Use of Physiological and Biochemical Markers for Assessing Air Pollution Stress in Trees
Authors: Richardson CJ , Sasek TW , Di Giulio RT 
Published: March 2009

Air pollutants such as O3, NOx, SO2, and H2O2 are powerful oxidants that can generate extremely reactive oxygen free radicals that may cause enzyme breakdown, membrane damage, and DNA alterations, all resulting in reduced growth. In this study, specific gas exchange measurements were used diagnostically as indicators of stress and as a means of separating stomatal from biochemical effects on photosynthesis. Similarly, biochemical antioxidants and oxidant stress indicators were hypothesized to be useful as early biomarkers of oxidant stress in trees.

Loblolly pine (Pinus taeda L.) seedlings were exposed to a range of ozone treatments in opentop chambers at two study sites in North Carolina. Treatments consisted of charcoal-filtered (CF) air and proportional additions of ozone to non-filtered air in relation to the ambient ozone concentration. Diagnostic gas exchange measurements included the response of photosynthesis and stomatal conductance to irradiance and to intercellular CO2 concentration (Ci). Biochemical measurements included the content of malondialdehyde (MDA), an indicator of lipid per-oxidation, and the activities of the enzymatic antioxidants Superoxide dismutase (SOD) and peroxidase (Px). These biochemical characteristics were also related to concurrent measurements of light-saturated photosynthesis.

In loblolly pine, elevated ozone (3.0 × ambient ozone) decreased photosynthetic capacity under light-saturating conditions by up to 40% and decreased the apparent quantum yield of photosynthesis by up to 54% compared to the charcoal-filtered control. The same elevated ozone concentrations also reduced the initial slope of the photosynthesis versus Ci relationship by almost 34% and reduced the CO2-saturated rate of photosynthesis by 28%. However, stomatal resistance reduced potential rates of photosynthesis by approximately 34% in both the elevated ozone treatment and the charcoal-filtered control. Thus reduced rates of photosynthesis at elevated ozone concentrations were due almost entirely to internal biochemical processes, not to stomatal effects. These biochemical effects may include light-harvesting and biochemical efficiencies of the photosystems, the activity of RuBP carboxylase, the regeneration rate of RuBP, and the electron transport capacity.

Malondialdehyde levels in loblolly seedlings exposed to 3.0 × ambient ozone were twice those of pines exposed to charcoal-filtered air, indicating a significant increase in lipid peroxidation. At the same time, the activities of the antioxidants SOD and Px were higher in pines exposed to intermediate levels of ozone (1.5 × ambient ozone), and more than twice the controls in the 3.0 × ambient ozone treatment.

Our results suggest that diagnostic gas exchange techniques can be used as a sensitive indicator for separating stomatal from biochemical control of photosynthesis in trees exposed to oxidizing air pollutants. Additionally, biochemical antioxidants, such as Px and SOD, and oxidant stress indicators, such as MDA, may be useful as early biomarkers of oxidant stress.