an index value from −1 to +1 was calculated. This value is indicativeof the shift in the modal value of the histogram of the fluorescenceimage pixels, where lower values represent a shift to the right andhigher values a shift to the left. This shift is significant in that thehue channel represents the colour or wavelength of the light, thescale running from 0 to 255 or red to violet. An index shift to lowervalues is representative of a shift in the histogram toward higherenergy or violet light and indicates lower photosynthetic efficiency,which is interpreted as greater stress. Histograms from the grey-scale images generated in the NIR chamber were divided into 10groups, representing a decrease in water content as the colour valueincreased from 0 to 255. For the analysis only the category with thelowest value, or the highest water content, was considered. In allcases, comparisons of the colour class data for the pixels in RGB,fluorescence, and NIR were made with the untreated control.