How far can sodium substitute for potassium in red beet?

Sodium (Na) movement between plants and humans is one of the more critical aspects of bioregenerative systems of life support, which NASA is studying for the establishment of long-term bases on the Lunar or Martian surface. T his study was conducted to determine the extent to which Na can replace pot assium (K) in red beet (Beta vulgaris L. ssp vulgaris) without adversely af fecting metabolic functions such as water relations, photosynthetic rates, and thus growth. Two cultivars, Ruby Queen and Klein Bol,were grown fbr 42 days at 1200 mu mol mol(-1) CO2 in a growth chamber using a re-circulating nutrient film technique with 0%, 75%, 95%, and 98% Na substitution for K in a modified half-strength Hoagland solution. Total biomass of Ruby Queen wa s greatest at 95% Na substitution and equal at 0% and 98% Na substitution. For Klein Bol, there was a 75% reduction in total biomass at 98% Na substit ution. Nearly 95% of the total plant K was replaced with Na at 98% Na subst itution in both cultivars. Potassium concentrations in leaves decreased fro m 120 g kg(-1) dwt in 0% Na substitution to 3.5 g kg(-1) dwt at 98% Na subs titution. Leaf chlorophyll concentration, photosynthetic rate, and osmotic potential were not affected in either cultivar by Na substitution for K. Le af glycinebetaine levels were doubled at 75% Na substitution in Klein Bol, but decreased at higher levels of Na substitution. For Ruby Queen, glycineb etaine levels in leaf increased with the first increase of Na levels and we re maintained at the higher Na levels. These results indicate that in some cultivars of red beet, 95% of the normal tissue K can be replaced by Na wit hout a reduction in growth.