Ridge-Furrow Planting and Water-Retaining Agents Improve Quinoa Yield via Enhanced Photosynthesis and Translocation

Sustainable quinoa production in semi-arid regions is constrained by water scarcity and inefficient photoassimilate partitioning. This study evaluated the interactive effects of planting patterns and superabsorbent polymer (SAP) application rates (0, 60, and 90 kg·ha−1) on ‘Longli 5’ quinoa source–sink dynamics. Results demonstrated that ridge-furrow planting coupled with a moderate SAP rate (60 kg·ha−1, D1) synergistically optimized the soil moisture environment, enhancing the two-year average leaf area index (LAI) by 52.6–66.9% and net photosynthetic rate (Pn) by 5.45–10.64% (p < 0.05). Unlike the high SAP rate (90 kg·ha−1), “water luxury consumption” and low input efficiency dynamic which induced “water luxury consumption” and low input efficiency, the D1 treatment effectively promoted photoassimilate remobilization to grains by 13.39–14.44%. Unlike high SAP rates (90 kg·ha−1), which induced ‘water luxury consumption’ and low input efficiency, the D1 treatment effectively promoted photoassimilate remobilization to grains by 13.39–14.44%.This mechanism optimized source–sink coordination, resulting in a significant increase in seed yield and thousand-grain weight (p < 0.05). These findings conclude that a balanced source–sink allocation, rather than maximal source capacity alone, is the definitive driver of productivity. Integrating ridge-furrow planting with moderate SAP application (60 kg·ha−1) is a highly efficient agronomic strategy for harmonizing quinoa growth and achieving sustainable, high yields in semi-arid environments.