Root nodule presence alters the dynamics of photosynthate translocation under varying soil moisture conditions

Abstract

This study investigated the dynamics of ¹¹C-labeled photosynthate translocation in nodulated and non-nodulated soybean plants across three time windows: during the low water condition, and at 0–3 h and 4–7 h after the recover water content. Advanced imaging techniques, including positron emission tomography (PET) and X-ray computed tomography (X-ray CT), enabled three-dimensional visualization of root structures and carbon allocation patterns. Under low water condition, nodulated soybeans prioritized carbon translocation to taproots and nodules. Using logistic modeling of ¹¹C accumulation, Vmax (steepest slope; maximum translocation rate) and Kmax (asymptote; accumulation capacity) were calculated. In nodulated plants, Vmax at lateral root tips increased by 56 % during 0–3 h after rewatering relative to the low-water condition; during 4–7 h, Vmax and Kmax at middle-layer nodules increased by 55 % and 49 %, respectively. Collectively, these results indicate a temporal reorganization of sink activity after rewatering—from lateral root tips early to middle-layer nodules later. These observations are consistent with a role for nodules as prominent sinks that may facilitate the redistribution of photosynthates after rewatering, whereas non-nodulated plants showed decreases in Vmax and Kmax across root regions under low-water conditions and exhibited little recovery during 0–3 and 4–7 h after rewatering. These patterns are consistent with a role for nodules in facilitating the reactivation and redistribution of carbon sinks under changing water availability. This study clarifies how water status modulates belowground carbon allocation in soybean and provides a basis for evaluating nodule-associated sink behavior under fluctuating moisture. These insights may inform crop management and guide breeding strategies aimed at improving resilience to water variability.