Loblolly pine seedling growth after inoculation with plant growth-promoting rhizobacteria and ozone exposure

Plant growth-promoting rhizobacteria promote plant growth and induce biocontrol, but are affected by soil type, water stress, microbial competition, and environmental conditions. One unexplored factor is the interaction of rhizobacteria-inoculated plants exposed to ozone. Loblolly pine (Pinus taeda L.) seeds were inoculated with either Bacillus subtilis (Ehrenberg) Cohn or Paenibacillus macerans (Schardinger) Ash. In field trials, 4-week-old seedlings were exposed for 12 weeks to carbon-filtered (CF ~~ 12 ppb), 1× (~~46 ppb), or 2× (~~97 ppb) ozone for 12 h·d^–1 in open-top chambers (OTC) in 1998 and 1999. In three greenhouse trials, 5-week-old seedlings were exposed to ozone at 0× (~~8 ppb), 1× (~~105 ppb), 2× (~~ 199 ppb), and 3× (~~298 ppb) for 4 h·d^–1, 5 d·week^–1 for 8 weeks in continuously stirred tank reactors (CSTR). In both the CSTRs and the OTCs, ozone-exposed seedlings exhibited 20%–50% less biomass and more foliar injury as compared with nonexposed seedlings. In CSTRs, at the 3× exposure, B. subtilis-inoculated seedlings had 12% less foliar injury than noninoculated seedlings. Foliar injury was 65% less for B. subtilis-treated seedlings in 1998, and root surface area, total root length, and root diameter was 25%–35% greater when seedlings were exposed to 2× ozone in the OTCs. This is the first report of rhizobacteria protecting seedlings against the negative effects of ozone exposure.