# Quantifying Nutrient Content in the Leaves of Cowpea Using Remote Sensing
## 3 Results and Discussion
### Spectral Reflectance  
The spectral properties of a leaf are a function of its chemical composition, morphology and internal structure [52]. Nutrient functions in plant physiology influence the spectral reflectance of the leaves. Each phenological stage of the crop was registered by spectral reflectance curves typical of healthy green vegetation [53], with reflectance values that differed only slightly (Figure 5). The reduction in plant nutrient content promotes greater leaf reflectance by significantly affecting the optical properties of the leaf [54]. A small difference in leaf nutrient values, which are lower at the R9 stage, reveals a slight change in reflectance. In the visible region, the vegetation presented, in the three phenological stages studied, lower reflectance factors when compared to the infrared region, with absorption valleys observed in the blue (490 nm) and red (680 nm). The study conducted by [55] is in accord with this result, affirming that the strong light absorption by pigments dominates the properties of the green leaf in the visible spectrum. Pigments that interfere heavily in reflectance are chlorophyll, caroten and xanthophylls. In the visible region, there is an interaction between radiant energy and the foliar structure by means of absorption and spread, where it is selectively absorbed by the chlorophyll and converted into heat or fluorescence, being photochemically stored in the form of organic components [56]. Figure 4: Nutrient content: phosphorus (**a**), potassium (**b**), zinc (**c**) and calcium (**d**) in the leaves of _Vigna unguiculata_ (L.) [PERSON]. Different letters indicate a statistical difference by [PERSON]’s test at 5% probability. At stage R9, there is an increase in reflectance in the visible region (Figure 5C) of approximately 20% compared to the other stages. This is due to the senescence of the plants, which have lower levels of chlorophyll and reduce radiation absorption by these pigments [57]. Similar results were obtained by [58] in the common bean, where the reflectance peak near 550 nm increased as the crop reached the end of the cycle. In the near-infrared (700 to 1000 nm), there is a small absorption of radiation; on the other hand, there is large internal radiation spread in the leaf, in which mesophyll structures have a major influence on reflectance [59]. Water availability is a factor that can also interfere in leaf reflectance in this region because it can cause alterations in the water--air relationship in the mesophyll. Reflectance peaks can be seen in the near-infrared region during each phenological stage of the study, caused by low EMR absorption (970 and 1180 nm) and spread within the foliar structures [60]. In this region, the mean reflectance was 0.5 (Figure 5). In the NIR, no great variations in reflectance were observed in the plants in the different phenological stages, considering that throughout the entire cycle, the crop presented no water or nutritional stress that might have caused alterations in the cellular structure of the cowpea leaf. Figure 5: Mean reflectance spectra and respective standard deviations for each phenological stage: (**A**) V4, (**B**) R6 and (**C**) R9. In the SWIR (1000 to 2400 nm), crop reflectance is more influenced by water [61]. A reduction in water quantity leads to a degradation of proteins and photosynthesis pigments, which restricts the absorption of electromagnetic radiation and, consequently, results in increased reflectance in this region. At the wavelengths of 1450 and 1930 nm, well-marked valleys can be observed, indicating the presence of water in the leaf (Figure 5). One can see that the valleys at 1450 and 1930 nm are quite accentuated, which indicates the presence of water.