summary Seedlings from four half‐sib families of Pinus elliottii var. elliottii Engelm. were planted in open‐top chambers, and exposed to ozone and simulated acidic precipitation. Treatments were applied for 11 months per year for 28 months. The ozone concentrations were: charcoal filtered air (CF), nonfiltered ambient air (1×), twice the nonfiltered level (2 ×) and three times the nonfiltered level (3 ×). Ozone exposures were computer controlled to follow natural diurnal fluctuations in ambient ozone concentrations. Simulated acid rain at pH 3.3, 4.3 or 5.3 was delivered weekly at the same volume as average weekly rainfall to all chambers. Leaf area, bud number and fascicle survival were all highest in the CF chambers. Genetic differences between half‐sib families were apparent in both leaf area and the number of live buds. Leaf area and fascicle survival were reduced with every increase in ozone exposure from CF to 3 ×. Leaf litterfall increased significantly with increasing ozone exposure. Leaf growth, measured by mean leaf area and mean dry weight per needle, was not affected by ozone exposure or by acid rain. It was concluded that ozone reduced leaf area mainly by causing premature leaf senescence. The only significant effect of acidic precipitation was enhanced leaf retention in trees irrigated with pH 3.3 solution. The results indicate that the increasing levels of trophospheric ozone may significantly reduce the growth of slash pine through foliage loss. The increased leaf retention caused by acid rain is unlikely to compensate for the detrimental effect of ozone.

summary Four half‐sib families of slash pine (Pinus elliottii var. elliottii) were planted in open‐top chambers and exposed to ozone and simulated acidic precipitation (3: 7 molar ratio of nitric and sulphuric acids). The 4 ozone treatments were (1) charcoal‐filtered air, (2) unfiltered ambient air, (3) twice ambient and (4) three times ambient ozone, with treatments applied 12 h per day, 11 months per year for 28 months. Seedlings were irrigated weekly with one of three rain acidities: pH 33, 4.3 or 5.3 at a total rainfall equivalent of 133 cm per year. Photosynthetic rates were measured approximately every 10 weeks, on the most recently emerged branches each year. In 1988 the 3rd flush was measured, and in 1989 and 1990 the 1st flush of each year was measured. Elevated ozone concentrations generally reduced photosynthetic rates, especially during the summer months, but there was no evidence of either a threshold dose or a simple linear relationship between photosynthesis and ozone exposure. There was a seasonal drop in photosynthetic rates from March to September each year. An ontogenetic decrease in photosynthesis was also evident as trees matured. Stomatal conductance also decreased as the needles aged. Seedlings irrigated with pH 3.3 solution typically exhibited the highest photosynthetic rates, presumably due to a fertilizer effect.

summary Seedlings from two half‐sib families of Pinus elliottii var. elliottii Engelm. differing in sensitivity to ozone were grown in open‐top chambers for 22 months under simulated acidic rain (pH 3‐3) and four concentrations of ozone. The ozone concentrations were: charcoal‐filtered air (CF); non‐filtered ambient air (NF); twice the NF level (2X), thrice the NF level (3X). Ozone exposures were computer controlled to follow diurnal and seasonal fluctuations in ambient ozone concentrations. On two days, 24 April and 29 May, in the spring of 1990, trees were sampled repeatedly for xylem water potential, needle conductance and photosynthesis from sunrise to sunset on the oldest needle age class attached to the main stem (first needle cohort produced in 1989, and therefore, exposed the longest to ozone). On 30 May, three needle age classes (first age class of 1989, the last age class of 1989 and first ape class of 1990) were sampled over the day for needle conductance in trees exposed to CF and 3X ozone. Soil moisture measurements indicated that the 3X trees used significantly more water from the 75–l00 cm soil depth even though leaf area on these trees was about 30% less. Tree response showed very consistent xylem water potential among sample days, families and ozone treatments. By contrast, needle conductance and photosynthetic rate showed significant ozone and family differences. In addition, the response of needle conductance to vapour pressure deficit lessened as ozone exposure increased. The putative ozone‐sensitive family (106‐56) exhibited less needle conductance in the 2X and 3X treatments as well as reduced photosynthesis on both sample days, suggesting prior ozone injury. The youngest needle age class of this same family when exposed to 3X ozone showed higher needle conductance in the afternoon which could lead to both excessive water loss and uptake of ozone. The interactive effects of chronic ozone exposure, limited soil moisture and genetics altered the water relations and gas exchange of slash pine growing in north Florida. Changes in sromatal conductance along with possible adjustments in the hydraulic properties of the roots and stems appeared to be responsible for the control of water potentials in trees of differing sizes and leaf area. The importance of the contribution of different needle age classes to the water and carbon economy of slash pine, and their modification by cumulative ozone exposure, cannot he overlooked in interpreting air pollution effects on evergreen trees.