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

Abstract

Ozone (O3) phytotoxicity is driven by the concentrations the plants are exposed to, and also by meteorological and physiological variables. Thus, O3 uptake might be a better indicator of the phytotoxicity of this pollutant than O3 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 O3 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 O3 accumulated dose (CUMFO3) for each biotype to be calculated. The biomass ratio of both clones was best correlated with indices based on O3 uptake, rather than with indices related to O3 exposure. In fact, the best performance of the system was achieved when O3 uptake > 40 nl l-1 (CUMFO3 40) was considered. The results support the need to relate O3 phytotoxicity to actual plant O3 uptake. © New Phytologist (2002).