Elevated ozone reduces photosynthetic carbon gain by accelerating leaf senescence of inbred and hybrid maize in a genotype-specific manner.

Exposure to elevated tropospheric ozone concentration ([O3 ]) accelerates leaf senescence in many C3 crops. However, the effects of elevated [O3 ] on C4 crops including maize (Zea mays L.) are poorly understood in terms of physiological mechanism and genetic variation in sensitivity. Using free air gas concentration enrichment, we investigated the photosynthetic response of 18 diverse maize inbred and hybrid lines to season-long exposure to elevated [O3 ] (~100 nl L-1 ) in the field. Gas exchange was measured on the leaf subtending the ear throughout the grain filling period. On average over the lifetime of the leaf, elevated [O3 ] led to reductions in photosynthetic CO2 assimilation of both inbred (-22%) and hybrid (-33%) genotypes. There was significant variation among both inbred and hybrid lines in the sensitivity of photosynthesis to elevated [O3 ], with some lines showing no change in photosynthesis at elevated [O3 ]. Based on analysis of inbred line B73, the reduced CO2 assimilation at elevated [O3 ] was associated with accelerated senescence decreasing photosynthetic capacity and not altered stomatal limitation. These findings across diverse maize genotypes could advance the development of more O3 tolerant maize and provide experimental data for parameterization and validation of studies modeling how O3 impacts crop performance.