Short-Term Changes in Spectral Properties of Phyllodes of Brigalow (Acacia harpophylla F Muell Ex Benth) in Response to Wetting

Abstract

The phyllodes of Acacia harpophylla (brigalow) possess a high reflectivity (0.20) in the visible wavelengths due to the presence of a dense covering of flattened trichomes. This results in a low value for phyllode absorptivity (0.79). While dry phyllodes appear grey-blue to the eye, wet phyllodes appear green and their reflection spectra are similar to those of 'typical' foliage elements. When wet, reflectivity in the visible wavelengths is reduced (0.09) and absorptivity is increased by 14%. Examination of the wetting and drying process, both visually and by measuring the time course of reflectivity, illustrates the importance of the wettable surfaces of both the epidermis and the individual trichomes. On drying, the reflectivity increase in the near infrared (NIR, 0.41-0.59) precedes that in the visible portion of the spectrum, although changes in the visible portion of the spectrum are larger than those in the NIR.