*4.4. Physiological Differences in Plants*

N synthesis is influenced by canopy structure, photosynthesis-related pigments, and water content, etc., and confounding effects are common in canopy reflectance under physiological stress, making N remote sensing monitoring challenging.

Water is the carrier of N transport in the crop and is closely related to the N status of the canopy. Exploring the spectral response to N and water shows that the spectral reflectance of wheat treated under low water conditions increases in both regions of SWIR, with less difference in the NIR region under water differences [27,59]. Thus, indices constructed based on SWIR and NIR reflectance are better able to show differences between N levels in different water treatment environments. The SWIR region contains more water-related information, but sensors that include SWIR are costly so it is relevant to explore watersensitive bands in the VIS–NIR range. Under the condition of constant N content, the red edge reflectance of crops with different water treatments tends to be the same [128]. Based on the red-edge correlation indices such as NDRE and normalized pigment chlorophyll index (NPCI), the introduction of water-related indices, such as floating-position water band index (FWBI), crop water stress index (CWSI), etc., can significantly improve the interaction between water deficit and N nutrition [128,175–177]. Whether the multi-analysis based on specific VIs or the whole spectrum, studies have been conducted to separate N and water information in the spectrum by reducing spectral mixing effects, thus improving the estimation accuracy of crop N under the influence of water.

Crop growth parameters are not independent of each other and may correlate under different circumstances, such as correlation between N, Chl and LAI when the canopy cover is small. However, when Chl and LAI change driven by other external conditions, there will be errors in estimating N status based on their correlation with N. Research needs indices that are both sensitive to N and resistant to interference from other factors. The combination of two VIs with different sensitivities to Chl and LAI, whose ratios can minimize the effect of LAI and have a better correlation with Chl, such as the joint indices modified chlorophyll absorption ratio index and second modified triangular vegetation index in ratio (MCARI/MTVI2) [131], the red-edge-chlorophyll absorption index and the triangular vegetation index in ratio (RECAI/TVI) [111], the transformed chlorophyll absorption reflectance index and optimized soil-adjusted vegetation index in ratio (TCARI/OSAVI) [177]. The red edge region is influenced by LAI which cannot estimate N well in complex situations. Chen et al. [127] found that there is a double-peaked phenomenon in the first-order derivative spectrum of the red edge region, and that changes in N concentration can be amplified as changes in the relative height of the double-peaked peaks, with which the proposed double-peaked canopy nitrogen index (DCNI) can overcome the influence of LAI. There is a certain similarity between different spectral indices and different physiological parameters, which show hierarchy and aggregation in statistical analyses. Exploring the different

relationships that may exist between parameters is therefore important for exploring N status in crops grown under different growing conditions.
