How do the Physiological and Spectral Behaviors of Eucalyptus Seedlings Respond to Different Water Restriction Conditions?

The use of remote sensing data from hyperspectral sensors offers advantages over other plant phenotyping methodologies. This technique enables non-destructive, largescale data acquisition, applicable not only in basic research but also in plant breeding. The objectives of this study were to: (I) evaluate the physiological behavior (photosynthetic rate, transpiration rate, stomatal conductance, and instantaneous water-use efficiency) of eucalyptus seedlings subjected to different irrigation volumes and frequencies; (II) assess spectral behavior using Reflectance Inflection Difference (RID); and (III) jointly relate physiological and spectral behaviors to irrigation volumes and frequencies. The experiment was conducted in 2023 at the Federal University of Mato Grosso do Sul (UFMS), Brazil, under controlled environmental conditions. The experimental design was a strip-plot arrangement, Irrigations were performed with fixed volumes of two (V2) and four (V4) liters per application, at irrigation frequencies of 1, 2, 4, and 8 days (IF1, IF2, IF4, and IF8, respectively), with four replicates per treatment. Following the irrigation period, physiological, spectral, and leaf pigment assessments were performed. Plants maintained stable carbon assimilation (A) despite a significant drop in stomatal conductance (Gs) and transpiration (E) under V2. This demonstrates early adaptive resilience and high water use efficiency (WUE). This confirms that an adequate water supply is essential to maximize the crop’s initial physiological performance. Conversely, under lower water availability, seedlings exhibited higher water-use efficiency, indicating the activation of adaptive mechanisms that maintain carbon assimilation with reduced water loss. The spectral responses were responsive to imposed water conditions, particularly in the NIR and SWIR regions, which were consistently associated with plant tissue water content and the degree of water restriction conditions. Our findings demonstrate that proper water management is critical to the initial physiological performance of eucalyptus seedlings and highlight the potential of spectral sensing as a practical tool for monitoring water status in forest nurseries.