Abstract
A stable isotope tracer technique was applied to a field experiment in order to establish a tracer technique for field experiments and to elucidate cadmium (Cd) uptake by soybean (Glycine max var. Enrei) and further translocation into its grain. The enriched ^<113>Cd (94.8%±0.3 content%) was used as a tracer. A hundred milliliters of tracer solution (^<113>Cd 10mg L^<-1>) was supplied to each soybean plant, totaling 1mg of ^<113>Cd application to each soybean plant. The Cd tracer (Cd_<tr>) was detected in all soybean leaves, pods and seeds sampled at the stages of R3 and R5, and it was also detected from almost all soybean pods, seeds and stems harvested at the stage of R8. These result indicates that 1mg of ^<113>Cd-tracer application is enough to detect it in soybean under field conditions. The total amount of Cd_<tr> injected into the soil is equivalent to 0.009mg kg^<-1>, which concentration is below one twentieth of the natural abundance of Cd in soil. An advantage of the stable isotope tracer compared with the radioisotope tracer is its high usability. This technique also carries the advantage that the ^<113>Cd-tracer and the soil-indigenous Cd in soybean plants can be determined simultaneously. A total of 6 ^<113>Cd applications was made at different growth stages of soybean. The Cd_<tr> concentration of leaves in the 1st injection plot (V3 stage) sampled at the R3 stage was significantly higher than that in the 2nd (R1) injection plots. This result indicates that leaves vigorously accumulate Cd only at the very early growth stage. The Cd_<tr> concentration in the soybean stems at harvesting was not significantly different up to the 5th injection plot (R6). The Cd_<tr> concentration of seeds, however, significantly fell after the 4th injection plot (R5), namely, Cd absorbed before the R5 stage causes an increase of Cd concentration in seeds. These results suggest that the Cd concentration in seeds hardly increases even if the Cd accumulated in leaves is translocated to the seeds. Additionally, the plant roots in the lower layer (30cm depth) and those in the upper layer (10cm depth) did not differ much in the Cd uptake activity until its R5 stage.
