Water Use Enhancement and Root Function Compensatory Regulation of Biomass Accumulation in Quinoa Under Salt Stress by Photosynthetic Drive Advantage

Water and salt stress significantly impact the accumulation of crop biomass (TB); however, the relative contributions of photosynthetic, physiological, and morphological factors remain poorly understood. This study aims to comprehensively investigate the effects of water and salt stress on crop growth physiology and identify the primary factors influencing biomass accumulation. We examined four quinoa varieties (<i>Qingli No.1, Qingli No.8, Gongza No.4, and Black quinoa</i>) under four salinity levels (s0: 0 mmol/L, s1: 100 mmol/L, s2: 200 mmol/L, and s3: 300 mmol/L) and two moisture levels (w1: 30% field capacity (FC), w2: 80% FC). Using principal component analysis (PCA) and correlation analysis, we constructed a random forest model (RF) and a partial least-squares path modeling (PLS-PM) framework to elucidate the effects of water and salt stress on quinoa growth physiology and clarify the adaptive mechanisms of quinoa under varying salinity conditions. The results indicate that (1) salinity has a more substantial regulatory effect on the accumulation of proline (Pro) and sodium ions (Na⁺) than water availability. Under conditions of adequate moisture (w2), the activity of antioxidant enzymes increased in response to mild salinity stress (s1). However, with escalating salinity levels, a significant decrease in enzyme activity was observed (<i>p</i> < 0.05). (2) PCA identified salinity as a key factor significantly influencing physiological changes in quinoa growth. The RF model indicated that, under severe salinity conditions (s3), intrinsic water-use efficiency (iWUE) emerged as a critical driver affecting biomass (TB) accumulation. (3) The PLS-PM model quantified the relative contribution rates of various factors to total biomass (TB). It revealed that, as salinity increased, the path coefficients of photosynthetic factors also rose, but their relative contribution diminished due to a corresponding reduction in the contribution of morphological factors. These findings offer a theoretical foundation and decision-making support for the integrated management of water-salt conditions in saline-alkali agricultural fields, as well as for the cultivation of salt-tolerant crops.