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 (O-3) effects on plants have commonly used short, acute [O-3] 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 O-3 effects induced by these two fumigation regimes? The leaf-level photosynthetic response of soybean to acute [O-3] (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 [O-3] 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 O-3 damage to photosynthetic production in the field.