Cadmium (Cd) accumulations in a Cd hyper-accumulator fern, Athyrium yokoscense (Ay), and tobacco, Nicotiana tabacum (Nt), were kinetically analysed using the positronemitting tracer imaging system under two medium conditions (basal and no-nutrient). In Ay, maximumly 50% and 15% of the total Cd accumulated in the distal roots and the shoots under the basal condition, respectively. Interestingly, a portion of the Cd in the distal roots returned to the medium. In comparison with Ay, a little fewer Cd accumulations in the distal roots and clearly higher Cd migration to the shoots were observed in Nt under the basal condition (maximumly 40% and 70% of the total Cd, respectively). The no-nutrient condition down-regulated the Cd migration in both species, although the regulation was highly stricter in Ay than in Nt (almost no migration in Ay and around 20% migration in Nt). In addition, the present work enabled to estimate physical and physiological Cd accumulation capacities in the distal roots, and demonstrated condition-dependent changes especially in Ay. These results clearly suggested occurrences of species-/condition-specific regulations in each observed parts. It is probable that integration of these properties govern the specific Cd tolerance/accumulation in Ay and Nt.
To maximize fruit yield of tomatoes cultivated in a controlled, closed system such as a greenhouse or a plant factory at a limited cost, it is important to raise the translocation rate of fixed carbon to fruits by tuning the cultivation conditions. Elevation of atmospheric CO 2 concentration is a good candidate; however, it is technically difficult to evaluate the effect on fruit growth by comparing different individuals in different CO 2 conditions because of large inter-individual variations. In this study, we employed a positron-emitting tracer imaging system (PETIS), which is a live-imaging technology for plant studies, and a short-lived radioisotope 11 C to quantitatively analyze immediate responses of carbon fixation and translocation in tomatoes in elevated CO 2 conditions. We also developed a closed cultivation system to feed a test plant with CO 2 at concentrations of 400, 1,500 and 3,000 ppm and a pulse of 11 CO 2 . As a result, we obtained serial images of 11 C fixation by leaves and subsequent translocation into fruits. Carbon fixation was enhanced steadily by increasing the CO 2 concentration, but the amount translocated into fruits saturated at 1,500 ppm on average. The translocation rate had larger inter-individual variation and showed less consistent responses to external CO 2 conditions compared with carbon fixation. Our experimental system was demonstrated to be a valuable tool for the optimization of closed cultivation systems because it can trace the responses of carbon translocation in each individual, which are otherwise usually masked by interindividual variation.
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