Root and soil respiration responses to ozone in Pinus taeda L. seedlings*†

Respiration rates of roots of loblolly pine (Pinus taeda L.) seedlings decreased in response to elevated ozone (O₃ ) concentrations. Estimated annual root respiration rates were 12% less in seedlings exposed to twice-ambient O₃ (7 h mean = 70.110 nl 1^-1 ) than in seedlings exposed to sub-ambient O₃ (7 h mean = 20.40 nl 1^-1 ). Measurements taken during periods of relatively rapid and of negligible root growth suggest that the reduced root respiration may be due to both reduced maintenance respiration and reduced growth respiration. Respiration rates of the soil substrate of the O₃ -exposed seedlings were also below those of the substrate of seedlings exposed to sub-ambient O₃ . The study supports the theory that there is a reduced supply of photosynthates to the roots of plants exposed to elevated O₃ . It is hypothesized that a reduced supply of photosynthates to the roots may affect soil respiration rates by reducing root exudation rates and, consequently, reducing rhizosphere microbial populations.