Trinitrotoluene (TNT) is released into the soil from many different sources, especially from military and mining activities, including buried landmines. Vegetative cover plays an important role in the movement of TNT contamination through the soil profile. Plants may absorb explosive residuals, causing stress. By understanding how plants respond to energetic compounds, we may be able to use non-invasive techniques to detect soil contamination. The objectives of this study were to examine the physiological responses of plants grown in TNT contaminated soils and to use remote sensing methods to detect uptake in plant leaves and canopies. Seedlings of Myrica cerifera were grown in 0, 30, 100, 250 and 500 mg of TNT/kg dry soil. Measurements of photosynthesis, stomatal conductance, chlorophyll fluorescence and reflectance were taken weekly for 3 months. At the end of the experiment, soils were allowed to dry out to examine the combined effects of TNT and drought.
Physiological effects of TNT contamination were apparent by week 3 as evidenced in decreased stomatal conductance and photosynthesis. Differences in light-adapted measurements of chlorophyll fluorescence were seen by week 2 in plants grown in 250 and 500 mg/kg TNT, prior to physiological responses. These differences remained throughout the experiment. Dark-adapted measurements were not significantly different until week 5, but were not constant over the experiment. Several reflectance indices were able to detect TNT contamination by week 5 including PRI, Curvature Index and Simple Ratio, but again these differences fluctuated throughout the experiment. Plants exposed to higher concentrations of TNT fluctuated between photochemical and non-photochemical quenching mechanisms for excess energy dissipation indicating differential responses to TNT and the constituent metabolites. Under conditions of a constant leaching of TNT into the soil, as in the case of a leaky landmine, TNT uptake would be continuous in the plants, potentially creating a distinct signature from remotely sensed plants. Thus, we may be able to use remotes sensing to detect TNT soil contamination in plant canopies before visible signs are apparent.