Growth responses of seedlings of six Betula pubescens Ehrh. Provenances to six ozone exposure regimes

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Seedlings of six provenances of Betula pubescens Ehrh. from different latitudes (59–70°N) were grown under six ozone exposure regimes by combining different concentrations and daily exposure periods. The different treatments at increasing O3 exposure over 40 nmol mol−1 (AOT40 given in parentheses) were: 19 nmol mol−1/24 h day−1 (0.1 μmol mol−1‐h), 42 nmol mol−1/12 h day−1 (2.5 μmol mol−1‐h), 44 nmol mol−1/24 h day−1 (7.1 μmol mol−1‐h), 76 nmol mol−1/6 h day−1 (9.4 μmol mol−1‐h), 75 nmol mol−1/12 h day−1 (17.8 μmol mol−1) and 116 nmol mol−1/6 h day−1 (19.8 μmol mol−1) for 40 days at a 24 h day−1 photoperiod in growth chambers placed in a greenhouse. The effect of increasing the O3 exposure from 19 nmol mol−1/24 h (0.1 μmol mol−1‐h AOT40) to 42 nmol mol−1/12 h (2.5 μmol mol−1‐h AOT40) was a decrease in root but not shoot dry weight. A further increase in the exposure to 44 nmol mol−1/24 h (7.1 μmol mol−1‐h AOT40) also decreased the shoot dry weight. An increase in the O3 concentration to 75 (9.4–17.8 μmol mol−1‐h AOT40) and 116 nmol mol−1 (19.8 μmol mol−1‐h AOT40) further decreased shoot and root dry weights. A moderate O3 exposure (42 nmol mol−1/12 h = 2.5 μmol mol−1‐h AOT40) increased the plant height and leaf size, while a further increase in O3 concentration and exposure time decreased both of these variables. The birch provenances generally had a similar response to the O3 treatments. The accumulated O3 exposure over the 0, 10, 20, 30, 40 and 50 nmol mol−1 concentrations (AOT0, AOT10, AOT20, AOT30, AOT40 and AOT50, respectively) was calculated for all O3 treatments. The shoot and root dry weights were correlated best with AOT40 and AOT30, and were estimated to decrease by 10% at an AOT40 of 7.0 and 5.5 μmol mol−1‐h, respectively. The development of O3 injury (yellow stipples/chlorosis) was most marked when correlated with AOT40.