Tracking drought-induced physiological trajectories in spring barley leaves using VIS–NIR spectroscopy across contrasting soil types

Drought stress is a major constraint on crop productivity, yet its impact on plant physiological status remains difficult to monitor non-destructively across contrasting soil conditions. In this study, we investigated drought-induced changes in the optical properties of spring barley (Hordeum vulgare L.) leaves using visible–near infrared (VIS–NIR, 350–2500 nm) spectroscopy in a controlled pot experiment.

Barley was grown on four mineral soils differing in texture and water retention capacity, which imposed contrasting soil water availability conditions. Plants were subjected to early drought stress, late drought stress, combined stress, and a well-watered control. Leaf reflectance spectra were collected repeatedly throughout the experiment using a PSR-3500 spectroradiometer with a leaf clip, allowing the analysis of temporal stress development under different soil settings.

Drought stress induced consistent spectral responses across all soils, including increased reflectance in the visible range associated with chlorophyll degradation, reduced near-infrared reflectance linked to changes in leaf internal structure, and pronounced changes in short-wave infrared water absorption features. Vegetation indices related to greenness and water content (e.g. NDVI, NDWI) declined progressively with increasing stress intensity.

Principal component analysis (PCA) revealed a clear and reproducible trajectory of spectral change corresponding to drought progression (control → early stress → combined stress → late stress). This temporal stress signal dominated the spectral variability, while soil type exerted a secondary influence compared to both drought intensity and measurement timing. Loadings indicated that red-edge dynamics and leaf water absorption bands (970, 1450 and 1940 nm) were the primary contributors to stress discrimination.

The results demonstrate that VIS–NIR spectroscopy provides a sensitive, non-destructive tool for tracking drought-induced physiological responses in barley leaves and for disentangling primary plant stress signals from secondary soil-mediated effects related to soil water availability. This approach offers strong potential for proximal and remote sensing applications aimed at improving drought monitoring and crop resilience assessment under changing climatic conditions.