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

British populations of Plantago major L. differ in ozone resistance. As stomata regulate ozone uptake it is possible that some of this variation in ozone resistance is the result of differences in stomatal behaviour. This paper reports a comparative study of the effects of ozone exposure on the stomatal conductance (g(s)), net CO2 uptake (A) and intercellular CO2 concentration (C-i) of 28 populations over a 5 d period 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 conductance in charcoal filtered air and ozone resistance. Ozone reduced g(s) in all populations within hours of the start of fumigation, but particularly so in the more sensitive ones, then over the next 4 d the pattern changed as the effect on the sensitive populations decreased. The effect 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 average 75% of that in control plants. Although the resistant populations showed a large reduction in g(s), ozone had no effect on A or C-i indicating 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 on g(s) but as the uptake of ozone would have increased at the same time it might have been related to acclimation.