Landmines: Plants as bio-indicators to what may lie beneath

Background/Question/Methods

Vegetative ecological processes plays an important role in the movement of explosives through soil. Vegetation near a buried landmine may uptake explosive compounds via bulk water movement into roots. Explosives accumulate in leaf tissues which may lead to physiological stress. Our objective was to determine the potential for using plants as bio-indicators to locate buried explosives. We used optical methods to detect stress induced by the presence of a widely used explosive compound. Plants were potted in soil contaminated with 500 mg kg^-1 of Composition B (Comp. B; 60/40 mixture of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT)). Leaf fluorescence and hyperspectral reflectance measurements were taken over a nine week period for three species of different functional types: Ulmus alata (tree), Vitis rotundifolia (vine), and Cyperus esculentus (sedge). These measurements were then analyzed within and across species using standard reflectance indices and related to plant functional traits (pigment content, specific leaf area (SLA), and biomass).

Results/Conclusions

Composition B induced changes in pigment content, SLA, and biomass. These changes were also observed in plant reflectance. At week five, C. esculentus showed significant differences relative to reference plants only in Modified Chlorophyll Absorption Ratio Index (MCARI) and at week 9 showed a difference in R₇₆₁ – R₇₅₇ index, where R represents reflectance and subscript denotes specific wavelength. Woody species U. alata and V. rotundifolia both had more indices with significant differences representing changes in fluorescence, chlorophyll content, and specific leaf area including the Photochemical Reflectance Index (PRI), and the Normalized Difference Vegetation Index (NDVI), and R₇₅₀ – R₇₁₀, R₇₄₀ – R₈₅₀,. By week nine, only two indices, representing chlorophyll fluorescence inidacted plant stress. This is most likely due to leaf drop and subsequent regrowth. As plants absorbed Comp. B, leaves became necrotic and eventually senesced with new leaves emerging shortly thereafter. By monitoring chemical changes in pigments, fluorescence, and structural changes within leaves due to uptake of explosive compounds, plants may be useful in remote detection of landmines. Further research is necessary to pinpoint specific responses of plants to the exposure of Comp. B over extended periods of time.