Comparative Ozone Dose Response of Gas Exchange in a Ponderosa Pine Stand Exposed to Long-Term Fumigations

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Abstract
Photosynthesis (by 14CO2 uptake) and stomatal conductance of attached ponderosa pine (Pinus ponderosa Laws.) needles were studied in relation to cumulative incident ozone dose in a stand of sapling trees in the San Bernardino National Forest. These trees have been exposed to long-term O3 fumigations throughout their history. A steady-state, ventilated, minicuvette system was used to measure photosynthesis and conductance at light saturation (photon flux density > 1000 μeinsteins m−2s−1), 20°C, and a vapor pressure gradient (needle to air) of 14 mb. Nine trees were stratified into three chronic injury classes (I, slight; II, moderate; III, severe) of three trees each, based on morphological O3 injury symptoms, and the trees were sampled monthly from May to October of 1977. Although these three injury classes grew in a similar environment, they had differential photosynthetic and stomatal responses, suggesting ecotypic variation in O3 sensitivity. The decline in photosynthesis and stomatal function normally associated with aging was accelerated as O3 injury symptoms increased. Photosynthesis in class I, II, and III trees was reduced to about 10% of the maximum rate of class I current needles after integrating 800, 700, and 450 ppm-h, respectively, at which point needle abscission occurred. At the October sampling, class I, II, and III trees retained 3, 2, and 1 annual needle whorls, respectively. Stomatal conductances tended to be somewhat larger in the more O3–sensitive trees during July and August, when the needles were rapidly expanding and possibly most vulnerable to the high O3 concentrations occurring during this period of the year. It cannot be stated conclusively that this trend in conductances is a determining factor that explains differential O3 sensitivity among the study trees. The relationship in conductances among injury classes reversed after the initial season of needle growth, primarily because stomatal function recovery during the relatively O3-free winter months was inversely related to O3 injury, in all trees and needle ages, losses in photosynthetic capacity exceeded reductions in stomatal conductance, suggesting that injury to the mesophyll, carboxylation, or excitation components of the CO2 diffusion pathway was greater than injury to the stomata.

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