STOMATAL-RESISTANCE AND THE RATIO OF INTERCELLULAR TO AMBIENT CARBON-DIOXIDE IN CONTAINER-GROWN YELLOW-POPLAR SEEDLINGS EXPOSED TO CHRONIC OZONE FUMIGATION AND WATER-STRESS

One-year old container-grown seedlings of yellow-poplar (Liriodendron tulipifera L.) were exposed to 0.15 ppm ozone (O3) fumigation and wate r stress (two 1-week drying cycles) in continuously stirred tank react ors prior to measuring stomatal resistance (R(s)), intercellular carbo n dioxide [CO2 (C(i))], and the ratio of C(i) and ambient CO2 (C(a)) a fter 20 and 36 days. Water stress increased R(s) by more than 250% and reduced C(i) and the C(i)/C(a) ratio by just over 20%. There were no significant effects of 0.15 ppm O3 on C(i) and the C(i)/C(a) ratio but R(s) was significantly increased by the interaction of water stress a nd O3 fumigation. For water-stressed seedlings fumigated with charcoal -filtered air, C(i) and the C(i)/C(a) ratio were significantly lower t han those of the other treatments. The data show that interactions inv olving low levels of ozone and short periods of water stress may have a substantial influence on stomatal physiology.