Abstract
Epidermal structures (ES) of leaves are known to affect the functional properties and spectral responses. Spectral studies focused mostly on the effect of hairs or wax layers only. We studied a wider range of different ES and their impact on spectral properties. Additionally, we identified spectral regions that allow distinguishing different ES. We used a field spectrometer to measure ex situ leaf spectral responses from 350 nm–2500 nm. A spectral library for 25 species of the succulent family Aizoaceae was assembled. Five functional types were defined based on ES: flat epidermal cell surface, convex to papillary epidermal cell surface, bladder cells, hairs and wax cover. We tested the separability of ES using partial least squares discriminant analysis (PLS-DA) based on the spectral data. Subsequently, variable importance (VIP) was calculated to identify spectral regions relevant for discriminating our functional types (classes). Classification performance was high, with a kappa value of 0.9 indicating well-separable spectral classes. VIP calculations identified six spectral regions of increased importance for the classification. We confirmed and extended previous findings regarding the visible-near-infrared spectral region. Our experiments also confirmed that epidermal leaf traits can be classified due to clearly distinguishable spectral signatures across species and genera within the Aizoaceae.
Highlights
The leaf epidermal anatomy, together with its biochemical and biophysical properties, strongly controls the functional properties and the adaptive fitness of plants
Out of twenty method combinations, we found that a cross-validated partial least squares discriminant analysis (PLS-DA) without a preceding Standard normal variate (SNV) transformation, applied on a full spectrum, delivered a Kappa value of 0.90 (Table 2), indicating an almost perfect concordance of the model prediction and the observed classes [50]
We demonstrated that five different functional types of epidermal leaf structures sampled across 25 species from a single plant family could be distinguished based on leaf spectral signatures
Summary
The leaf epidermal anatomy, together with its biochemical and biophysical properties, strongly controls the functional properties and the adaptive fitness of plants. Optically-measured spectral signatures of leaves are commonly used for analyzing these properties. Spectral signatures were used to measure biochemical components, such as pigment concentration [1,2] nutrient content [3] or other biochemicals, such as tannins [4,5]. Cellular leaf structure [6] or leaf water content [7] are typical biophysical properties of leaves that can be captured by spectral signatures. Epidermal structures (ES) are known to affect the spectral signatures of leaves [8,9,10], they were mostly neglected in recent studies of leaf optical properties
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