On the use of spectral reflectance indices to assess agro-morphological traits of wheat plants grown under simulated saline field conditions

Successful breeding of plants for salinity stress tolerance requires realistic growing conditions and fast, non-destructive evaluation techniques for phenotypic traits associated with salinity tolerance. In this study, we used subsurface water retention technique (SWRT) as a growing condition and spectral measurements as an evaluation method to assess different agro-morphological traits of salt-tolerant (Sakha 93) and salt-sensitive (Sakha 61) wheat genotypes under three salinity levels (control, 60, and 120mm NaCl). The effects of salinity on agro-morphological traits were evaluated and related with forty-five published vegetation- and water-spectral reflectance indices (SRIs) taken at both the heading and grain milk growth stages for each salinity level, genotype, and growth stage. In general, the agro-morphological traits gradually decreased as salinity levels increased; however, the reduction in these traits was more pronounced in Sakha 61 than in Sakha 93. The effect of salinity levels and their interaction with genotypes on the SRIs was only evident at the grain milk stage. The performance of the spectral reflectance indices depicted that the closest associations with agro-morphological traits depended on salinity level, degree of salt tolerance of the genotypes, and growth stage. The SRI-based vegetative indices correlated better with growth and yield of Sakha 93 than SRI-based water indices and vice versa for Sakha 61. The SRI-based vegetative and water indices are effective for assessment of agro-morphological traits at early growth stages under high salinity level. The functional relationship between grain yield per hectare and the best SRIs was linear for the high salinity level and Sakha 61; however, the quadratic model was found to best fit this relationship for the control, moderate salinity level, and Sakha 93. The overall results indicate that the usefulness of the SRIs for assessment of traits associated with salinity tolerance is limited to salinity level and growth stage.