## 4 Discussion
This finding indicates that effective genetic gain in plant biomass under salinity stress could be obtained by evaluating genotypes based on SRIs and that these SRIs could be used as alternative selection criteria for fast and accurate assessments of salinity tolerance in a spring wheat-breeding program. However, we observed the interaction of growth stages with genotypes for different SRIs in this study (Table 2). This emphasizes that the appropriate growth stage for assessing plant biomass under salinity conditions using SRIs must be precisely identified for the accurate assessment of the salt tolerance of wheat genotypes in breeding programs. The results presented in Table 4 confirmed this finding and further revealed that the appropriate growth stage for assessing plant biomass also depends on the type of SRIs. At the BT stage, all water-related SRIs, except NDWI-M1, and only one index from vegetation-related SRIs (RENDVI-P) were found to be efficient for assessing the three traits; the opposite was true at the EMG stage. However, nearly all vegetation- and water-related SRIs were similarly well suited to assess the three traits at the AN stage (Table 4). All of these results implied that the variation in plant biomass among genotypes can be assessed at the early growth stage (BT) using the water-related SRIs, which detect plant water status, and the late growth stage (EMG) is appropriate for vegetation-related SRIs, which reflects the chlorophyll and growth status of plants. The AN stage is the appropriate developmental stage for plant biomass assessment, using a combination of vegetation- and water-related SRIs. These results confirmed the importance of two phases of plant response to salinity stress described by [PERSON] and [PERSON] [24]. Additionally, the visible wavelengths incorporated in the vegetation-related SRIs (especially red light) are usually saturated at a high leaf area index and high concentration of different pigments; this may explain why vegetation-related SRIs failed to assess plant biomass at the BT stage. However, as growth progresses from the BT to EMG stage, these kinds of indices become effective for assessing plant biomass because necrosis and senescence symptoms start to appear on the leaves as a real phenomenon of salinity damage, and genotypic differences in biomass and chlorophyll contents are also high at later stages. This also confirms why the mean values of vegetation-related SRIs decrease from the BT to the EMG stage (Table 3). Furthermore, because multiple measurements of spectral reflectance of genotypes across multiple growth stages capture overall the effects of salinity stress on various physiological attributes that ultimately cumulatively affect plant biomass and yield, this may be the reason that nearly all SRIs successfully assess plant biomass when the SRI data are analyzed together across the three growth stages (Table 4). Therefore, in all cases, our results suggest that it is possible to detect genotypic variation in plant traits related to biomass accumulation under high-salinity stress at early growth stages based on SRIs. The results of the PCA presented in Figure 5, in which all indices and their relationship with the three plant traits are shown together, fully confirmed the abovementioned findings and showed that the angle of the vectors of the three traits was acute in three cases: (1) with three water-related SRIs (WI-M, NDWI-M2, and NDWI-P) and one vegetation-related SRI (RENDVI-P) at the BT stage; (2) with all vegetation-related SRIs, except RENDVI-P, at the EMG stage; and (3) with a combination of these former indices at the AN stage and across three stages. However, the angle between the vectors of the remaining indices and the three traits was obtuse or straight at each growth stage or across three stages (Figure 5). Generally, the acute angle between SRIs and traits reflects the ability of these indices to effectively assess genotypic variation in plant biomass.