Linking Tissue Damage to Hyperspectral Reflectance for Non-Invasive Monitoring of Apple Fruit in Orchards

Alexei Solovchenko,Igor Smirnov,Vitaly Velichko,Andrey Kuzin,Alexei Dorokhov,Boris Shurygin,Dmitriy Khort,Aleksandr Aksenov,Alexandr Nikolenko

doi:10.3390/plants10020310

Abstract

Reflected light carries ample information about the biochemical composition, tissue architecture, and physiological condition of plants. Recent technical progress has paved the way for affordable imaging hyperspectrometers (IH) providing spatially resolved spectral information on plants on different levels, from individual plant organs to communities. The extraction of sensible information from hyperspectral images is difficult due to inherent complexity of plant tissue and canopy optics, especially when recorded under ambient sunlight. We report on the changes in hyperspectral reflectance accompanying the accumulation of anthocyanins in healthy apple (cultivars Ligol, Gala, Golden Delicious) fruits as well as in fruits affected by pigment breakdown during sunscald development and phytopathogen attacks. The measurements made outdoors with a snapshot IH were compared with traditional “point-type” reflectance measured with a spectrophotometer under controlled illumination conditions. The spectra captured by the IH were suitable for processing using the approaches previously developed for “point-type” apple fruit and leaf reflectance spectra. The validity of this approach was tested by constructing a novel index mBRI (modified browning reflectance index) for detection of tissue damages on the background of the anthocyanin absorption. The index was suggested in the form of mBRI = (R640−1 + R800−1) − R678−1. Difficulties of the interpretation of fruit hyperspectral reflectance images recorded in situ are discussed with possible implications for plant physiology and precision horticulture practices.

Highlights

Light reflected by plants carries ample information about its biochemical composition, tissue architecture, and physiological condition
Fruits bearing no visual symptoms of damage were selected as well as fruits affected to a different extent by physiological disorders commonly encountered in orchards and phytopathogen lesions (Figure 1a)
The obtained findings allowed us to bridge the gap between point-type and field imaging-based measurement approaches, at least for the detection of damages associated with tissue browning

Summary

Introduction

Light reflected by plants carries ample information about its biochemical composition, tissue architecture, and physiological condition. Developmental changes in pigment composition as well as those induced by the environmental stresses and attacks of phytopathogens, manifest themselves as specific changes in plant reflection properties [1,2,3,4,5]. Chloroplasts of mature apple peel cells possess photosynthetic pigments including chlorophylls a, b and carotenoids [4,6], so these cells are capable of photosynthesizing at a rate close to that documented in leaves [7]. Carotenoids are important for light harvesting and for photoprotection [8]. Anthocyanins responsible for the red color of apples increase their consumer acceptance [9] serving as a protection against excessive solar radiation [10,11]. Non-destructive assessment of pigments in plants in general as well as in apple fruits is complicated by overlapping absorption spectra of individual pigments and the non-linear relationship of reflectance vs. pigment content [2,12]

Methods

Results

Conclusion