Cadmium stress reshapes photosynthetic carbon allocation and phytoremediation efficiency in C3 and C4 Cd-accumulators

Cadmium (Cd) contamination modifies both soil processes and plant physiology, but the relationship between photosynthetic carbon allocation and Cd uptake in C3 and C4 accumulator plants remains unclear. Here, the C3 accumulator Solanum photeinocarpum (SP) and the C4 accumulator Pennisetum americanum × P. purpureum (PA) were studied using a ¹³C pulse-labeling technique. Results showed that fixed carbon was mainly transferred to soil as labile organic carbon (LOC) and light fraction organic carbon (LFOC). In SP, Cd pollution promoted Cd accumulation in shoots and roots, increased soil available Cd and pH, and enhanced carbon transfer to soil, leading to greater remediation efficiency. In PA, Cd pollution decreased soil pH and microbial biomass carbon, but increased available Cd and soil organic carbon, with shoot-fixed carbon positively correlated with Cd uptake. These findings demonstrate that Cd pollution differentially regulates carbon allocation in C3 and C4 accumulators through different mechanisms, supporting both phytoremediation efficiency and carbon sequestration.