Screening soybean cultivars for resistance to iron-deficiency chlorosis in culture solutions containing magnesium or sodium bicarbonate

Hydroponic culture solutions containing bicarbonate (HCO3-) may be used to screen crops such as soybeans (Glycine max) for resistance to iron (Fe) deficiency or chlorosis. Some successful methods use sodium bicarbonate (NaHCO3) in combination with elevated partial pressures of carbon dioxide (CO2) to buffer pH and elevate bicarbonate. Replacing NaHCO3 with magnesium bicarbonate [Mg(HCO3)(2)] as the form of bicarbonate alkalinity has the potential to produce culture solutions that simulate soil solutions more closely and eliminate any potential for specific sodium (Na) toxicities in sensitive plants. A modified screening solution based on Mg(HCO3)(2)-CO2 was tested against the successful NaHCO3-CO2 method, using three soybean varieties of known resistance to Fe-deficiency chlorosis. Alkalinity was 10 mM [added as NaHCO3 or Mg(HCO3)(2)], solutions were aerated with 3% CO2, and Fe was provided as FeDTPA (diethylenetriamine-pentaacetic acid) at 15 mu M (low Fe) or 60 mu M (adequate Fe). Leaf chlorophyll, visual chlorosis index, and leaf Fe concentration were closely related. Solutions based on NaHCO3 or Mg(HCO3)(2) provided identical chlorosis-susceptibility rankings for the three cultivars.