Nondestructive determination of carotenoids in kiwifruit leaves infected with Pseudomonas syringae pv. actinidiae by surface‐enhanced Raman spectroscopy combined with chemical imaging

Abstract

AbstractSurface‐enhanced Raman spectroscopy (SERS), a noninvasive and nondestructive technique, was used to distinguish healthy and cankered kiwifruit leaves. Silver nanoparticles with iodide and calcium ions (Ag@ICNPs) were synthesized as SERS substrates and significantly enhanced the characteristic signal peaks of carotenoids. Due to its simple operation and high sensitivity, the detection process is straightforward and efficient. Using a 532 nm laser excitation source, spectra were collected from the leaf surface over a wavelength range of 400–2000 cm−1, and the characteristic carotenoid peak at 1520 cm−1 was selected for integral imaging. Detection of changes in carotenoid spectral characteristics on the surface of kiwifruit leaves could be used as an early warning of kiwifruit canker. Raman spectroscopy and high‐performance liquid chromatography confirmed the existence of two main carotenoids, lutein and β‐carotene, and their intensity changed significantly following infection, especially in the later stages of infection. The results of principal component analysis and support vector machine (SVM) analysis of preprocessed spectra showed that samples of healthy, early disease and advanced disease leaves clustered separately, and the accuracy of SVM classification was up to 98%. The results of chemical imaging showed that the average carotenoid signal decreased with extended incubation time and there were significant differences among healthy, early disease and advanced disease stages. Therefore, SERS can be used as a general tool for in situ nondestructive determination of surface carotenoid changes in kiwifruit leaves during disease, and thereby effectively monitor the nutritional quality of kiwifruit before harvest.