Bicarbonate Directly Induces Iron Chlorosis in Susceptible Soybean Cultivars

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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. NaHCO3 (0 or 10 mM), P (10 or 400 µM as NaH2PO4, and NH+4 (0 or 300 µM) as (NH4)2SO4 were factorial treatments in a nutrient solution. Low Fe availability was maintained by supplying 5 µM Fe + 10 µM EDDHA, and excess CaCO3(pH 7.5). With no HCO‐3 added, ‘T203’ (extremely chlorosis susceptible) was green at 10 µM P, but severely chlorotic at 400 µM 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 µM P, 0 HCO‐3 + 400 µM P and 10 HCO‐3 + 400 µM 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 µM P, but decreased with HCO‐3 at 400 µM 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.
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