Growth and photosynthetic response of two larches exposed to O3 mixing ratios ranging from preindustrial to near future

In this study, we questioned whether ground-level ozone (O 3 ) induces hormesis in Japanese larch ( Larix kaempferi ) and its hybrid F 1 ( L. gmelinii var. japonica × L. kaempferi ). In order to answer the question, we exposed seedlings of both taxa to four O 3 treatments [ranging from ≈10 to 60 nmol(O 3 ) mol –1 ] in open-top chambers for two consecutive growing seasons. We found a hormetic response in maximum photosynthetic rate ( P Nmax ) at 1700 μmol(CO 2 ) mol –1 and maximum rates of carboxylation ( V cmax ) and electron transport ( J max ) in both larches. Stimulation of P Nmax , V cmax , and J max did not lead to suppressed plant productivity in Japanese larch, which followed a stress-tolerant strategy, but it did lead to suppressed plant productivity in hybrid larch which followed a competitive strategy. These findings are the first to suggest that stimulation of physiological functions by low O 3 exposures may have negative consequences for larch reproduction.