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Herbicides inhibit plant growth by affecting different bio-chemical pathways or bio-physical states. Stress that was induced by inhibiting lipid metabolism or photosynthesis on corn and sunflower was detected using leaf spectral reflectance. Leaf spectral signatures were measured at six time points during two weeks by a spectro-radiometer. Phenotypes were evaluated by an expert at the same time points. Spectral processing models were used to compare the spectral response of a control group to treated plants using statistical T-tests. Results revealed that spectral reflectance can be used to detect the stress induced by inhibiting photosynthesis on corn and sunflower at the same time that the first visual symptoms appeared. Also, early detection can be accomplished with a lower confidence two days before visual symptoms appear.