EFFECTS OF OZONE ON STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS IN POPULATIONS OF PLANTAGO-MAJOR L

British populations of Plantago major L. differ in ozone resistance. A s stomata regulate ozone uptake it is possible that some of this varia tion in ozone resistance is the result of differences in stomatal beha viour. This paper reports a comparative study of the effects of ozone exposure on the stomatal conductance (g(s)), net CO2 uptake (A) and in tercellular CO2 concentration (C-i) of 28 populations over a 5 d perio d under controlled conditions. The ozone exposure was 70 nl l(-1) for 7 h d(-1) while controls in charcoal filtered air received < 5 nl l(-1 ). There was no relationship between mean or maximum stomatal conducta nce in charcoal filtered air and ozone resistance. Ozone reduced g(s) in all populations within hours of the start of fumigation, but partic ularly so in the more sensitive ones, then over the next 4 d the patte rn changed as the effect on the sensitive populations decreased. The e ffect on the resistant populations remained more constant. Conductance measurements made in the dark period showed that the stomata did not close completely, and that in ozone fumigated plants g(s) was on avera ge 75% of that in control plants. Although the resistant populations s howed a large reduction in g(s), ozone had no effect on A or C-i indic ating that closure was not caused by reduced photosynthesis and higher C-i. It is argued that closure was probably caused by direct effects on the guard cells. In sensitive populations ozone reduced A to around 50-60% of the controls on day 1 but only to 70-80% of the controls on day 5. The recovery might have been caused by the decreasing effect o n g(s) but as the uptake of ozone would have increased at the same tim e it might have been related to acclimation.