4.4 Root tissue metabolites
A total of 14 annotated metabolic pathways were found in this study, and the most differentially expressed (upregulated) metabolite was S-adenosylmethionine (SAM) (in eight of the 14 metabolic pathways). SAM is second only to ATP in biological utilization and is involved in numerous intracellular metabolic reactions (Fontecave et al., 2004). Studies have shown that SAM has an important role in regulating plant adaptations to various abiotic stresses, such as iron deficiency (Lan et al., 2011), drought (Mayne et al., 1996) and salinity (Gong et al., 2016; Fujimoto et al., 2011) as well as biotic stresses (e.g. resistance to pathogenic bacteria) (Fujimoto et al., 2011) and has a complex time-dependent role in the senescence processes (Owiti et al., 2011). However, the effect of planting density on the abundance of SAM in roots has not been reported previously. In cucumber, soaking broken-root seedlings in SAM-containing solution significantly promoted root development, chlorophyll accumulation, enhanced photosynthetic rate, and improved plant uptake of N, P, and K (Liu et al., 2018). In the study presented here, the average abundance of SAM in quinoa roots in low-density planting was twice that in high-density planting. It is suggested that SAM can improve the adaptation of quinoa to coastal saline soils and may have some growth-promoting effects. Further research is needed to shed some light on the mechanism underpinning SAM-related promotion of quinoa growth.