Objective Considerable researches have documented the negative effects of O3 on woody species in North America and Europe, however, little is known about how woody tree species respond to elevated O3 in subtropical China. The objectives of this study are to investigate the effects of elevated O3 on two evergreen tree species in subtropical China, and compare the O3 sensitivity between them and explore potential mechanisms on differential responses to O3.
Method One-year-old seedlings of Phoebe zhennan S. Lee et F. N. Wei and Phoebe bournei (Hemsl.) Yang were exposed to non-filtered air, 100 nmol mol-1 O3 air (E1) and 150 nmol mol-1 O3 air (E2) to investigate the O3 effects on photosynthesis, antioxidant capacity and biomass of these two species.
Result The results are as follows:(1) O3 fumigation reduced net photosynthesis rate (Pn) mainly through non-stomatal factors in both species. Pn was lower under E2 than that under E1 in P. zhennan. Whereas for P. bournei, Pn was greater under E2 than that under E1 in August measurement, while the reverse patterns were observed in the measurements in September and October. (2) O3 fumigation increased measured antioxidant substances levels in both species. The levels of total phenols and total antioxidant capacity increased with O3 concentrations increasing. However, reduced ascorbic acid contents were lower under E2 than that under E1. (3) O3 fumigation decreased root, stem, foliage and total biomass in both species. The reduced below/above ground biomass ratio was only observed in P. bournei.
Conclusion O3 fumigation decreases photosynthesis, increases the antioxidant capacity, and ultimately inhibits the growth of the two species. The O3 sensibility of P. bournei is stronger than that of P. zhennan based on Pn and biomass. The differential sensitivity to O3 is related to (i) stomatal conductance, and (ii) antioxidant substances responses to elevated O3 and their background level.
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