In vitro protein synthesis of sugar beet (Beta vulgaris) and maize (Zea mays) is differentially inhibited when potassium is substituted by sodium.

The substitution of potassium ions (K+) by sodium ions (Na+) in the nutrition of plants is restricted. It was shown earlier that net protein synthesis is the process which is most sensitive to the substitution of K+ by Na+ in young sugar beet. We hypothesized that the activity of ribosomes is inhibited by the substitution. This hypothesis was tested in an in vitro approach. Cytosolic polysomes were isolated from growing leaves of sugar beet and maize by means of differential centrifugation. In vitro systems of both plant species were tested for functionality and comparability. Translation was quantified by the 35S-methionine incorporation in TCA-precipitable products. The effect of different substitution levels (0%, 20%, 40%, 60%, and 80% substitution of K+ by Na+) on in vitro translation was measured. Translation by polysomes of both plant species was significantly inhibited by the substitution. However, the translation by maize polysomes was more negatively affected by the substitution. A significant decrease in the translation by maize polysomes was observed already when 20% of K+ were replaced by Na+, whereas in the case of sugar beet, the translation was inhibited firstly at the substitution level of 40%. The in vitro results show that the process of translation itself is disturbed by the substitution and indicate a higher tolerance of sugar beet polysomes to increased Na+ concentrations and Na+/K+ ratios compared to polysomes of maize. We propose that this tolerance contributes to the salt resistance of sugar beet.