Diurnal water relations and gas exchange of two slash pine (Pinus elliottii) families exposed to chronic ozone levels and acidic rain *

Abstract

summarySeedlings 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.