Non-destructive assessment of chloride in persimmon leaves using a miniature visible near-infrared spectrometer

Abstract

New methods for the non-destructive, rapid and reliable assessment of leaf chloride (Cl−) are needed in many agricultural salt-threatened areas throughout the World. Although portable visible near-infrared spectrometers could be useful for this task, accompanying models for leaf Cl− assessment on basis their visible near-infrared (Vis-NIR) spectra had yet to be developed. To study the feasibility of such models, a persimmon orchard was grown in E Spain and irrigated with eight waters ranging in Cl− concentration from 70 to 350 mg/L. Then, six times throughout the growing season the Vis-NIR foliar reflectance spectra of various representative leaves per Cl− treatment were recorded with a portable miniature Vis-NIR spectrometer. The leaf Cl− was subsequently determined in the same leaves with a laboratory reference method. Next, both the Cl− and spectral data were analyzed using descriptive statistics, mathematical data transformations, principal components analysis (PCA) and partial least squares regression (PLSR). A wide, sufficiently normal-distributed and representative range of leaf Cl− content from 0.17 to 3.14%Cl− was obtained. With correlation coefficients exceeding +0.65 between 390 and 472 nm and between 690 and 692 nm, the foliar reflectance was the spectral data correlated the most with the leaf Cl−. Remarkably, both wavelengths intervals are where chlorophyll a has its two absorbance maxima in the visible region. A PLSR model including all the wavelengths in the Vis-NIR reflectance spectra plus the day of the year, attained values of R2 = 0.78, RMSE = 0.34%Cl− and RPD = 2.18 in the external validation. However, for leaf Cl− below 1.5%Cl−, i.e., when Cl− assessment is most needed, this model presented R2 = 0.55, but RMSE = 0.08%Cl− and RPD = 5.5, in addition to a bias of +0.04%Cl− and a standard error of 0.05%Cl−. These results show that portable Vis-NIR spectrometers can be useful for the non-destructive, rapid and even reliable assessment of leaf Cl−. The potential of this methodology for other crop plants and inorganic ions should be tested.