ozone on below-ground carbon allocation in spri ng wheat seedlings (Triticum aestivum L. 'ANZA'). Wheat seedlings were grow n in a sand-hydroponic system and exposed to either high ozone (38-40 ppm-h ) or low ozone (23-31 ppm-h) for 21 days in a series of replicated experime nts. Following the ozone exposures, the plants were pulsed with (CO2)-C-14 and allocation of C-14-labeled photosynthate was measured in the plant and growth media. Soluble root exudates were measured, without disturbing the p lant roots, 24 h after the (CO2)-C-14 pulse. Shoot biomass was reduced by 1 7% for the high ozone and 9% for the low ozone exposures, relative to contr ol treatments. Root biomass was reduced by 9% for the high ozone exposures, but was not significantly different than the controls for the low ozone. T he amount of C-14 activity in the shoot and root tissue 24 h after the (CO2 )-C-14 pulse, normalized to tissue weight, total (CO2)-C-14 uptake, or the total C-14 retention in each plant, was not affected by either high or low ozone exposures. The amount of C-14 activity measured in the growth media s olution surrounding the roots increased 9% for the high ozone exposures, an d after normalizing to root size or root C-14 activity, the growth media so lution C-14 activity increased 29 and 40%, respectively. Total respiration of (CO2)-C-14 from the ozone-treated plants decreased, but the decrease was not statistically significant. Our results suggest that soluble root exuda tion of C-14 activity to the surrounding rhizosphere increases in response to ozone. Increased root exudation to the rhizosphere in response to ozone is contrary to reports of decreased carbon allocation below ground and sugg ests that rhizosphere microbial activity may be initially stimulated by pla nt exposure to ozone. Published by Elsevier Science Ltd.