Responses of white clover to gaseous pollutants and acid mist: implications for setting critical levels and loads

summary White clover plants (Trifolium repens L. cv. ‘Grasslands Huia’) were exposed to combinations of gaseous pollutants and acid mists for 15 wk. The treatments were (a) charcoal‐filtered air (control), (b) 40 ppb SO2+ 40 ppb NO2, (e) 40 ppb Oa with additional peaks of 2 × 3 h at 80 ppb and 1 × 1 h at 110 ppb O3 and (d) SO2+NO2+O3 (a combination of treatments b and c), these being combined with acid mist treatments (6 mm wk1) with solutions at pHs of 2.5, 3.5, 4.5 and 5.6. All gaseous pollution treatments resulted in substantial reductions in plant dry weights with additive effects of SO2+ NO2 and O3 in the SO2+ NO2+O2 treatment. There was a lower dry weight, averaged over all gas pollution treatments, for plants exposed to pH 2.5 compared with pH 4.5 or pH 5.6 mists. An interactive effect of gas x mist treatments revealed that effects of acid mists on plant dry weights were overridden by the more toxic O3 and SO2+ NO2+ O3 gas exposures. Net photosynthesis was reduced in plants exposed to pH 2.5 and pH 3.5 mists compared with plants exposed to pH 5.6 mist. Rates of stomatal conductance were reduced by exposure to the gaseous pollution treatments but not to acid mists. Water loss was greater after 55 h for leaves detached from plants exposed to all gaseous pollution treatments compared with ones from control plants. There was a greater loss of water from detached leaves exposed to pH 2.5 compared with pH 5.6 mist. The quantities (loads) of S + N in gaseous form resulting in decreased growth of T. repens were calculated to be much lower than those applied in mists with no adverse effects. It is concluded that the concepts of critical levels and critical loads of pollutants need considerable refinement for realistic assessments of the threat of pollutants to vegetation.