Bicarbonate Directly Induces Iron Chlorosis in Susceptible Soybean Cultivars

Abstract

Abstract Four soybean [ Glycine max (L.) Merr.] cultivars differing in susceptibility to iron (Fe) chlorosis in wet calcareous soils were grown in nutrient solutions to characterize the effects of phosphate (P) and bicarbonate (HCO ‐ 3 ) in inducing chlorosis. NaHCO 3 (0 or 10 m M ), P (10 or 400 µ M as NaH 2 PO 4 , and NH + 4 (0 or 300 µ M ) as (NH 4 ) 2 SO 4 were factorial treatments in a nutrient solution. Low Fe availability was maintained by supplying 5 µ M Fe + 10 µ M EDDHA, and excess CaCO 3 (pH 7.5). With no HCO ‐ 3 added, ‘T203’ (extremely chlorosis susceptible) was green at 10 µ M P, but severely chlorotic at 400 µ M P; ‘Wayne’ (chlorosis susceptible) was green at both P levels. Addition of HCO ‐ 3 caused chlorosis in T203 and Wayne at 10 or 400 µM P. Chlorosis resistant ‘AP9’ and ‘Hawkeye’ remained green with added P or HCO ‐ 3 . Chlorosis ratings (1 = green to 5 = severely chlorotic) in the 0 HCO ‐ 3 + 10 µ M P, 0 HCO ‐ 3 + 400 µ M P and 10 HCO ‐ 3 + 400 µ M P treatments, respectively, were: 1.0, 1.0, and 1.5 for AP9; 1.0, 1.0, and 1.4 for Hawkeye; 1.0, 1.2, and 3.9 for Wayne; and 1.0, 4.2, and 4.6 for T203. Low Fe concentrations in young leaves corresponded with high chlorosis ratings. Phosphorus concentration in young leaves did not change with HCO ‐ 3 at 10 µ M P, but decreased with HCO ‐ 3 at 400 µ M P. In the absence of HCO ‐ 3 , Wayne was not chlorotic at either concentration of solution P, although this cultivar is known to be highly susceptible to chlorosis in the field. Bicarbonate‐induced chlorosis in Wayne was not a result of increased solubility or plant uptake of P. Thus, HCO ‐ 3 was a direct factor in causing soybean chlorosis, and likely is also a cause in soils.