Quantification of chlorophyll and carotenoid pigments in eucalyptus foliage with the radiative transfer model PROSPECT 5 is affected by anthocyanin and epicuticular waxes

Precision monitoring of plant health via remote sensing benefits from accurate quantification of attributes such as foliar pigments related to stress. For example, pigment content (the ratio of chlorophyll to carotenoids) can be indirectly related to photosynthetic efficiency in plants and hence productivity. Optical remote sensing can potentially provide this data with high spatial accuracy via spectral reflectance. In a previous study we found that the leaf radiative transfer model PROSPECT 5 could estimate chlorophyll content in Eucalyptus globulus leaves reasonably well but prediction of carotenoids was poor. To examine the basis for poor model performance, we conducted a targeted study using both a pot experiment with E. globulus and field samples of E. globulus and E. nitens. We examined the influence of anthocyanins and waxes on the performance of the model by manipulating these variables with shade, nutrition and a wax removal treatment. The relationship between measured to modelled leaf chlorophyll concentration was strongest when plants were shaded then exposed to several days sun before assessment, (r²=0.88), in which anthocyanins were 3-fold lower than the “sun” treatments. The fit was also improved for “sun” treatment plants when wax was removed from leaves (r²=0.77). This highlights the impact of anthocyanins and wax on performance of PROSPECT5 to predict chlorophyll in eucalypts. Surprisingly, the fit between measured and modelled carotenoid concentrations was not greatly improved by any of the treatments - the best fit achieved was for the treatment with wax removed (r²=0.42). For the adult leaves collected in the field, we investigated the impact of anthocyanin content on the fit of measured to modeled carotenoids, but results were not greatly improved when using only leaves with low anthocyanin content. During this study we refined our UV spectroscopy methods for both chlorophyll and carotenoid estimation, and these refinements were validated by analysis of the same samples with ultra-high performance liquid chromatography. After method refinement, data generated from the two methods were closely related (carotenoids, r²=0.99; chlorophyll, r²=0.97) and method problems were not contributing to poor relationships between measured pigments and those modelled by PROSPECT5. We conclude that the presence of waxes in eucalypts leaves is the main source of error in the estimation of pigment concentrations using PROSPECT 5. These results suggest that application of PROSPECT 5 to eucalypt canopies across different seasons may be problematic, particularly for quantification of carotenoids, as both anthocyanins and waxes can change with season, ontology and stress. However, these could be accounted for in future versions of the model.