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

Short-term 14CO2 pulse and chase experiments were conducted in order to investigate the effect of ozone on below-ground carbon allocation in spring wheat seedlings (Triticum aestivum L. `ANZA\u27). Wheat seedlings were grown 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 experiments. Following the ozone exposures, the plants were pulsed with 14CO2 and allocation of 14C-labeled photosynthate was measured in the plant and growth media. Soluble root exudates were measured, without disturbing the plant roots, 24 h after the 14CO2 pulse. Shoot biomass was reduced by 17% for the high ozone and 9% for the low ozone exposures, relative to control treatments. Root biomass was reduced by 9% for the high ozone exposures, but was not significantly different than the controls for the low ozone. The amount of 14C activity in the shoot and root tissue 24 h after the 14CO2 pulse, normalized to tissue weight, total 14CO2 uptake, or the total 14C retention in each plant, was not affected by either high or low ozone exposures. The amount of 14C activity measured in the growth media solution surrounding the roots increased 9% for the high ozone exposures, and after normalizing to root size or root 14C activity, the growth media solution 14C activity increased 29 and 40%, respectively. Total respiration of 14CO2 from the ozone- treated plants decreased, but the decrease was not statistically significant. Our results suggest that soluble root exudation of 14C 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 suggests that rhizosphere microbial activity may be initially stimulated by plant exposure to ozone