Plant physiological responses for genotypic evaluation of iron efficiency in strategy I and strategy II plants—A review

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Abstract
Iron (Fe)‐deficiency chlorosis is widespread in numerous crops grown on calcareous soils and results in significant yield losses. Examples of affected crops are soybean (Glycine max L.), chickpea (Cicer arietinum L.), peanut (Arachis hypogaea L.), dry bean (Phaseolus vulgaris L.), tomato (Lycopersicon esculentum Mill.), oat (Avenae sativa L.), wheat (Triticum aestivum L.), sorghum [Sorghum bicolor (L.) Moench], corn (Zea mays L.), rice (Oryza sativa L.), and various fruit species. Other than with high return fruit crops, the problem is not economically solved with fertilizer application. A more reasonable solution is the introduction of genotypes which are resistant to Fe‐deficiency chlorosis development (physiologically ‘Fe efficient')‐ Field selection for resistant cultivars is complicated by heterogeneous soil and environmental conditions, and breeding programs implementing these techniques are slow and have reached a plateau. The physiological responses to Fe‐deficiency stress include release of H+ ions and reductants from roots and increased reduction of ferric Fe at the plasmalemma of roots (dicots and non‐graminaceous monocots) or release of phytosiderophores from roots (grasses). A few workers have proposed selecting cultivars using these physiological responses to avoid the expense and time‐consuming effort of field selection. Use of one or more of these physiological responses for screening has been attempted with peach [Prunus persica (L.) Batsch] and grape (K vinifera) rootstocks, oat, wheat, soybean, and subterranean clover (Trifolium subterraneum L.). Reduction of Fe by roots or release of H ions or a combination of the two show promise in dicots and release of phytosiderophore shows potential in graminaceous monocots as selection criteria. Simplification of these techniques and direct testing in breeding programs are needed.