Two methods for the analysis of colorimetric components applied to plant stress monitoring

Several authors have shown the possibility of early detection of many stress conditions in plants by analysing leaf colour. The availability of portable instruments, increasingly accurate and reliable, leads to the belief that, in the near future, this technique could be used for routine analysis in the field. Nevertheless, due to the variability of the leaf colour parameter and consequently to the large amount of data to be collected to obtain a representative sample, direct analysis of chromaticity coordinate values, as generated by an instrument, could give results that are hard to interpret, especially when the colour variation to be detected is subtle, such as in leaves at the first stages of stress. The availability of standard methodologies to analyse colorimetric data would allow them to be interpreted correctly and, in a short time, to widen their fields of application. In this work two methods of elaboration, with related software (PCA and COLORE) that give an objective evaluation of colorimetric variations of stressed plants by means of specific indexes of alteration, are illustrated. PCA, based on principal components analysis, allows two `indexes', Tij and D, to be calculated, that can describe the differences between two or more treatments. It also allows the location of the different treatments on the chromaticity diagram to be graphically represented. COLORE calculates the values of the dominant wavelength and purity of the colour. From these, it computes the CI index that can be related to degree of stress. The two softwares were applied to data from different studies of thermal and water stress in Sorghum bicolor ((L.) Moench) and Citrus macrophylla (L). The results showed a clear correlation between the colorimetric indexes and the ecophysiological parameters chosen as indicators of stress.