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The Effect of Technetium-99 and Environmental Conditions on Soybean Physiology

Dep. of Radiation Oncology, Scripps Clinic and Res. Foundation, 10666 North Torrey Pines Road, La Jolla, CA 92037;

James W. Neel

Dep. of Biology, San Diego State Univ., 5300 Campanile Dr., San Diego, CA 92182-0712.

Patrick J. Papin

Dep. of Physics, San Diego State Univ., 5300 Campanile Dr., San Diego, CA 92182-0712.

^* Corresponding author.

ABSTRACT

Technetium-99 is considered a long-term risk of nuclear energy because it has a long half-life, mass 99 is produced at a high yield during nuclear fission, and Tc-99 is taken up readily by aquatic and terrestrial organisms. Plants are an important intermediate of Tc-99 food chain transfer to animals and humans. The present study investigated adenosine triphosphate (ATP), photosynthesis and structural responses of intact soybean seedlings [Glycine max (L.) Merr. cv. Williams] to micromolar amounts of Tc-99 under different ambient light levels. Adenosine triphosphate was measured directly in primary leaf extract after 6 d of exposure to Tc at varying concentrations (up to 5 µM Tc-99). The results indicated that under full light conditions (320 µmol photon m^–2 s^–1), soybean primary leaves contained nanomolar concentrations of ATP almost twice as high as for untreated plants. With a reduction of light intensity by 50%, primary leaf pertechnetate uptake and leaf ATP content were reduced regardless of Tc-99 exposure levels. Gas exchange measurements with a Li-Cor 6200 Portable Photosynthesis system indicated a significant decline of photosynthetic rate and a higher internal CO₂ accumulation under full light conditions, while stomatal conductance decreased as a function of Tc-99 concentration. Several possible mechanisms of Tc interaction with chloroplast ultrastructure are proposed. A hypothesis related to the metabolic behavior of Tc in higher plants is presented.