Foliar Senescence Is The Most Sensitive Response To Ozone In Bromus Hordeaceus And Is Modulated By Nitrogen Input

A study was conducted on the effect of tropospheric ozone (O-3) on soft brome (Bromus hordeaceus) and the modulation of its response by nitrogen (N). Two assays were conducted using open-top chambers (OTCs). 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 Iberian Peninsula. 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.