Ability of remotely sensed solar-induced chlorophyll fluorescence (SIF) to serve as a vegetation productivity and stress indicator is impaired by confounding factors, such as varying crop-specific canopy structure, changing solar illumination angles, and SIF-soil optical interactions. This study investigates two normalisation approaches correcting diurnal top-of-canopy SIF observations retrieved from the O2-A absorption feature at 760 nm (F 760 hereafter) of summer barley crops for these confounding effects. Nadir SIF data was acquired over nine breeding experimental plots simultaneously by an airborne imaging spectrometer (HyPlant) and a drone-based highperformance point spectrometer (AirSIF). Ancillary measurements, including leaf pigment contents retrieved from drone hyperspectral imagery, destructively sampled leaf area index (LAI), and leaf water and dry matter contents, were used to test the two normalisation methods that are based on: i) the fluorescence correction vegetation index (FCVI), and ii) three versions of the near-infrared reflectance of vegetation (NIRV). Modelling in the discrete anisotropic radiative transfer (DART) model revealed close matches for NIRv-based approaches when corrected canopy SIF was compared to simulated total chlorophyll fluorescence emitted by leaves (R2 = 0.99). Normalisation with the FCVI also performed acceptably (R2 = 0.93), however, it was sensitive to variations in LAI when compared to leaf emitted chlorophyll fluorescence efficiency. Based on the results modelled in DART, the NIRvH1 normalisation was found to have a superior performance over the other NIRv variations and the FCVI normalisation. Comparison of the SIF escape fractions suggests that the escape fraction estimated with NIRvH1 matched escape fraction extracted from DART more closely. When applied to the experimental drone and airborne nadir canopy SIF data, the agreement between NIRvH1 and FCVI produced chlorophyll fluorescence efficiency was very high (R2 = 0.93). Nevertheless, NIRvH1 showed higher uncertainties for areas with low vegetation cover indicating an unaccounted contribution of SIF-soil interactions. The diurnal courses of chlorophyll fluorescence efficiency for both approaches differed not significantly from simple normalisation by incoming and apparent photosynthetically active radiation. In conclusion, SIF normalisation with NIRvH1 more accurately compensates the effects of canopy structure on top of canopy far red SIF, but when applied to top of canopy in-situ data of spring barley, the effects of NIRvH1 and FCVI on the diurnal course of SIF had a similar influence.

Cadmium (Cd) toxicity poses a significant threat to soil health and sustainable food production. Its bioaccumulation in plant tissues induces phytotoxicity by affecting physiological and biochemical attributes, leading to a reduction in plant biomass and production. Recently, nanotechnology has emerged as a promising approach for addressing heavy metal toxicity in an eco-friendly manner to enhance crop production. However, the comparative role of foliar applied calcium oxide nanoparticles (CaO-NPs) and bulk calcium fertilizer under Cd stress in alfalfa remains unexplored. Herein, we studied the ameliorative role of CaO-NPs and bulk calcium (50 and 100 mg L- 1) to alleviate Cd stress (30 mg kg- 1) in alfalfa seedlings. Plants exposed to Cd exhibited significant decreases in morpho-physiological traits, gas exchange attributes, and pigment contents as well as increase in Cd bioaccumulation in plant tissues. Notably, exogenous application of CaO-NPs ameliorates the toxic impact of Cd by enhancing plant biomass (45%), fluorescence efficiency and gaseous exchange attributes. The maximum dose of CaO-NPs induced Cd-tolerance response accompanied by a significant increase in antioxidative enzyme activities, such as superoxide dismutase (SOD; 29%), peroxidase (POD; 41%), catalase (CAT; 36%) and ascorbate peroxidase (APX; 49%), which play positive roles in ROS scavenging. TEM examination further revealed the protective role of these NPs in averting Cd-induced damage to leaf ultrastructure and mesophyll cells. Furthermore, CaO-NPs had a substantial influence on both Cd and Ca2+ accumulation in plant tissues, while qRT-PCR analysis demonstrated higher expression of antioxidant defense genes viz. Cu/ZnSOD (0.38 fold change (FC)), MtPOD (0.51 FC), MtCAT (0.61 FC) and MtAPX (0.79 FC) under CaO-NPs application, over Cd control. Overall, our findings suggested that exogenous CaO-NPs could be effective in alleviating the adverse effects of Cd on alfalfa seedlings to ensure food safety and support sustainable agriculture.