GROWTH-RESPONSES OF SEEDLINGS OF 6 BETULA-PUBESCENS EHRH. PROVENANCESTO 6 OZONE EXPOSURE REGIMES

Seedlings of six provenances of Betula pubescens Ehrh. from different latitudes (59-70 degrees N) were grown under six ozone exposure regime s by combining different concentrations and daily exposure periods. Th e different treatments at increasing O-3 exposure over 40 nmol mol-(1) (AOT40 given in parentheses) were: 19 nmol mol(-1)/24 h day(-1) (0.1 mu mol mol(-1)-h), 42 nmol mol(-1)/12 h day(-1) (2.5 mu mol mol(-1)-h) , 44 nmol mol(-1)/24 h day(-1) (7.1 mu mol mol(-1)-h), 76 nmol mol(-1) /6 h day(-1) (9.4 mu mol mol(-1)-h), 75 nmol mol(-1)/12 h day(-1) (17. 8 mu mol mol(-1)) and 116 nmol mol(-1)/6 h day(-1) (19.8 mu mol mol(-1 )) for 40 days at a 24 h day(-1) photoperiod in growth chambers placed in a greenhouse. The effect of increasing the O-3 exposure from 19 nm ol mol(-1)/24 h (0.1 mu mol mol(-1)-h AOT40) to 42 nmol mol(-1)/12 h ( 2.5 mu mol mol(-1)-h AOT40) was a decrease in root but not shoot dry w eight. A further increase in the exposure to 44 nmol mol(-1)/24 h (7.1 mu mol mol(-1)-h AOT40) also decreased the shoot dry weight. An incre ase in the O-3 concentration to 75 (9.4-17.8 mu mol mol(-1)-h AOT40) a nd 116 nmol mol(-1) (19.8 mu mol mol(-1)-h AOT40) further decreased sh oot and root dry weights. A moderate O-3 exposure (42 nmol mol(-1)/12 h = 2.5 mu mol mol(-1)-h AOT40) increased the plant height and leaf si ze, while a further increase in O-3 concentration and exposure time de creased both of these variables. The birch provenances generally had a similar response to the O-3 treatments. The accumulated O-3 exposure over the 0, 10, 20, 30, 40 and 50 nmol mol(-1) concentrations (AOT0, A OT10, AOT20, AOT30, AOT40 and AOT50, respectively) was calculated for all O-3 treatments. The shoot and root dry weights were correlated bes t with AOT40 and AOT30, and were estimated to decrease by 10% at an AO T40 of 7.0 and 5.5 mu mol mol(-1)-h, respectively. The development of O-3 injury (yellow stipples/chlorosis) was most marked when correlated with AOT40.