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

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 (CUMFO3) 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.