Quantification of chlorophyll and carotenoid pigments in eucalyptus foliage with the radiative transfer model PROSPECT 5 is affected by anthocyanin and epicuticular waxes
Barry, KM and Newnham, GJ, Quantification of chlorophyll and carotenoid pigments in eucalyptus foliage with the radiative transfer model PROSPECT 5 is affected by anthocyanin and epicuticular waxes, Proceedings of the Geospatial Science Research Symposium - GSR_2, 10-12 December 2012, RMIT University, Melbourne, Victoria, pp. 1-7. ISBN 978-0-9872527-1-5 (2012) [Refereed Conference Paper]
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. We have found that a leaf radiative transfer model (PROSPECT 5) can accurately fit chlorophyll content in Eucalyptus globulus leaves but prediction of carotenoids is 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. 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 fit of measured to modeled chlorophylls was best for leaves of plants in the "shaded" treatment, (r2=0.84) where anthocyanins were 3-fold lower that the "sun" treatments. The fit was also improved for "sun" treatment plants when wax was removed from leaves (r2=0.78). This highlights the impact of anthocyanins and wax on performance of PROSPECT5 to predict chlorophyll in eucalypts. Surprisingly, the fit between measured and modeled carotenoid concentrations was not greatly improved - the best fit achieved was for the treatment with wax removed (r2=0.44). 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. Comparison of laboratory methods to quantify carotenoids (UV spectroscopy versus ultra-high performance liquid chromatography with mass spectroscopy) and several method improvements resulted in a strong fit between the methods (carotenoids, r=0.99; chlorophyllab, r=0.97). We conclude that the presence of waxes in eucalypts leaves is the main interference with the performance of 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.