Occurrence Of Iron Deficiency In Field Crops

Iron deficiencies have been reported for many plant species and geographical regions. The most common problems correspond to the cultivation of sensitive crop species (Table 2-1) in arid and semi-arid regions with calcareous soils (presence of free carbonates). Iron deficiency is widely reported in soybean [Glycine max (L.) Merr.], peanut (Arachis hypogaea L.), dry bean (Phaseolus vulgaris L.), sorghum [Sorghum bicolor (L.) Moench], and rice (Oryza sativa L.). Additional Fe deficiency is reported under unique or specialized conditions with corn (Zea mays L.), wheat [Triticum aestivum (L.).Thell.], oat (Avenae sativa L.), and other minor crops.

Table 2-1. The relative sensitivity or tolerance of crops to low levels of available Fe in soil. Some crops are listed under more than one category because of variations in the response of different varieties of a given crop. Modified from Tisdale et al. (1985).

Sensitive

Moderately Sensitive

Tolerant

Field bean

Alfalfa

Barley

Forage sorghum

Corn

Cotton

Grain sorghum

Cotton

Millet

Peanut

Field pea

Oat

Soybean

Forage legume

Potato

Sudangrass

Rice

Sugar Beet

Soybean

Wheat

One of the most widely studied species suffering Fe deficiency chlorosis among field crops is soybean. Iron deficiency in soybean has been a significant yield-limiting factor for decades. Despite extensive study, the incidence of Fe deficiency is increasing primarily due to expansion of soybean production into areas where soil conditions that exacerbate deficiency prevail. The acreage of soybean affected by Fe deficiency has been estimated to have expanded by 160% in the last three decades, potentially affecting nearly two million hectares and reducing yields an average of 0.75 Mg ha-1 (Figures 2-1 and 2-2; Hansen et al., 2004). The decades of research with Fe deficiency in soybean have elucidated an understanding of Fe deficiency stress response mechanisms (Jolley and Brown, 1994) and identified a wide variation among soybean varieties in susceptibility to Fe deficiency. A strong correlation exists between the susceptibility of varieties in the field to Fe deficiency and the degree to which these varieties express Fe-deficiency response mechanisms under controlled conditions (Jolley et al., 1992; Stevens et al., 1993). Private companies and public institutions routinely rank soybean varieties for susceptibility to Fe deficiency. Despite the extensive research and variety screening efforts, Fe deficiency continues to be a challenge in soybean production (Franzen and Richardson, 2000; Goos and Johnson, 2000; Hansen et al., 2003; Inskeep and Bloom, 1984).

Figure 2-1. Geographical distribution of Fe deficiency in field crops in the U.S. Modified after Berger and Pratt (1963).

Peanut is another dicotyledenous species sensitive to Fe deficiency and there are production challenges associated with Fe deficiency in various regions (Frankel et al., 2004). Peanut cultivated in the southern Great Plains (Reed and Ocumpaugh, 1991) of the U.S., northern China (Zuo et al., 2004) and Israel (Hartzook et al., 1974; Hartzook, 1984) often develops Fe deficiency. The problem is particularly prominent with the presence of a caliche layer near the soil surface, which can induce perched water conditions in surface soils. When grown as a monocrop on calcareous soils, Fe deficiency commonly limits peanut yield in Northern China production areas (Zuo et al., 2004), while the problem is rare when peanut is intercropped with corn. This difference is attributed to a complex of interactions that improve Fe uptake and nitrogen (N2) fixation (Zuo et al., 2004).

J 970 2002

J 970 2002

Figure 2-2. These maps of the North Central U.S show the states of North Dakota, South Dakota, Iowa and Minnesota. The maps show the land area in each county where soybean was planted on soils with pH >7.2 in 1970 and 2002. A 160% increase in soybean production on high pH soils between 1970 and 2002 is partially responsible for an increase in the importance of iron deficiency chlorosis in soybean in this region.

Figure 2-2. These maps of the North Central U.S show the states of North Dakota, South Dakota, Iowa and Minnesota. The maps show the land area in each county where soybean was planted on soils with pH >7.2 in 1970 and 2002. A 160% increase in soybean production on high pH soils between 1970 and 2002 is partially responsible for an increase in the importance of iron deficiency chlorosis in soybean in this region.

Iron deficiency is also a problem in dry bean. Dry bean most often suffers Fe deficiency in the same soil conditions as soybean. Research in the central U.S. (Nuland et al., 1997) shows differing responses to high pH soils among 24 dry bean cultivars. The most tolerant cultivars remained green all season with normal yields and the least resistant cultivars suffered extensive yield loss. These observations were confirmed by Ellsworth et al. (1997, 1998) when they measured the physiological response to Fe deficiencies. Their results correlated with field scores (r = -0.71 to -0.49). In Tunisia, Krouma et al. (2003) observed differences in physiological responses and chlorosis development in five common bean cultivars, but did not relate the differences to field conditions. These studies indicate that some varieties are more tolerant of conditions that make Fe less available and effort needs to be placed in management and choice of the cultivar rather than in-season fertilization.

Another widely impacted crop is sorghum. Monocots, such as sorghum, release naturally occurring chelates (phytosiderophores) in response to Fe deficiency (Clark et al., 1988; Lytle and Jolley, 1991), but the magnitude of phytosiderophore release is minor compared to more tolerant crops, such as corn and wheat (Onyezili and Ross, 1993). Thus, grain sorghum is particularly sensitive to Fe deficiency problems in soils with low available Fe. Significant areas of Fe deficiency have been reported for sorghum in the Southern Great Plains region of North America including large areas in the states of Texas and Oklahoma. Iron-deficiency symptoms in sorghum are typically expressed in young plants, but become less apparent as the plant matures. The visual symptoms of Fe deficiency are spatially variable in fields, often associated with calcareous outcroppings. In addition to yield losses, sorghum that is deficient in Fe at early growth stages experiences other production challenges such as uneven flowering and delayed grain ripening (Livingston et al., 1996). In severe cases, Fe deficiency can cause a total crop failure.

Rice is moderately sensitive to Fe deficiency, but the problem is relatively rare (Dobermann and Fairhurst, 2000). When rice is produced in flooded conditions, soil Fe is reduced to the more soluble Fe+2 forms, even in calcareous soils. Zinc (Zn) deficiency is a more common problem in rice production, while in some cases there may be simultaneous deficiencies in both Zn and Fe. There are occurrences of Fe deficiency in rice in the U.S. under certain conditions: inadequate soil reduction due to low organic matter levels, calcareous subsoil conditions, and large ratios of phosphorus (P) to Fe, where low solubility iron phosphates form (Morikawa et al., 2004). Iron deficiency occurs in the Texas and Louisiana rice production areas in high pH, sandy soils with low soil Fe levels (Fred Turner, Texas A&M, Beaumont, TX, personal communication) and in areas of the Florida Everglades region on peat soils with low total Fe content where Fe fertilizer must be applied (Snyder and Jones, 1988). Iron deficiencies have also been observed in areas of rice production in Indo Gangetic Plains area of Pakistan and India. The common rice production system in this region is a rice wheat rotation, with rice culture being done by transplant (Agrawal and Srivastava, 1984; Singh et al., 2004; Timsina and Connor, 2001). Iron deficiency occurs in early transplants and depends highly on irrigation and flooding practices. Yields can be dramatically reduced, even though plants regreen as they mature. Alternatively, Fe deficiency is rare in the Arkansas and California rice production areas.

Corn is only moderately sensitive to Fe deficiency problems (Table 2-1). However, Fe deficiency of corn has been reported as a production problem in some areas of the central Great Plains and Western U.S., but is generally not a problem in the majority of the corn growing areas. However, Fe deficiency in corn is observed in the flood plains of the river valleys of Nebraska on high pH and sodium affected soils and reduces corn yields on as many as 500,000 ha (Nordquist et al., 1992) and is reported to occur on millions of hectares on a global scale (Nordquist et al., 1996). The occurrence of Fe deficiency in sodic soils suggests that the Fe uptake mechanisms are impaired as a result of root stress from soil salts. Plant selection efforts have identified moderately productive corn hybrids for these soils, but have not been able to completely eliminate yield losses associated with Fe deficiency (Nordquist et al., 1992, 1996). Due to the heterogeneous nature of Fe deficiency, one production concern is whether to select the highest yielding hybrids even if they are sensitive to Fe deficiency, or to select moderately yielding hybrids that are tolerant to Fe deficiency.

Wheat and oat are tolerant to Fe deficiency, even when grown in soils where Fe deficiency may be a problem with other crops (Table 2-1). However, both oat and wheat show significant susceptibility to Fe deficiency under animal grazing practices, a major management scheme used on much of the wheat acreage in Southern Plains region of the U.S and in the oat acreage in South Texas. Grazing semi-dwarf winter wheat during the vegetative stage is common in the Southern Plains region of the U.S. This practice can reduce grain yields, but allows for economic gains through its use as forage (Winter and Thompson, 1990). Factors such as reduced leaf area and seed weight and increased winterkill contribute to lower grain yield in grazing systems, but in some instances, the grazing has also triggered Fe-deficiency chlorosis in the vegetative regrowth for wheat cultivars that do not normally exhibit chlorosis when not grazed (Berg et al., 1993). Most of the oat grown in south Texas is grazed and grown in semiarid areas where soils are calcareous and Fe-deficiency chlorosis is a potential problem (Anderson, 1982). Grazing or clipping is known to intensify Fe chlorosis in oat in these areas as chlorosis is often enhanced following clipping in oat forage tests (Ocumpaugh et al., 1992). Differential susceptibility to Fe-deficiency chlorosis among wheat and oat cultivars likely contribute to some of the unexplained cultivar x grazing interactions reported in yields of grazed wheat and oat (Winter and Thompson, 1990; Ocumpaugh et al., 1992). In hydroponic studies it was shown that the ability to release adequate phytosiderophore following grazing is critical to Fe-deficiency chlorosis resistance in winter wheat (Hansen et al., 1995).

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