Plants can utilize iron form Fe‐N,N'‐di‐(2‐hydroxybenzoyl)‐ethylenediamine‐N,N'‐diacetic acid, a ferric chelate with 10⁶ greater formation constant than Fe‐EDDHA.

Abstract

HBED [N,N'‐di‐(2‐hydroxybenzoyl)‐ethylenediamine‐N,N'‐diacetic acid] was synthesized in 1967, but it only recently became available in the quantities needed for plant nutrition research. This chelator is a close relative of EDDHA, but has a Fe³⁺‐chelate formation or stability constant (at ionic strength, μ+ 0.1) of 39.68 compared to 33.91 for EDDHA. Even the Fe³⁺ selectivity (i.e. Fe³⁺/Ca²⁺ stability constant ratios) is greater for HBED than for EDDHA. These greater stabilities with Fe³⁺ appear to result from better steric fit of HBED to Fe³⁺. FeHBED has Fe³⁺ stability constant and Fe³/Fe²⁺ and Fe³⁺/Ca²⁺ stability constant ratios much lite the Fe³⁺ selective bacterial siderophores.

Soybean, corn, and nutsedge species were grown in complete Hoagland type nutrient solution containing 0 Fe, 10 μM FeHEDTA, 10 μM FeEDDHA (1:1 and 1:3 Fe:EDDHA ratios), or 10 μM FeHBED, and containing CaCO₃ to control pH at 7.5. Soybean and nutsedge species obtained adequate Fe and remained green, while corn was severely chlorotic on all treatments except FeHEDTA. The results show the extreme ability of non‐graminae plant roots to reduce Fe³⁺ chelates and absorb iron in aerated solutions where Fe²⁺ can not persist long after Fe³⁺ chelates have been reduced. Nutsedge was found to be a typical Strategy‐1 species, unlike corn, even though the Cyperaceae and Poaceae are placed together in the order Cyperales by some taxonomic authorities.

GEOCHEM, a computer program designed to calculate equilibria for chemical species in soil solutions, was adapted to nutrient solutions to allow comparison of HBED with EDTA, DTPA, and EDDHA. It now appears that previous attempts to speciate EDDHA nutrient solutions used incorrect formation constants for CuEDDHA chelate species. Using published formation constants, GEOCHEM indicated that Cu would displace Fe across a wide pH range, but that other elements do not displace Fe at practical nutrient solution pH. The greater Fe³⁺ selectivity of HBED prevents Cu from displacing Fe from FeHBED. Chemical studies of Cu displacement of Fe from FeEDDHA at pH 5.5 (where the kinetics of Fe³⁺ exchange allow the reaction to be studied) showed that the formation constant for CuEDCHA is about 22.3 (μ = 0.1 M), not 23.9 as previously estimated. Cu can displace Fe from EDDHA within the normal nutrient solution pH range.

HBED has several advantages for use as an Fe‐chelator for nutrient solutions compared to EDDHA. EDDHA catalyzes oxidation of Mn²⁺ and Co²⁺ to strongly chelated Mn³⁺ and Co³⁺ species with unknown stability constants, but HBED does not. Thus, formation constants are available for nutrient elements with HBED, but not EDDHA. No element can displace Fe from FeHBED in practical nutrient solutions, but Cu can displace Fe from FeEDDHA. However, Fe in EDCHA is more easily available to plants than is Fe in FeHBED, probably because of stronger chelation of Fe²⁺ by HBED. HBED provides a more predictable reagent to control activities of micronutrient elements in nutrient solutions than does EDDHA. HBED is more expensive than EDDHA, but this should not interfere with use of HBED in plant nutrition research.

Key words:

• EDDHA
• HEDTA
• corn
• soybean
• tomato
• nutsedge
• Cyperus. chelators
• chelates
• EDTA
• DTPA
• chlorosis
• GEOCHEM
• speciation