PARENT SESSION
Posters P1D Photosynthesis and global change. Abstracts (699-710)

Chlorophyll a fluorescence and thermal imaging of leaf-level variation in photosynthetic responses to O₃ exposure. Charles Chen^*,1, Thomas Frank², Stephen Long³, ¹ Program of Ecology and Evolutionary Biology, Urbana, IL, USA² Department of Geography, Urbana, IL, USA³ Departments of Crop Sciences and Plant Biology, Urbana, IL, USA

ABSTRACT- Global mean tropospheric ozone concentrations today are already at levels which have a negative impact on plants in natural ecosystems. Ozone (O₃)enters the plant through the stomata and causes oxidative stress, leading to a decrease in photosynthesis. This air pollutant is such a highly reactive oxidizing agent that the effect of ozone exposure can be variable even at the leaf scale. However, at low levels of ozone exposure, this variability may be invisible to the naked eye, and thus ignored. The purpose of this study was to describe the spatial variability of the effect of O₃ on photosynthesis within a leaf. A chlorophyll a fluorescence imaging system was used to examine the photosynthetic efficiency of leaves. Spatial heterogeneity of the images was calculated as the standard deviation of neighboring pixels. Soybean plants grown in a controlled environment were exposed to a single acute dose of O₃ (400ppb for 4hrs). The maximum and the operating efficiency of PSII decreased after the O₃ exposure; however, the spatial heterogeneity of the photosynthetic parameters increased (p<0.05). Plants grown under controlled chronic exposure conditions (70ppb-6hr/day for 14 days) showed increasing spatial heterogeneity in photosynthetic efficiency over the course of the exposure. Spatial heterogeneity was also assessed on plants from the SoyFACE (Free-Air Concentration Enrichment) field site, where soybeans were chronically exposed to elevated levels of O₃ (1.2 x ambient) under open canopy conditions. In addition, thermal images were taken to examine the spatial variability of stomatal conductance in the leaves. The non-uniform decrease in the photosynthetic efficiency of leaves under O₃ exposure indicates that the use of classical methods of measuring photosynthesis such as gas exchange or integrated chlorophyll fluorescence, which integrate over relatively large areas of the leaf, may not be ideal for examining the effects of O₃ at the leaf level.

KEY WORDS: thermal imaging, heterogeneity, chlorophyll fluorescence imaging, ozone