Pecan (C. illinoinensis) is the only widely grown commercial nut tree species native to North America. The native distribution of pecan is primarily along the Mississippi River Valley as far north as southeastern Iowa and south to Mississippi and Louisiana. Isolated populations of native pecans exist in Mexico, extending even as far south as Zaachila, Oaxaca, and Mexico (Janick and Paull, 2008; Thompson and Grauke, 1991). The native pecan region includes humid and semiarid areas with harsh to mild winters and annual rainfall between 660 and 1300 mm (Sparks, 2002, 2005).
In the southwestern United States, improved pecan cultivars have become a major driver for the agricultural economy with combined in-shell production value of $182 million in 2014 for New Mexico and Arizona (USDA-NASS, 2015). One important way in which the growing environment in the southwestern pecan production region differs from that of the pecan native range and southeastern U.S. growing areas is soil pH. Southwestern pecan orchard soils typically have alkaline pH and are calcareous, whereas southeastern U.S. soils tend to be neutral to acidic (Chang, 1953; Worley, 1994). Plant available Mn, more specifically Mn2+, and other micronutrients are less soluble and poorly available for root uptake by pecan trees in those alkaline soils (Chang, 1953; Lindsay, 1979; Sims, 1986; Sparks, 2000) and this results in micronutrient deficiency symptoms that are commonly visible in southwestern U.S. pecans orchards. The complexity of Mn availability in soil was suggested by Millaleo et al. (2010) and Rule and Graham (1976) who found soil Mn availability increased with high pH under white clover (Trifolium repens L.), but Mn availability decreased with higher pH under tall fescue (Festuca elatior L.), indicating that soil chemistry, plant species responses at the plant root to soil interface, and microorganism activity also may affect Mn availability (Barker and Pilbeam, 2015).
Zinc nutrition has been the subject of much intensive research in pecan since the early 1930s, when the “little leaf” and “rosette” symptoms were first identified as symptoms of zinc (Zn) deficiency in pecan (Alben et al., 1932a, 1932b). Commercial pecan producers in the southwest today typically make multiple Zn fertilizer applications each growing season as part of their regular mineral nutrition programs (Heerema, 2013; Walworth and Pond, 2006). Manganese, on the other hand, has received relatively little attention from pecan researchers or growers, despite the roles it plays in plant carbon fixation.
Manganese is best known for its essential role in Pn, specifically on the oxidation side of the PS II complex in the oxygen reaction center, and as a coenzyme for biosynthesis of chlorophyll (Epstein and Bloom, 2005; Marschner, 1995). According to Kriedemann et al. (1985), inhibition of Pn occurs in wheat even with moderate Mn deficiencies but chloroplast ultrastructure and chlorophyll breakdown (leading to visible symptoms of deficiency) occurs only with severe deficiencies. In the 1960s, Mn deficiency and its negative impact on Pn was measured in spinach and tomato where there was a 50% to 70% reduction in photophosphorylation in isolated chloroplasts and rapid declines in Hill reaction activity (Spencer and Possingham, 1960, 1961).
Sufficient Mn concentration ranges in pecan leaflet tissue have been reported to be between 200–500 µg·g−1 (Jones et al.,1991) and 150–500 µg·g−1 (Robinson et al.,1997). Smith et al. (2001) published the first report of Mn deficiency in pecan. In their study, they showed that trees growing in a Texas soil with pH 7.2 had suppressed Mn uptake such that leaf concentrations were only 1 to 18 µg·g−1 and “shoot growth was short with pale green foliage but no discernible pattern of chlorosis.” Foliar-applied Mn sulfate fertilizer increased leaf Mn concentrations to 138 µg·g−1 and eliminated the chlorotic symptoms (Smith et al., 2001). Henriques (2003) showed that with decreasing Mn availability in field grown ‘Kiowa’ pecan trees, total chlorophyll content and Pn rates decline dramatically, below 11 µg·g−1 Mn leaf tissue concentrations. In that study, without any Mn fertilizer applications, the “mildly deficient” plants (mean leaf Mn 11 µg·g−1) showed a 44.3% reduction of Pn and the “severely deficient” plants (mean leaf Mn 7 µg·g−1) showed a 62.9% reduction of Pn when compared with plants with mean leaf Mn concentration of 79 µg·g−1. In a second study, Henriques (2004) suggested that pecan adapt to Mn deficiency by reducing the chloroplast numbers while preserving PS II function in existing chloroplasts, which in turn reduces metabolic costs associated with low Mn concentrations. Other plant species form disorganized chloroplasts with low chlorophyll content, such as Spinacia oleracea and Senna obtusifolia (Homann, 1967).
Visible Mn toxicity is very rare in the southwestern United States, but Núñez-Moreno (2009) reported reduced shoot growth and fruiting of ‘Western’ pecan in Arizona because of increased leaf Mn levels. Severely affected trees in that study had leaf Mn concentration of 4034 µg·g−1, whereas unaffected trees had 1620 µg·g−1 leaf Mn. Similarly, in a study involving potted ‘Desirable’ pecan trees, the toxic Mn levels in leaf tissue were 4525 µg·g−1 (O’Barr et al., 1987). In Oklahoma, leaf Mn levels in ‘Western’ pecan trees have been reported as high as 2244 µg·g−1 with no visible phytotoxicity symptoms (Smith and Cotton, 1985). Among other plant species, maize has a much lower tolerance to Mn toxicity with a critical threshold of only 200 µg·g−1 resulting in leaf death (Marschner, 1995). Soybean has a threshold of up to 600 µg·g−1, but sunflower can tolerate as high as 5300 µg·g−1 Mn in plant tissue before leaf tissue becomes chlorotic and eventually dies (Edwards and Asher, 1982; Marschner, 1995).
The NMSU Cooperative Extension Service recommends 100–300 µg·g−1 Mn in July sampled leaflet tissue for New Mexico pecan trees and University of Arizona Cooperative Extension recommends 104–674 µg·g−1 (Heerema, 2013; Walworth et al., 2011). New Mexico pecan orchards showed, on average, only 85 µg·g−1 Mn in leaf tissue (McCaslin and Boyse, 1999; Pond et al., 2006), but the level of Mn at which Pn is optimum is not known. Symptoms of micronutrient deficient plants are not always visible even when the plant is deficient and this well-documented phenomenon is known as “hidden hunger” (Brady and Weil, 2007; Heerema, 2013). On a nutrient response curve, hidden hunger occurs when the concentration of the nutrient in the leaf is low enough to suppress plant performance (e.g., growth, yield) but not low enough for visible symptoms to be expressed.
The objective of our study was to investigate Pn rates in relation to increasing Mn concentrations. To accomplish this, we characterized the effect of foliar Mn fertilizer applications on leaf Mn and Pn and related physiological parameters over a broad range of leaf Mn concentrations.
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