Analysis of Interrelationships Among Yield Source, Flow, and Sink Traits in Soybean Cultivars

To evaluate the contributions of source, flow and sink traits to soybean yield, clarify the relationships among these traits during yield formation, reveal the key physiological mechanisms underlying soybean yield formation, and provide a theoretical basis for soybean breeding.

Qihuang 34, Lindou 9, Weidou 9, and Hedou 12 were investigated during the pod-setting and seed-filling stages. The photosynthetic performance, assimilate partitioning, and dry matter accumulation were analyzed throughout plant development by using ¹³C isotope labeling. Source–flow–sink traits were comprehensively evaluated, and radar chart analysis was applied to characterize the coordination of these traits among cultivars during yield formation.

(1) The correlations between source, flow and sink traits and yield varied considerably. The contributions of these traits to yield ranked as follows: leaf area index > 100-seed weight > net photosynthetic rate > palisade tissue thickness > effective quantum yield of PSII (ΦPSII) > glutamine synthetase activity > glutamate synthase activity > mean sieve tube diameter > stomatal conductance > midrib diameter. (2) Significant interactions were observed among source, flow and sink traits. Source–sink interactions were reflected by positive correlations of net photosynthetic rate, stomatal conductance and transpiration rate with 100-seed weight. Source–flow interactions were indicated by positive correlations of palisade tissue thickness and leaf thickness with net photosynthetic rate, whereas flow–sink interactions were represented by a positive correlation between mean sieve tube diameter and 100-seed weight. (3) Source, flow and sink traits differed markedly among cultivars. Qihuang 34 exhibited higher leaf area index, palisade tissue thickness, leaf thickness, net photosynthetic rate, ΦPSII, sustained dry matter accumulation in main stem pods, yield, 100-seed weight, glutamine synthetase activity, glutamate synthase activity, 3-day assimilate translocation to main stem pods and sustained assimilate translocation than the other three cultivars. Hedou 12 showed the largest midrib diameter, Weidou 9 had the highest seed number per plant, and Lindou 9 exhibited the greatest carbon assimilation capacity of branch pods. These results indicate that high soybean yield depends on the coordinated regulation of source, flow and sink traits rather than on any single trait.

Among the four soybean varieties tested, Qihuang 34 exhibited a significantly higher yield than the other varieties, which was mainly attributed to its higher leaf area index, greater photosynthetic efficiency, stronger assimilate translocation capacity, higher 100-seed weight, and greater activities of nitrogen-assimilation enzymes. Breeding high-yield soybean cultivars with coordinated source, flow and sink characteristics should prioritize germplasm with high photosynthetic efficiency based on ΦPSII, net photosynthetic rate and leaf area index, followed by evaluation of assimilate transport efficiency using leaf and vascular bundle anatomical traits and assessment of yield potential through nitrogen metabolism enzyme activities and sink capacity.