Is a short, sharp shock equivalent to long-term punishment? Contrasting the spatial pattern of acute and chronic ozone damage to soybean leaves via chlorophyll fluorescence imaging

Experimental investigations of ozone (O₃) effects on plants have commonly used short, acute [O₃] exposure (>100 ppb, on the order of hours), while in field crops damage is more likely caused by chronic exposure (<100 ppb, on the order of weeks). How different are the O₃ effects induced by these two fumigation regimes? The leaf-level photosynthetic response of soybean to acute [O₃] (400 ppb, 6 h) and chronic [O ₃] (90 ppb, 8 h d^-1, 28 d) was contrasted via simultaneous in vivo measurements of chlorophyll a fluorescence imaging (CFI) and gas exchange. Both exposure regimes lowered leaf photosynthetic CO₂ uptake about 40% and photosystem II (PSII) efficiency (F_q′/ F_m′) by 20% compared with controls, but this decrease was far more spatially heterogeneous in the acute treatment. Decline in F _q′/F_m′ in the acute treatment resulted equally from decreases in the maximum efficiency of PSII (F_v′/F _m′) and the proportion of open PSII centres (F _q′/F_v′), but in the chronic treatment decline in F_q′/F_m′ resulted only from decrease in F_q′/F_v′. Findings suggest that acute and chronic [O₃] exposures do not induce identical mechanisms of O ₃ damage within the leaf, and using one fumigation method alone is not sufficient for understanding the full range of mechanisms of O₃ damage to photosynthetic production in the field.