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) O(3) (ozone) exposure (70ppb over 8h 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 O(3) exposure and a reason for increased O(3) sensitivity in snap bean cultivars, as it implied higher O(3) uptake during times of disequilibrium. Sluggishness increased the time to open (thus limiting CO(2) 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 O(3)-induced stomatal sluggishness is a common trait among different plant physiognomic classes.