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Digestion, nutrition & metabolism

Spectrometric prediction of secondary metabolites and nitrogen in fresh Eucalyptus foliage: towards remote sensing of the nutritional quality of foliage for leaf-eating marsupials

Ebbers, MJH, Wallis, IR, Dury, S, Floyd, R & Foley, WJ  2002, Australian Journal of Botany, vol. 50, no. 1, pp. 761-768.

Near-infrared reflectance spectroscopy was used to obtain spectra from whole fresh and dry leaves for E. melliodora and E. globulus, two common Eucalyptus species, to determine concentration of total nitrogen, 1,8 cineole (a terpene) and sideroxylonal A (a phenolic antifeedant compound). This technique was successful, with the water absorbance peaks not obscuring the peaks on 1,8 cineole’s spectrum and obscuring some non-vital peaks on nitrogen and sideroxylonal A’s spectra, meaning that their concentrations could be reliably predicted. The concentrations for E. globulus were less accurate than for E. melliodora, likely due to wax on the leaf surface interfering in the experiment.

  Spectra were taken of the pure compounds to allow comparisons to the spectra obtained from whole fresh and freeze-dried leaves. The peaks in cineole did not overlap with the expected water peak area, but a prominent sideroxylonal A peak overlapped. The fresh and freeze-dried leaf spectra were compared against each other to confirm that no significant differences arose due to preservation, and were found to be very similar. A partial least squares regression was the most successful approach for modelling cineole, sideroxylonal and nitrogen concentrations from the spectral data. The difference between fresh and freeze-dried leaves for E. globulus was similar to E. melliodora; however, in E. globulus, the cineole content ranged from 7.5-24.6 mg/g in adult leaf samples to 15.1-48.2 mg/g in juvenile leaf samples, as E. globulus has morphologically different adult and juvenile leaves. The model was slightly less accurate for E. globulus in both cineole and nitrogen concentrations, likely due to less samples and waxes on juvenile leaves interfering with the spectrometer reading.

  The current process for spectroscopy of Eucalyptus leaves is to cut the foliage from trees, dry and grind it, then manually collect spectra. Drying reduces the terpene concentration, eliminating data useful for understanding koala diet selection, which the method described here avoids. The current process is also time and labour-intensive, and cannot be applied for landscape scale analysis, whereas fresh leaf spectroscopy could allow aerial measurements of tree crowns. Other chemical data had previously been collected remotely, but not plant secondary metabolites. There is no clear consensus on whether fresh leaves can be reliably used for spectra collection, so this experiment gives more evidence that it can be used successfully.

  By confirming that near-infrared reflectance spectroscopy is successful on whole fresh leaves rather than just dried, ground samples, the possibility is raised of portable and/or airborne spectrophotometry, rather than painstaking collection and sample preparation. This would greatly speed up spectra collection, allowing larger areas to be surveyed and therefore a greater understanding of secondary metabolite variation in Eucalyptus.


Summarised by Laura Wait


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