Effects of Iron Complexation and Indiscriminate Uptake on Shoot Iron of Barley

Abstract

The free-ion model (FIM) describes iron (Fe) uptake by barley [Hordeum vulgare(L.) ‘Grammett’] as being controlled by the activity of the buffered, free, uncomplexed Fe^{3 +} in solution. Chelators' effect on Fe uptake by barley was evaluated and the rate of exchange of Fe between chelators was examined. Barley was grown for two weeks in a low-Fe nutrient solution and transferred to solutions varying in Fe and chelators for 6 h assays. Shoot ⁵⁹Fe was higher in barley grown in citrate (7743 and 1928 ⁵⁹Fe Bq g^{− 1}) than in NTA (3220 and 1113 ⁵⁹Fe Bq g^{− 1}; P = 0.045) despite similar free-Fe^{3 +} activities. A comparison of Fe uptake by barley from solutions with pFe^{3 +} activities of 17.1 and 24.6 showed < 5% was from indiscriminate apoplastic-flow uptake (3250 ⁵⁹Fe Bq g^{− 1} vs. 160 ⁵⁹Fe Bq g^{− 1}). Using nutrient solutions from the barley studies but without plants, Fe exchange between chelators and a simulant for the barley phytosiderophore occurred within hours (for NTA and citrate), or days (EDTA and HEDTA). Results were similar between the barley and Fe-exchange experiments for the two nutrient-solution treatments where the same Fe^{3 +} activities but different total-Fe concentrations were used: the higher total-Fe treatment resulted in six-fold higher shoot ⁵⁹ Fe, while in the Fe-exchange study that treatment had six-fold more Fe bound to the phytosiderophore simulant after 2 d. Results indicated deviations from the FIM were not explained by indiscriminate-flow uptake, and that sluggish Fe-exchange reactions between chelate and phytosiderophore-simulant, not FIM guidelines, may be more important in explaining Fe uptake from synthetic chelates by Fe-deficient barley.

Keywords:

• free-ion model
• apoplast
• hydroponics
• solution culture

ACKNOWLEDGMENTS

Funds were received from the University of Queensland Travel Awards in 1998 for International Collaborative Research.

Notes

^°Geometric means are presented.