High spectral resolution spectrometers were used to detect optical signals of ongoing plant stress in potted white clover canopies subjected to ozone fumigation. The case of ozone stress is used in this manuscript as a paradigm of oxidative stress. Steady-state fluorescence (Fs) and the Photochemical Reflectance Index (PRI) were investigated as advanced hyperspectral remote sensing techniques able to sense variations in the excess energy dissipation pathways occurring when photosynthesis declines in plants exposed to a stress agent. Fs and PRI were monitored in control and ozone fumigated canopies during a 21-day experiment together with the traditional Normalized Difference Vegetation Index (NDVI) and physiological measurements commonly employed by physiologists to describe stress development (i.e. net CO2 assimilation, active fluorimetry, chlorophyll concentration and visible injuries). It is shown that remote detection of an ongoing stress through Fs and PRI can be achieved in an early phase, characterized by the decline of photosynthesis. On the contrary, NDVI was able to detect the stress only when damage occurred. These results open up new possibilities for assessment of plant stress by means of hyperspectral remote sensing.