Iron deficiency stress response of various C-3 and C-4 grain crop genotypes: strategy II mechanism evaluated

The relative amount of phytosiderophore produced by various Strategy II plants has been categorized as follows: barley (Hordeum vulgare L.) > wheat (Triticum aestivum L.) > oat (Avena byzantina C. Koch.) > rye (Secale cereale L.) much greater than corn (Zea mays L.) much greater than sorghum (Sorghum bicolor (L.) Moench) > rice (Oryza sativa L.). With the exception of rice, these plants developed under oxidized soil conditions, and the C-3 species produce more phytosiderophore than C-4 species under Fe-deficiency stress. Iron-efficient Coker 227 oat produced phytosiderophore in response to Fe-deficiency stress, while Fe-inefficient TAM 0-312 oat did not. Although Fe-efficient WF9 corn and Fe-inefficient ys1 corn differed in their ability to obtain Fe, neither produced sufficient quantities of phytosiderophore to explain these differences. The objectives of this research were to determine: (a) if phytosiderophore production of Fe-deficiency stressed C-4 species millet (Panicum miliaceum L.) and corn is low or absent compared to identically stressed C-3 species oat and barley, and (b) if native, inbred and hybrid corn cultivars differ in ability to produce and utilize phytosiderophores. Although release of phytosiderophore for Fe-stressed corn and millet was generally lower than oat, quantity of release was not always related to obtaining Fe and maintaining green plants. Barley maintained high leaf Fe and low chlorosis with a minor release of phytosiderophore. Oat with increased release acted similarly to barley, whereas a relatively high release of phytosiderophore from White maize did not effect Fe uptake or greening. Likewise, small amounts of phytosiderophore were produced by all corn types, but corn was generally unable to obtain adequate Fe from the growth medium. Two of the native corns, Coneso and Tepecintle, maintained relatively low chlorosis, but they differed in phytosiderophore release. Thus, it appears that the C-4 plants studied herein generally release a lower amount of phytosiderophore than do C-3 species, but overcoming Fe-deficiency chlorosis is not guaranteed by such release. The Strategy II mechanism of mere release of phytosiderophore and consequential Fe acquisition appears simplistic. There is a need for understanding what other factors are involved.