DIURNAL WATER RELATIONS AND GAS-EXCHANGE OF 2 SLASH PINE (PINUS-ELLIOTTII) FAMILIES EXPOSED TO CHRONIC OZONE LEVELS AND ACIDIC RAIN

Seedlings from two half-sib families of Pinus elliottii var. elliottii Engelm. differing in sensitivity to ozone were grown in open-top cham bers for 22 months under simulated acidic rain (pH 3.3) and four conce ntrations of ozone. The ozone concentrations were: charcoal-filtered a ir (CF); non-filtered ambient air (NF); twice the NF level (2X), thric e the NF level (3X). Ozone exposures were computer controlled to follo w diurnal and seasonal fluctuations in ambient ozone concentrations. O n two days, 24 April and 29 May, in the spring of 1990, trees were sam pled repeatedly for xylem water potential, needle conductance and phot osynthesis from sunrise to sunset on the oldest needle age class attac hed to the main stem (first needle cohort produced in 1989, and theref ore, exposed the longest to ozone). On 30 May, three needle age classe s (first age class of 1989, the last age class of 1989 and first age c lass of 1990) were sampled over the day for needle conductance in tree s exposed to CF and 3X ozone. Soil moisture measurements indicated tha t the 3X trees used significantly more water from the 75-100 cm soil d epth even though leaf area on these trees was about 30% less. Tree res ponse showed very consistent xylem water potential among sample days, families and ozone treatments. By contrast, needle conductance and pho tosynthetic rate showed significant ozone and family differences. In a ddition, the response of needle conductance to vapour pressure deficit lessened as ozone exposure increased. The putative ozone-sensitive fa mily (106-56) exhibited less needle conductance in the 2X and 3X treat ments as well as reduced photosynthesis on both sample days, suggestin g prior ozone injury. The youngest needle age class of this same famil y when exposed to 3X ozone showed higher needle conductance in the aft ernoon which could lead to both excessive water loss and uptake of ozo ne. The interactive effects of chronic ozone exposure, limited soil mo isture and genetics altered the water relations and gas exchange of sl ash pine growing in north Florida. Changes in stomatal conductance alo ng with possible adjustments in the hydraulic properties of the roots and stems appeared to be responsible for the control of water potentia ls in trees of differing sizes and leaf area. The importance of the co ntribution of different needle age classes to the water and carbon eco nomy of slash pine, and their modification by cumulative ozone exposur e, cannot be overlooked in interpreting air pollution effects on everg reen trees.