Investigating indices to explain the impacts of ozone on the biomass of white clover (Trifolium repens L. cv. Regal) at inland and coastal sites in Spain

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Abstract
Summary: Ozone (O 3 ) phytotoxicity is driven by the concentrations the plants are exposed to, and also by meteorological and physiological variables. Thus, O 3 uptake might be a better indicator of the phytotoxicity of this pollutant than O 3 exposure. An ozone biomonitoring system based on the biomass ratio between an ozone‐resistant (NC‐R) and an ozone‐sensitive (NC‐S) biotype of white clover ( Trifolium repens cv. Regal) has been tested for 4 yr at four Spanish sites experiencing wide differences in their climatic and ozone exposure profiles. A complementary experiment was also carried out in open‐top chambers exposing plants of both biotypes to three O 3 treatments. Ozone exposure, air temperature, relative humidity and leaf stomatal conductance to water vapour were measured during the life span of both clover clones. These measurements enabled the total O 3 accumulated dose (CUMFO 3 ) for each biotype to be calculated. The biomass ratio of both clones was best correlated with indices based on O 3 uptake, rather than with indices related to O 3 exposure. In fact, the best performance of the system was achieved when O 3 uptake > 40 nl l −l (CUMFO 3 40) was considered. The results support the need to relate O 3 phytotoxicity to actual plant O 3 uptake.