Nickel is an often-overlooked plant (Brown et al., 1987, 1990) and animal (Welch and Graham, 2005) essential micronutrient. Although Ni deficiency in plants severe enough to trigger visual symptoms is relatively rare, compared with other essential micronutrients, both visual and non-visual deficiencies may be more common than generally supposed. This is partially because of antagonistic interactions between Ni and certain first-period transition metals (Wood, 2010). There is a dearth of information regarding the physiology of Ni’s interaction with other essential and beneficial micronutrients; however, excessive tissue zinc (Zn) or copper (Cu)—i.e., a high Zn:Ni or Cu:Ni ratio—can trigger symptoms of Ni deficiency (Wood, 2010). Because of relatively great physiochemical similarity between Fe and Ni, it is likely that excessive endogenous Fe can disrupt Ni-dependent physiology enough to trigger economic crop loss. A natural consequence of insufficient understanding is accidental induction of Ni deficiency in crops resulting from either excessive supplemental fertilization with certain trace metals and/or cropping on mineralized soils relatively rich in these metals.
Pecan trees growing in commercial orchards, yards, gardens, and nurseries often exhibit Ni deficiency during early spring when canopies are rapidly expanding. Ni deficiency often manifests itself as a potentially fatal orchard replant malady when young transplants replace missing trees in mature orchards or are planted in second-generation orchard sites (Wood et al., 2003a, 2003b, 2004, 2006a). Incidence and severity of deficiency vary with tree age, or size, and on the nature of the action and interaction of several biotic and abiotic soil factors (Wood et al., 2006b). Severe Ni deficiency can kill young pecan trees (Wood et al., 2004), supporting conclusions by Brown et al. (1987, 1990) that Ni is an essential nutrient element for higher plants. The fundamental cause(s) of Ni deficiency in soils containing sufficient Ni to meet plant needs vary but include nematode damage to feeder roots (Nyczepir et al., 2006), excessively cool and/or dry soils during early spring (Wood et al., 2006b), excessive Zn and/or Cu (Wood, 2010), and possibly excessive long-term use of glyphosate (Yamada et al., 2009).
Iron fertilizers are typically “chelates” that bind Fe3+ (ferric, or oxidized Fe). A common form is Fe-DPTA. Iron (Fe3+) chelates bind to the cytoplasmic plasmalemma, where, in dicots, sequestered Fe3+ is chemically reduced to Fe2+ before release from the chelant molecule and subsequent transport across the plasma membrane into the cytoplasm (Chaney et al., 1972; Romheld and Marschner, 1986). Roots can also absorb small amounts of chelants (Tiffin and Brown, 1961; Tiffin et al., 1960; Weinstein et al., 1951), which in turn can disrupt plant processes by sequestering divalent or trivalent metal ions needed for physiologically active complexes such as metalloenzymes. Pecan orchards, especially those established on relatively high pH soils, occasionally receive Fe-DPTA sprays for correction of Fe deficiency. Other field, vineyard, nursery, and hydroponic crops also receive Fe-DPTA on occasion.
Excessive orchard fertilization can trigger Ni deficiency, especially if excessively high soil/tissue Zn and/or Cu reduces the physiological availability of Ni within the plant (Wood, 2010; Wood et al., 2003a). Pecan foliage often exhibits visible Ni deficiency symptoms although the absolute foliar Ni concentration exceeds the apparent “lower critical” concentration (Nyczepir et al., 2006) of ≈0.85 μg·g−1 dry weight, thus indicating that other nutrient elements can affect endogenous Ni bioavailability/use. Such micronutrient interactions are common in plants, especially in situations of extreme soil pH or metal composition (Kabata-Pendias, 2001). These interactions can trigger chemical stress linked to either antagonistic or synergistic effects on root uptake and/or cellular/enzymatic bioavailability/use.
There are reports of Ni on Fe antagonism in which high Ni reduces endogenous Fe concentration and/or bioavailability (Chen et al., 2009; Ghasemi et al., 2009; Hewett, 1953; Khalid and Tinsley, 1980; Koch, 1956; Kovacik et al., 2009; Misra and Dwivedi, 1977; Nicholas and Thomas, 1954; Nishida et al., 2012). However, there is little information regarding the reverse Fe on Ni antagonism, especially in woody perennials. Cataldo et al. (1978) found that Fe2+ suppresses Ni2+ absorption and translocation in soybean (Glycine max), whereas Wallace et al. (1977a) found that Fe3+ (as Fe-EDDHA) did not suppress Ni concentration in foliage of bush bean (Phaseolus vulgaris). Khalid and Tinsley (1980) concluded that in annual rye grass (Lolium multiflorum), it is the Ni:Fe ratio, rather than absolute concentration of either, in plant tissues and organs that is most tightly associated with reduced Fe bioavailability/use under high Ni conditions. The reverse Fe on Ni antagonism merits investigation. This study reports the effect of Fe-DPTA and DPTA on induction of Ni deficiency in pecan and documents a Fe on Ni antagonism in a long-lived woody perennial crop.
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