Quantitative Separation of the Bicarbonate from the Counter-cation Effect in Bean Plants

Abstract

The effect of Rb+ and Na+ as counter-cations of HCO3 − was evaluated on bean (Phaseolus vulgaris L. cv. Poncho) plants using mixture experiment statistical methodology in a series of experiments set up in a controlled environment chamber. Mixture experiments using three components (Rb+, K+, and Na+) or two components (K+ and Na+) were conducted to delineate the toxicity of HCO3 − versus the counter-cation effect. The quantitative separation of the toxic effects was possible only when the individual stress had an additive effect when combined with the other stress. Potassium mixtures were used as reference for comparison with other mixtures because plants did not respond to K+, probably because it was included at a minimum concentration of 2.5 mm K+ or because it was supplied in the preestablishing solution. Rubidium caused a decrease in shoot dry weight (SDW), but SDW accumulation was even lower when HCO3 − was added to the Rb+ solutions. However, Rb+ was not included in follow-up experiments because the response of plants to Rb+ was very similar to that of Na+. The toxic effect of Na+ caused SDW to decrease at a rate of 3.7% per millimolar increase of Na+. However, the effect of HCO3 − was dependent on its concentration, because at 2.5 mm HCO3 −, the decrease in SDW was 12.7% per millimolar HCO3 −, whereas at 3.75, 5, and 5.65 mm, the decrease was 11.0%, 7.8%, and 10.7% per millimolar HCO3 −, respectively. At 7.5 mm HCO3 −, the decrease in SDW was 4.2% to 8.2% per millimolar increase in HCO3 −, respectively. The decreasing HCO3 − rate may be explained by the nonadditive effect between HCO3 − and Na+ at high alkalinity levels.