PHYSIOLOGICAL CONDITIONS AND UPTAKE OF INORGANIC C-14 BY PLANT-ROOTS

The uptake of inorganic C-14 by bean plant roots was measured. The plants were grown in a nutrient solution culture at pH 6 and a (NaHCO3)-C-14 tracer was added to the growth medium. Photosynthesis and transpiration were varied by exposing the aerial portions of the plants to different atmospheric CO2 concentrations, humidities and light levels in a cuvette system. Leaf concentrations of C-14 were measured at the end of the experiments using liquid scintillation counting. Plant uptake of C-14 via the roots was independent of the photosynthetic rate and, in most cases, could be predicted by knowing the transpiration rate and the nutrient solution concentration. However, when a less efficient root-medium aeration system was used, C-14 uptake was greater than that predicted using transpiration, a phenomenon observed by other researchers. This contrasted to results of another experiment where the measured uptake of iodine was much slower than that predicted using transpiration. Knowledge of transpiration rates is useful in predicting inorganic carbon uptake via the roots and in estimating C-14 transport from contaminated soils to biota. Also, the independence of the uptake from photosynthesis and ambient CO2 concentrations suggests that future increases in atmospheric CO2 concentrations may not have a direct effect on root uptake of soil carbon.