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.

Protein synthesis is the most sensitive process when potassium is substituted by sodium in the nutrition of sugar beet (Beta vulgaris).

Potassium ions (K(+)) and sodium ions (Na(+)) share many physical and chemical similarities. However, their interchangeability in plant nutrition is restricted. Substitution studies showed that K(+) can be replaced by Na(+) to a large extent in the nutrition of Beta vulgaris L. However, the extent of substitution without negative impacts is not unlimited. The aim of the present study was to identify the process which is most sensitive during the substitution of K(+) by Na(+) in nutrition of young sugar beet plants. We focused on transpiration, growth, and net protein synthesis. Plants were grown under controlled environmental conditions. With transfer of seedlings into nutrient solution, plants were cultivated in different substitution treatments. For all treatments the sum of K(+) and Na(+) (applied as chloride) was fixed to 4 mM. The extent of substitution of K(+) by Na(+) in the nutrient solution was varied from low (0.25% substitution: 3.99 mM K(+), 0.01 mM Na(+)) to almost complete substitution (99.75% substitution: 0.01 mM K(+), 3.99 mM Na(+)). The supply of 3.99 mM K(+) in 0.25% substitution treatment guaranteed the absence of K(+) deficiency. Transpiration was not affected by the substitution. Growth was inhibited at a substitution level of 99.75%. Net protein synthesis was already affected at a substitution level of 97.50% (0.10 mM K(+), 3.90 mM Na(+)). Hence, net protein synthesis was most sensitive to the substitution and limited the extent of substitution of K(+) by Na(+) in the nutrition of young sugar beet plants.