Pixel-level hyperspectral metrics for chlorophyll fluorescence and pigment assays in zucchini leaves across fertilisation treatments

Plant stress can arise before it is visible while fertilizers such as compost, widely used in nursery substrates, can promote growth but may also induce unfavorable physiological changes. In this context, indicators for non-destructive tests based on hyperspectral imaging (HSI) can be used to detect changes and correlate them with destructive analyses. This study evaluated whether a series of HSI indices and descriptors might capture a compost dose-dependent response: a set of 21 zucchini plants grown under three compost treatments (0, 5, and 10% v:v) were employed to collect leaf hyperspectral images and verify their consistency with laboratory destructive tests. Homogeneous leaf areas have been isolated through a region-growing algorithm at pixel level to obtain 4 regions of interest (ROI) for each sample, and then processed to reduce noise and bias; from each ROI, spectral indices from scientific literature and metrics have been calculated and compared to laboratory data for the estimation of Spearman correlations across treatments. Results showed that spectral patterns were largely non-linear, since at 5% compost doses most spectral metrics indicated an improved physiological configuration and showed consistent associations with photochemical and pigment laboratory measurements. Metrics based on visible-range spectral integrals were the most informative for distinguishing physiological patterns across compost doses (ρ = +0.536 at 0%, ρ = −0.286 at 5%, ρ = +0.643 at 10%), together with metrics based on green region (ρ = +0.714 at 0%, ρ = −0.214 at 5%, ρ = +0.857 at 10%). In addition, at 10% compost several descriptors shifted in magnitude, aligning with laboratory results in identifying stress symptoms or unfavorable leaf conditions, with ρ values up to 0.714 for pigment-based window metrics. These findings support HSI to track compost dose–response in leaves without destructive tests, providing a basis to guide nutrient management and compost optimization in other plants.