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Unlike most horticultural crops, blueberry (Vaccinium spp. section cyanococcus) prefers low-pH (4.2–5.5) soils. Other plants can acidify their rhizosphere to create a hospitable microenvironment. Southern highbush blueberry (SHB; Vaccinium corymbosum interspecific hybrids) plants do not acidify their rhizosphere in response to Fe deficiency, but other factors that affect rhizosphere pH have not been elucidated. We report results from two hydroponic experiments exploring N uptake effects on the rhizosphere pH of ‘Emerald’ SHB. Ammonium (NH4 ) uptake led to rhizosphere acidification, whereas nitrate (NO3 ) uptake led to rhizosphere alkalization. When grown in a split-root hydroponic system, roots that took up NH4 + acidified the rhizosphere to a greater extent that roots not exposed to NH4 . Rhizosphere acidification was observed even in a nontreated control. These results suggest that NH4 + uptake is the main driver of rhizosphere pH in SHB. N form effects suggest that fertilization with NO3 – might lead to undesirable rhizosphere alkalization. Soil pH is a major factor mediating plant agricultural performance. Plants in family Ericaceae (i.e., acid-loving plants) thrive in acidic soils (pH 4.2–5.5) where NH4 + is the most abundant form ofN (Marrs andBannister, 1978). When acid-loving plants are grown in neutral or high-pH soils, they exhibit stress, nutritional deficiencies, stunted growth, and low survival (Finn et al., 1993; Marrs and Bannister, 1978; PayaMilans et al., 2017; Tsuda et al., 2014). Acidic soils are rare in agricultural and landscape settings. Thus, growers must find naturally acidic soils or rely on costly soil amendments (Almutairi et al., 2017; Williamson et al., 2015) or soilless substrates (Kingston et al., 2017) for the production of ericaceous crops, such as blueberries (Vaccinium spp. section cyanococcus). Development of cultivars and practices that reduce dependence on these inputs requires a better understanding of the processes that affect rhizosphere pH. Some herbaceous and woody plants can acidify their rhizosphere (Gogorcena et al., 2001; Hawkins et al., 2008; Lei et al., 2015; Santi and Schmidt, 2009; Suzko et al., 2018; Valentinuzzi et al., 2015). Previous research suggests that plants acidify their rhizosphere to create more hospitable microenvironments for nutrient uptake and growth (Lei et al., 2015; Suzko et al., 2018). Plants acidify their rhizosphere by using plasma membranebound H-ATPases (Santi and Schmidt, 2009) and/or by exuding organic acids (Marschner et al., 1987). Cells have a basal level of H-ATPase activity, but several stimuli—such as cation uptake—can enhance this activity (Haynes, 1990). P and Fe deficiencies also increase H-ATPase activity in some plants, as roots acidify the rhizosphere to solubilize these nutrients (Gogorcena et al., 2001; Jimenez et al., 2007; Lei et al., 2015). N uptake is a major factor affecting rhizosphere pH because this element can be taken up as a cation and/or an anion. NH4 + uptake leads to rhizosphere acidification as cells extrude H to maintain their electrical charge balance (Marschner et al., 1991). On the other hand, NO3 – uptake leads to rhizosphere alkalization as the symport mechanism responsible for NO3 – movement across the plasma membrane removes H from the rhizosphere (Meharg and Blatt, 1995). Thus, the N form taken up by the plant can enhance or counteract H-ATPase-mediated rhizosphere acidification. SHB (Vaccinium corymbosum interspecific hybrids) is an economically important acid-loving plant (Perez and Ferreira, 2017). SHB plants take up N preferentially in the NH4 + form (Merhaut and Darnell, 1995). Although SHB plants do not acidify their rhizosphere in response to Fe deficiency (Nunez et al., 2015), other factors that affect SHB rhizosphere pH are unknown. This research aims to quantify the effect of NH4 + uptake on SHB rhizosphere pH change. We hypothesized that NH4 + uptake is the main driver of rhizosphere acidification by SHB. We tested this hypothesis in two separate hydroponic experiments. Materials and Methods Plant material and acclimation period. One-year-old rooted cuttings of ‘Emerald’ SHB were washed clean of substrate using tap water. Cuttings were transplanted to a previously described hydroponic system (Poonnachit and Darnell, 2004) in which each plant was exposed to 2 L of constantly aerated nutrient solution. Plants were acclimated in a nutrient solution containing 0.5 mM KNO3, 0.5 mM K2PO4, 1.0 mM MgSO4, 0.5 mM CaCl2, 0.045 mM H3BO3, 0.01 mM MnSO4, 0.01 mM ZnSO4 with 0.3 mM CuSO4, 0.2 mM Na2MoO4, and 45 mM Fe provided as Sequestrene 330 (10% iron(III)-diethylenetriamine pentaacetic acid) (Becker Underwood, Inc., Ames, IA). Nutrient solution was buffered to pH 5.5 using 5.0 mM 2-(4-morpholino)-ethane sulfonic acid (MES). Nutrient solution pH was adjusted back to pH 5.5 using 1MHCl or 1M KOH as needed, three times each week. Fresh nutrient solution was provided every 7 d to maintain nutrient concentrations. Debris, algae, and detached roots were removed from the reservoir to minimize impact on solution pH. Acclimation lasted 21 d in both experiments. Plant fresh weight (FW) was measured nondestructively at the end of acclimation. All experiments were carried out in a temperature-controlled greenhouse set to maintain air temperatures between 15.6 and 26.7 C. Natural light was supplemented by metal halide lamps with a photosynthetic photon flux density output of 367 nmol·m·s for 14 h every day. Expt. 1: Nitrogen form effect. After acclimation, one half of the plants were transferred to a nutrient solution in which 5 mM NO3 – (delivered as KNO3) was the only form of N. The other half of the plants received a nutrient solution in which 5 mM NH4 + [delivered as (NH4)2SO4] was the only form of N. Additional salts were not added to compensate for K and SO4 – concentrations. Considering that the magnitude of N uptake is much greater than K and SO4 – uptake (Havlin et al., 2013), it is reasonable to assume that most changes in rhizosphere pH were the result of N form as opposed to these counterions. In addition, KNO3 and (NH4)2 SO4 have been used as blueberry fertilizers in previous scientific studies with SHB (Nunez et al., 2015). Nutrient solutions were adjusted to pH 5.5 during nutrient solution preparation, but they did not contain any pH buffer. All other nutrient concentrations were the same as during the acclimation period. Nutrient solution in reservoirs with no plants was used as the control for each treatment. Nutrient solution pH, N uptake, and leaf chlorophyll content were measured periodically. The treatment period lasted 35 d. This experiment followed a completely randomized design with one variable (N form) and two treatments (NH4 + and NO3 ). Individual plants were the experimental unit. There were four single-plant replications per treatment. All data except for rhizosphere pH Received for publication 19 Nov. 2018. Accepted for publication 25 Feb. 2019. This research was supported by the Thad Cochran Southern Horticultural Laboratory, U.S. Department of Agriculture Agricultural Research Service under NACA agreement no. 58-6062-5-005. Corresponding author. E-mail: g.nunez@ufl.edu. HORTSCIENCE VOL. 54(5) MAY 2019 955 were analyzed using one-way analysis of variance (ANOVA) in R version 3.5.1 (R Core Team, 2017). Rhizosphere pH data were analyzed using a linear mixed model (see section Rhizosphere pH measurement). Expt. 2: Ammonium uptake effect. After acclimation, plants were transplanted to a split-root hydroponic system. Plants were held upright using garden stakes and Velcro straps (Supplemental Fig. 1). Root systems were separated into two parts of similar volume. Each half of the root system was placed in a different 1.5-L reservoir (termed reservoir A and reservoir B). In reservoir A, all plants were exposed to a complete, buffered nutrient solution that contained 5 mM NH4 . NH4 + concentration treatments were delivered in reservoir B. Seven plants were exposed to a nutrient solution containing 5 mM NH4 + in reservoir B, whereas seven different plants received a nutrient solution without N in reservoir B. NH4 + was delivered as (NH4)2SO4 in all solutions. Nutrient solutions were adjusted to pH 5.5 during nutrient solution preparation. Solutions in reservoir A were buffered with 5.0 mMMES. Solutions in reservoir B were not buffered to track pH change. All other nutrient concentrations were the same as during the acclimation period. In summary, treatments were (reservoir A/reservoir B) NH4 /NH4 + and NH4 / 0N. Reservoirs with no plants were used as controls for each treatment. Nutrient solution pH, N uptake, and leaf chlorophyll content were measured periodically. Root vitality was measured 30 d after transplant to the split-root system. The treatment period lasted 35 d. At the end of this period, plants were harvested destructively to measure plant and organ dry weight and tissue N content. This experiment followed a completely randomized design with one variable (NH4 + concentration in reservoir B) and two treatments (0 mM NH4 + and 5 mM NH4 ). Individual plants were the experimental unit. There were seven single-plant replications per treatment. All data except for rhizosphere pH were analyzed using one-way ANOVA in R version 3.5.1 (R Core Team, 2017). When appropriate, reservoir A and reservoir B were compared via paired one-tail t tests (a = 0.05). Rhizosphere pH data were analyzed using a linear mixed model (see section Rhizosphere pH measurement). Rhizosphere pH measurement. Nutrient solution pH was measured three times per week during the treatment period in both experiments using a standardized portable pH meter (Accumet AP110; Fisher Scientific, Hampton, NH). Solution crosscontamination and disturbance of fine roots was avoided during pH measurement. Rhizosphere pH for a single treatment on a single day was compared with controls using onesample single-tail t tests (a = 0.05)

Ammonium Uptake Is the Main Driver of Rhizosphere pH in Southern Highbush Blueberry