Ozone exposure and stomatal sluggishness in different plant physiognomic classes

Gas exchange responses to static and variable light were tested in three species: snap bean (Phaseolus vulgaris, two cultivars), California black oak (Quercus kelloggii), and blue oak (Q. douglasii). The effects of 1-month (snap beans) and 2-month (oaks) O3 (ozone) exposure (70 ppb over 8 h per day in open-top chambers) were investigated. A delay in stomatal responses (i.e., ‘sluggish’ responses) to variable light was found to be both an effect of O3 exposure and a reason for increased O3 sensitivity in snap bean cultivars, as it implied higher O3 uptake during times of disequilibrium. Sluggishness increased the time to open (thus limiting CO2 uptake) and close stomata (thus increasing transpirational water loss) after abrupt changes in light level. Similar responses were shown by snap beans and oaks, suggesting that O3-induced stomatal sluggishness is a common trait among different plant physiognomic classes.