Blueberry is a high-value specialty crop and production has undergone significant growth in the last several decades, with further increases projected (Strik and Yarborough, 2005). Blueberry has great potential to become a much more important U.S. and global crop because of their high yield potential, long postharvest life as a fresh berry, health benefits, and wide consumer acceptance. However, blueberry production costs are high, constraining sustainability and profitability of the fresh market industry; thus, increased production efficiency is necessary for continued competitiveness in a global marketplace. Both establishment and maintenance costs are high in blueberry production. Estimates for field preparation and first-year establishment cost average ≈$10,000/acre for northern highbush blueberry in the Pacific northwestern United States (Eleveld et al., 2005) and as much as ≈$20,000/acre for SHB in the southeastern United States (Williamson et al., 2012). Blueberry has strict soil requirements for satisfactory growth; this is reflected in the establishment and maintenance costs. Blueberry cultivation is limited to acidic soils, usually with high organic matter, where iron is readily available and ammonium (NH4+) is the predominant N form (Erb et al., 1993; Williamson and Lyrene, 1998). Although blueberry can be grown on more typical mineral soils (i.e., higher native pH and lower organic matter), the soil inputs required to maintain productivity are extensive (Eleveld et al., 2005; Goulart et al., 1995; Korcak, 1986; Williamson et al., 2006). On sandy soils in the southeastern United States, pine bark application costs up to $6000 per acre during site preparation. Additional applications are required every 2 to 3 years to maintain plant vigor and productivity, and cost ≈$4000/acre. Among other things, these modifications lower soil pH and result in increased iron and NH4+ availability. Without these modifications, blueberry exhibits symptoms of iron deficiency (Brown and Draper, 1980), N deficiency, or both, and poor growth (Korcak et al., 1982). However, soil amendments and mulches may immobilize N, requiring higher N fertilization rates for optimum productivity (Hart et al., 2006; Williamson and Miller, 2009). The effect of soil pH on blueberry growth has been studied extensively, particularly as it relates to nutrient availability. High soil pH decreases NH4+ concentration due to increased rates of microbial conversion of NH4+ to NO3− at soil pH above 6.0 (Miller and Hawkins, 2007). Since blueberry takes up NH4-N more rapidly than NO3-N [Fig. 1 (Merhaut and Darnell, 1995; Poonnachit and Darnell, 2004)], blueberry exhibits lower tissue N concentrations when fertilized with NO3-N compared with NH4-N (Merhaut and Darnell, 1996). This, in turn, contributes to growth reductions, as containerized SHB grown with NH4-N exhibited significantly greater leaf, stem, and whole plant dry weight compared with plants grown with NO3-N [Table 1 (Merhaut, 1993; Poonnachit and Darnell, 2004)]. Limited uptake of NO3-N in blueberry has been attributed in part to low activity of NR, the enzyme responsible for reducing NO3− to nitrite. Lack of detectable leaf NR activity and low root NR activity are characteristic of cultivated blueberry species (Claussen and Lenz, 1999; Darnell and Cruz-Huerta, 2011; Darnell and Hiss, 2006; Poonnachit and Darnell, 2004).
Dry weight of ‘Sharpblue’ southern highbush blueberry grown for 16 weeks in sand culture with ammonium (NH4+) vs. nitrate (NO3−) nitrogen.
High soil pH also decreases iron availability in the soil and decreases activity of ferric chelate reductase (FCR), the enzyme responsible for cleaving and reducing Fe in the soil and making it available for plant uptake. The reduction and cleavage of the ferric-chelate at the apoplastic surface of the root epidermis has a pH optimum of 5.5–6.0 (Bagnaresi and Pupillo, 1995; Cohen et al., 1997). Thus, the pH of the rhizosphere can have a profound effect on FCR activity and subsequent ferrous iron uptake.
Although soil pH effects on nutrient availability and uptake are known, it is unclear if the requirement for low soil pH in blueberry production is due to effects on nutrient availability/uptake. Additionally, it is unclear if the requirement for high organic matter (soil amendments) is related directly to nutrient availability/uptake. Regardless of the reasons for the strict soil requirement of cultivated blueberry, the question arises as to whether there are blueberry species that exhibit wider soil adaptation than the cultivated species. If so, there is the possibility of using such species as rootstocks for commercial production to reduce soil inputs.
Sparkleberry is a wild blueberry species native to the southeastern United States (Camp, 1945), which has many characteristics that would be desirable in cultivated blueberry. Sparkleberry has a deep root system and is therefore less susceptible to drought. It grows well on soils with pH 4.5–6.5 (Stockton, 1976), low organic matter (Lyrene, 1997), low Fe availability, and N primarily in the NO3− form; i.e., soils that cultivated northern highbush and SHB blueberry tolerate poorly (Lyrene, 1997). In hydroponic systems buffered at pH 5.5, sparkleberry exhibits higher rates of NR activity (Fig. 2) and greater rates of NO3-N uptake (Fig. 3) compared with SHB (Darnell and Cruz-Huerta, 2011; Darnell and Hiss, 2006; Poonnachit and Darnell, 2004). Similarly, FCR activity and Fe uptake are both greater in sparkleberry compared with SHB (Darnell and Cruz-Huerta, 2011), although this is not consistent across all plants within those species (G. Nunez, J.W. Olmstead, and R.L. Darnell, unpublished data).
More recent work indicates that nutrient solution pH affects NO3− and Fe uptake differently in sparkleberry and SHB (R.L. Darnell, unpublished data). Nitrate uptake rate was significantly greater in hydroponically grown sparkleberry at pH 7.0 compared with uptake at pH 4.5 or compared with uptake in SHB at either pH. The increased NO3− uptake rate correlated with a 20% to 60% increase in root NR activity in sparkleberry grown at pH 7.0. Similarly, there was an increase in both Fe uptake and FCR activity in roots of sparkleberry grown at pH 7.0 compared with sparkleberry grown at pH 4.5 or SHB grown at either pH. These preliminary results suggest that both NO3− and Fe uptake, and assimilation in sparkleberry are greater than in SHB at higher rhizosphere pH and may, at least partially, account for why sparkleberry is better adapted to mineral soils than in SHB.
Varying tolerances among blueberry species to high soil pH and low soil organic matter present possibilities for rootstock use to increase adaptation of blueberries to nontraditional blueberry soils. Galletta and Fish (1971) reported broader adaptation to nontraditional blueberry soils for northern highbush blueberry grafted on rabbiteye blueberry (Vaccinium virgatum) compared with own-rooted northern highbush blueberry, with no indications of graft incompatibility after 7 to 10 years. In general, northern highbush blueberry grafted on rabbiteye blueberry was more vigorous, larger, and had lower mortality rates than own-rooted northern highbush blueberry. Ballington (1998) compared ‘Premier’ rabbiteye blueberry on its own roots with ‘Premier’ grafted on sparkleberry for four harvest seasons on a sandy soil with irrigation applied only during the establishment year. For the first harvest, yield of grafted plants was about twice that of own-rooted plants and nearly 2.5 times greater during the subsequent three harvest seasons. In southeastern China, Xu et al. (2014) compared own-rooted ‘Sharpblue’ SHB with ‘Sharpblue’ grafted on sea bilberry (Vaccinium bracteatum), a native species known to have a broad ecological adaptation. Grafted ‘Sharpblue’ had greater yield, plant height, and crown diameter than own-rooted plants for the first 3 years following planting.
Casamali et al. (2013) compared own-rooted ‘Meadowlark’ and ‘Farthing’ SHB with the same cultivars grafted onto sparkleberry rootstocks, grown in either pine-bark amended or nonamended soil in two north-central Florida locations (Citra and Archer). Own-rooted plants in pine-bark amended soil had larger canopy volumes than own-rooted plants in nonamended soil, or grafted plants in either soil treatment for both cultivars and both locations following the first complete growing season. The following year (2.5 growing seasons from planting), treatment effects on canopy volume in ‘Farthing’ were similar to what was observed in the first year (Casamali et al., 2013). However, canopy volumes of grafted ‘Meadowlark’ on either amended or nonamended soil were the same or slightly smaller than canopy volumes of own-rooted plants on amended soil (Fig. 4). Furthermore, canopy volumes of grafted ‘Meadowlark’ on nonamended soil were greater than own-rooted plants on nonamended soil (B. Casamali, R.L. Darnell, and J.G. Williamson, unpublished data) and severe chlorosis was observed in own-rooted, but not grafted plants, of both cultivars in nonamended soil (Fig. 5).
During the first fruiting year, own-rooted plants in amended soil produced the greatest yields for both cultivars (Casamali et al., 2013). However, during the second fruiting year, grafted ‘Meadowlark’ produced greater yields than own-rooted ‘Meadowlark’ at Citra, while at the Archer location, the greatest yields were obtained in both cultivars grown on their own roots in amended soil followed by grafted plants in either soil type, and own-rooted plants on nonamended soil (B. Casamali, R.L. Darnell, and J.G. Williamson, unpublished data).
Certain blueberry species, including sparkleberry, may have potential as rootstocks for northern highbush and SHB blueberry to expand commercial blueberry production into soils that are currently considered unsuitable (i.e., high soil pH and low soil organic matter). Further, the use of rootstocks may increase the productivity and longevity of blueberry plantings, especially under marginal soil conditions. Generally, tolerance of northern and SHB blueberry to adverse soil conditions such as high soil pH and low organic matter appears to be substantially greater when plants are grafted on certain blueberry species than when grown on their own roots. However, it is unclear whether size, vigor, and yield of grafted plants grown under adverse soil conditions will equal or exceed that of own-rooted plants grown under optimum soil conditions. Furthermore, scion by rootstock interactions were noted for northern highbush blueberry grafted on rabbiteye blueberry (Galletta and Fish, 1971) and SHB grafted on sparkleberry (Casamali et al., 2013), indicating a need for additional long-term evaluations of a range of blueberry cultivars on a variety of rootstock species.
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