Winter Wheat Genotype Effect on Canopy Reflectance: Implications for Using NDVI for In‐Season Nitrogen Topdressing Recommendations

Abstract

Active optical sensors (AOSs) measure crop refl ectance at specifi c wavelengths and calculate vegetation indices (VIs) that are used to prescribe variable N fertilization. Visual observations of winter wheat (Triticum aestivum L.) plant greenness and density suggest that VI values may be genotype specifi c. Some sensor systems use correction coeffi cients to eliminate the eff ect of genotype on VI values. Th is study was conducted to assess the eff ects of winter wheat cultivars and growing conditions on canopy refl ectance, as measured by red or amber normalized diff erence vegetative indices (NDVIs) derived from AOSs. Variations in NDVI values among three wheat cultivars were measured at three growth stages (Zadoks 31, 37, and 65) during 3 yr at three sites in Poland. GreenSeeker Model 505 and Crop Circle ACS-210 sensors were utilized to measure red and amber NDVIs, respectively. Signifi cant (p < 0.05) diff erences in both forms of NDVI associated with wheat genotypes were observed across years and sites at Zadoks 31, the time when canopy sensing and N fertilization decisions are oft en made. Lack of a genotype ´ site interaction for both red and amber NDVIs and the presence of a signifi cant genotype ´ year interaction for both VIs suggested that (i) canopy greenness and density of the same genotype measured at the same growth stage are likely to be stable across diff erent growing conditions, and (ii) NDVI values for a particular genotype tend to vary more across years than across sites. Because developing temporally variable correction coeffi cients is not practical, we strongly recommend that an in situ calibration (based on in-fi eld or a virtual reference strip) is utilized to normalize NDVI across genotypes, years, and sites.

Assessing wheat spatial variation based on proximal and remote spectral vegetation indices and soil properties

“…One of the most diffused is the assessment of the normalised difference vegetation index (NDVI), which is the ratio between the difference and the sum of canopy reflectance in the near infra-red (NIR) (780 nm) and red (670 nm), providing an indication of crop health and vigour (Tucker, 1979). The NDVI has been widely used in the wheat crop to support biomass and grain yield predictions (Erdle et al, 2011;Grohs et al, 2011;Quebrajo et al, 2015;Bushong et al, 2016), assess optimum N fertiliser doses (Macnack et al, 2014;Calvo et al, 2015;Samborski et al, 2015), and for other uses as mapping crop areas (Patil et al, 2010;Jin et al, 2016), and estimating water requirements (Chattaraj et al, 2013;Ozcan et al, 2014).…”

“…The NDVI is still at present the reference spectral vegetation index, although limits have been evidenced compared to other indices operating in the same wavebands (Erdle et al, 2011;Samborski et al, 2015). Sometimes, better estimates of grain yield and quality were obtained when NDVI was integrated by environmental parameters as soil moisture, precipitation and cumulated heat during wheat growth (Macnack et al, 2014;Bushong et al, 2016).…”

Sensors for measuring plant phenotyping: A review

“…But plant height, measuring distance, temperature, and reflectance from soil or adjacent rows affect the performance of active sensors, the optimal measuring distance should be adjusted depending on plant architecture and the growth stage, and the distance of sensors beyond 40 cm from the canopy is appropriate [88][89][90][91] . In addition, reflectance indices were less sensitive at the late growth stages of plants with the reduction in the NIR reflectance from canopy [83,87] . Among all of the active-type and passive-type spectral sensors, GreenSeeker, CropCircle, and N-sensor are the most commonly used for on-the-go in real-time measurement of plant chlorophyll, all of them could be mounted on a platform and suitable for high-throughput phenotyping.…”

“…Conversely, GreenSeeker, CropCircle and other devices equipped with active spectral sensors have been widely used in agriculture, because they can tolerate severe environments. The active sensors usually emit amber or red and NIR wavebands light due to their link to chlorophyll [83] and record the reflectance of several wavebands to calculate some vegetation indices to measure chlorophyll, primarily NDVI. GreenSeeker and CropCircle were used by Barker [84] mounted on field-based phenotyping platforms to measure plant chlorophyll, the change in NDVI from night to the brightest noon was 0.046 for GreenSeeker and 0.0013 for CropCircle, which indicated that they were all not significantly affected by ambient light.…”

“…They also used RGB image analysis as a reference method and proposed a novel index, the early plant vigor index (EPVI), using single wavelengths values (670 nm, 750 nm and 862 nm) to evaluate early plant vigor. GreenSeeker Model 505 (red at 656 nm and NIR at 774 nm) and CropCircle ACS-210 (amber at 590 nm and NIR at 880 nm) were used to collect red and amber canopy NDVI values of winter wheat at three growth stages by Samborski et al [83] , they found that genotype has an effect on both red NDVI values and amber NDVI values at Zadoks 37 to 39 growth stages, and only on amber NDVI values at 55 to 71 growth stages. CropCircle ACS-210 and ACS 430 (red at 630 nm, red-edge at 730 nm and NIR at 780 nm) were compared by Taskos et al [86] , different NDVI values were calculated and analyzed in each individual waveband.…”