Foliar senescence is the most sensitive response to ozone in B romus hordeaceus and is modulated by nitrogen input

A study was conducted on the effect of tropospheric ozone ( O 3 ) on soft brome ( B romus hordeaceus ) and the modulation of its response by nitrogen ( N ). Two assays were conducted using open‐top chambers ( OTC s). Three O 3 treatments were considered: filtered air, with concentrations below background levels (charcoal‐filtered air), non‐filtered air ( NFA ) that simulates ambient O 3 concentrations, and unfiltered air to which 40 nL L −1 O 3 above‐ambient concentrations was added ( NFA +), simulating elevated values recorded in natural areas of annual pastures in the I berian P eninsula. Three N rates were used, simulating the increase in soil N through atmospheric deposition and excreta from livestock grazing. Ozone caused an augmentation in foliar senescence, whereas green biomass was not altered; consequently, an increased senescent/green biomass ratio was produced. A stronger O 3 effect was detected in the second assay compared with the first. This was related to the estimated absorbed O 3 fluxes, which were double the value calculated in the former. Increasing N input enhanced biomass production, but its effectiveness was greater in the first assay, under less‐favourable weather conditions and lower plant growth. In the first assay, the O 3 response was modulated by N availability, which mitigated the effects of O 3 to medium concentration values. In the first assay, O 3 reduced the aerial/subterranean biomass ratio, caused by a positive‐trend effect on roots. Foliar concentration of lignin was increased by O 3 , and in vitro digestibility of aerial biomass and the plant cell wall fraction tended to decrease with increasing O 3 .