Chronic vs. short-term acute O3 exposure effects on nocturnal transpiration in two Californian oaks

We tested the effect of daytime chronic moderate ozone (O-3) exposure, short-term acute exposure, and both chronic and acute O-3 exposure combined on nocturnal transpiration in California black oak and blue oak seedlings. Chronic O-3 exposure (70 ppb for 8 h/day) was implemented in open-top chambers for either 1 month (California black oak) or 2 months (blue oak). Acute O-3 exposure (similar to 1 h in duration during the day, 120-220 ppb) was implemented in a novel gas exchange system that supplied and maintained known O-3 concentrations to a leaf cuvette. When exposed to chronic daytime O-3 exposure, both oaks exhibited increased nocturnal transpiration (without concurrent O-3 exposure) relative to unexposed control leaves (1.8x and 1.6x, black and blue oak, respectively). Short-term acute and chronic O-3 exposure did not further increase nocturnal transpiration in either species. In blue oak previously unexposed to O-3, short-term acute O-3 exposure significantly enhanced nocturnal transpiration (2.0x) relative to leaves unexposed to O-3. California black oak was unresponsive to (only) short-term acute O-3 exposure. Daytime chronic and/or acute O-3 exposures can increase foliar water loss at night in deciduous oak seedlings.