Mapping stressed wheat plants by soil aluminum effect using C-band SAR images: implications for plant growth and grain quality

Abstract

Under toxic aluminum (Al) levels in the soil, wheat (Triticum aestivum L.) suffers stress and plant growth is affected. A method for diagnosis of plants is proposed that includes the following as a strategy: to analyze total Al in the soil, employ satellite radar imagery and calculate a vegetation index. The objective of this research, conducted at the field scale, was to explore how radar backscattering coefficients from a winter wheat canopy, combined with the normalized difference vegetation index (NDVI) and geographic information system (GIS) technology, can be used as a mapping tool for the variability of Al-stressed canopies. As a result, an analysis of covariance showed significant differences, and the lowest plant height was obtained at a high level of soil Al, as well as the minimum grain weight and magnesium content. It was found that a simple model could be used to estimate plant height from the backscattering coefficient of vertical transmit-vertical receive polarization (σ°VV), with a strong correlation (r − 0.84). In turn, a third-order polynomial regression model (R2 0.70) was proposed to estimate the NDVI from σ°VV. This model provided a good estimate of the NDVI at the stem elongation stage of growth (50 days after sowing). Detected NDVI patterns were associated with variation in canopy stress depending on polarimetric information, which, in turn, was related to soil Al levels. Thus, the maps derived from the model can monitor spatial variability, where NDVI values < 0.68 indicate stressed areas. This study provides guidance for in-season stress spatial variability caused by Al.