In 2018, New Mexico represented 34% of United States total production of improved cultivar pecan nuts (USDA-NASS, 2019). Pecan represents a major economic crop for the southwest growing region, and research focusing on plant mineral nutrition is crucial to improve pecan productivity. The nutritional status for pecan orchards may be assessed by comparing foliar tissue nutrient concentrations with published recommended levels (Flynn et al., 1999; Heerema, 2013). A shortage of N inhibits flower induction and can cause flower abortion in pecan trees (Acuña-Maldonado et al., 2003). Trees show leaf chlorosis (Epstein and Bloom, 2005) as a response to N deficiency, and in severe cases, leaves will prematurely senesce and abscise (Heerema, 2013; Taiz and Zeiger, 2010). In pecan trees, inadequate N levels in soils can compromise nut development and quality (Heerema, 2013; Heerema et al., 2014).
Pecan leaf greenness and Pn activity are affected by leaf tissue concentrations of N and micronutrients. Heerema et al. (2017) found that zinc (Zn) application via fertigation increased Pn on immature pecan trees. Sherman et al. (2017) observed an improvement in Pn under foliar applications of manganese (Mn) on nonbearing pecan trees. Heerema et al. (2014) suggested monitoring leaf greenness (i.e., SPAD) on pecan fruiting shoots to plan for an efficient N application in late-season to sustain optimum Pn levels. However, the interactions between N and Ni has not been studied.
Additionally, the application of N and micronutrients can positively influence yield and growth. For example, Caliskan et al. (2008) concluded that an application of N combined with an iron (Fe) chelate compound (FeEDTA) can positively improve early growth and final yield of soybean in Mediterranean-type soil. For pecan, Walworth et al. (2017) reported pecan nut yield increases and improved tree growth due to soil Zn fertilization. Furthermore, Badshah and Gohar (2013) reported that N (5% concentration) and Zn (0.25% concentration) significantly increase pecan seedling height, number of leaves, leaf area, root diameter, and number of roots.
Increased yield has long been reported in certain systems when Ni fertilizers are applied; for example, Roach and Barclay (1946) found that potato (Solanum tuberosum L.) had higher yields under foliar Ni applications. Years later, Brown et al. (1987) demonstrated that barley (Hordeum vulgare L. cv. Onda) failed to complete its life cycle, and plants did not produce viable grain, under solution culture growing conditions with low Ni concentration. The results of this study helped to support the role of Ni as an essential plant micronutrient.
Currently, the most beneficial response to Ni was observed when N was supplied as urea (Bai et al., 2006). Ojeda-Barrios et al. (2016) observed that urease activity was positively related to the foliar level of Ni in pecan leaves. Because pecan trees use a ureide-transporting mechanism that requires Ni for the proper functioning of urea metabolism, Ni deficiency might affect ureide catabolism and affect the availability of N for growth and development processes (Bai et al., 2006, 2008). In addition, low urease activity in leaves will result in large urea accumulation that causes leaf tip necrosis (Bai et al., 2006).
Nickel absorption by plant roots can be limited in alkaline soils that exhibit a significantly high pH (Wood and Reilly, 2007b) and causes Ni deficiency that reduces plant growth, Fe uptake, disrupts N metabolism, and accelerates plant senescence (Bai et al., 2006). Ni deficiency can predispose trees to diseases that contribute to inconsistency in quality and quantity of nut production, which causes a reduction in pecan orchard profit (Wood and Reilly, 2007c). Various authors had proposed a pecan leaf Ni concentration to be above 2.5 mg·kg−1 (Heerema, 2013; Smith et al., 2012) and a normal leaf Ni concentration to be within 8.5 to 14.2 mg·kg−1 (Pond et al., 2006).
Nickel deficiency symptoms have been reported in pecan orchards from New Mexico and Arizona, further adding to the need to evaluate Ni in pecan nutrition (R. Heerema, personal communication). Nickel deficiency is expressed as a disorder known as mouse ear (ME; Wood and Reilly, 2007a; Wood et al., 2004a, 2004b). Mouse ear in pecan is characterized by small, roundish leaflets (Wood et al., 2004a, 2004b), and it was originally thought to be caused by Mn, copper (Cu), or Zn deficiency (Gallaher and Jones, 1976; Gammon and Sharpe, 1956; Wood et al., 2004b). Wood et al. (2004b) recommended the application of Cu or phosphorus (P; which contained trace amounts of Ni) to correct ME. Wood et al. (2004c) provided evidence of a possible role of Ni in higher plants. Further evidence from that study indicated that foliar Ni applications on pecan trees, soon after budbreak, prevent or minimize the incidence of ME.
Another disorder related to pecan Ni deficiency is water stage fruit split (WSFS) (Wood and Reilly, 2006a). This disorder expresses itself as splits over the length of the shell; the affected nuts eventually drop prematurely, typically occurring after periods of heavy rain and high humidity (Heerema, 2013; Heerema et al., 2010). Nickel deficiency might influence WSFS (Wells and Wood, 2008), because the shell is mostly composed of lignin, a molecule that provides rigidity to different plant structures. If Ni deficiency is present, the shell might not be hard enough to retain the liquid endosperm, creating a shell rupture (Wood and Reilly, 2006a). Nickel may act as a cofactor agent in precursor metabolic processes that produce lignin (Wood and Reilly, 2006a); consequently, insufficient Ni condition may cause a measurable reduction in lignin formation.
Lignin is a complex molecule formed in the cell walls of living plants (Taiz and Zeiger, 2010). Lignin provides strength to cell walls, facilitates water transport (Hatfield and Fukushima, 2001), and serves as protection to the plant when it binds to cellulose and proteins. This protection blocks the growth of pathogens by acting as a barrier to further infection or wounding (Taiz and Zeiger, 2010). In pecan trees, lignin is found in the nut shell and in large amounts in the trunk, branches, leaves, fruit, and in xylem cells (Kutscha and Gray, 1970).
Wood et al. (2006b) demonstrated that, under Ni deficiency conditions, pecan branches and shoots showed visible symptoms of brittle, and they were easily broken by hand or in the presence of high winds. A potential side effect of improving Ni nutrition is the impact that might have on lignin content; therefore stronger woody tissue will be expected (Wood and Reilly, 2007b) and other metabolic process (e.g., Pn) might be affected.
The objective of this study is to determine the effects of Ni and N application rates on Pn activity, leaf greenness (SPAD), and shoot lignin concentration over time in nonbearing pecan trees. This study hypothesizes that pecan trees receiving higher amounts of N and Ni will have higher Pn levels, leaf greenness, and branch lignin concentration than trees not receiving Ni and N over a 2-year period.
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