Chronic vs. Short-Term Acute O(3) 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 (~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 (1.8× and 1.6×, 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 (2.0×) 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.