A methodical approach for non-destructive estimation of plant pigments by means of remission spectroscopy applied in fruit and vegetable analyses

The biological variability of plant material often leads to perturbations when optical recordings are carried out non-destructively on living horticultural products or even within the photometric analysis of plant extracts. Perturbations are related to varying and coinciding absorption and scattering coefficients. Changing spectral properties appear due to different cultivars, and growing season and also occurs during the development of fruit. An instrumental analysis of varying optical attributes consequently requires accurate methodological specifications. To date, spectral measurements have already been introduced in practice through extensive research in the field of plant spectroscopy and through the recent increase in the availability of low-cost devices. It can be expected that the use of optical sensor systems may contribute to an economic and sustainable use of natural resources as a part of the concept for precision horticulture. In terms of optical phytomonitoring approaches, technologies which address variable amounts of different and thus individual chromophoric plant components, better known as plant pigments, become important. Their wavelength-selective light absorption makes pigments specifically responsive to reflection or transmission recordings in the ultraviolet and visible (UV/VIS) range of the electromagnetic spectrum. Along with their chromophoric attributes, pigments serve as indicators for physiological stages of leaf and fruit. Consequently, the instrumental acquisition of changing pigment contents has high potential with regards to dynamic plant-adapted processes in the production of fruit and vegetables. To date, some applications have been tested that monitor the physiological state of horticultural products along the entire supply chain. This begins with plant-related production control, followed by an estimation of the optimum harvest time, up to fruitdependent adjustments of optimal storage conditions. According to the known issues of non-destructive spectroscopy, a new approach was figured out in the present work for the analysis of strongly coinciding spectral remission signals. The tool developed contributes to a more precise analysis of individual pigment contents, which vary during the cultivation of horticultural crops. Furthermore, the potential optical sensor can be applied without the need for re-calibration for different cultivars and seasonal effects. The tool is based on an iterative algorithm, which separates coinciding pigment spectra from in-situ as well as in-vitro readings from the sum spectrum of individual pigments. Finally, the algorithm was integrated into a stand-alone application containing a library of chlorophyll a (CHLa) and b (CHLb), as well as signatures of lycopene (LYC), β-carotene (bCAR), α-carotene (aCAR), lutein (LUT) and violaxanthin (VIO). The new approach was initially validated through standardised spectrophotometric analysis of adjusted pigment compositions in comparison with established equation systems for calculating bCAR, LYC and total carotenoids (CARtotal). It was shown that the developed iterative multiple linear regression (iMLR) provides quantitative determinations of chlorophylls (r2=1.00; rmse<8.88%) and high correlation also for the single carotenes LYC (r2=0.99; rmse=5.03%) and bCAR (r2=0.96; rmse=7.38%). In contrast to the other methods, iMLR was capable of determining the xanthophyll LUT (r2=0.98; rmse=20.91%) in highly-spectral