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.