elocation-id: e4138
Blueberry (Vaccinium corymbosum L.) production in Mexico is expanding rapidly, yet nitrogen management and salinity stress remain major challenges. This study evaluated the effects of ammonium (NH4+) and nitrate (NO3-) fertilization, with or without sodium chloride (NaCl, 30 mM), on growth, yield, and fruit quality of ‘Biloxi’ blueberry grown in coconut fiber substrate. A completely randomized 3x3 factorial design plus control was applied, varying nitrogen source, concentration (75% and 100%), and salinity. NH4+ significantly increased biomass (121.2%), flower production (316%), fruit number (231%) and yield (162.7%) compared with NO3-. A 100% N rate enhanced shoots (19.5%) and fruit count (43.4%) but reduced fruit size. Salinity reduced fruit number (-70.3%) and yield (-53.1%) without affecting vegetative growth. Significant interactions among nitrogen source, concentration and salinity influenced flowering, quality and agronomic traits. Results indicate NH4+ based fertilization improves blueberry productivity under saline conditions, supporting more efficient nitrogen management strategies.
Vaccinium corymbosum L., ammonium, nitrate, stress, quality, yield.
Over the past decade, Mexico has become one of the world’s top five producers of blueberry (Vaccinium corymbosum L.), with annual growth in cultivated area and yield exceeding 15%, mainly concentrated in Jalisco, Michoacán, Sinaloa, Baja California, and Guanajuato (Trejo-Pech et al., 2024). This expansion is driven by increasing international demand and the recognized nutraceutical properties of the fruit (Krishna et al., 2023), encouraging cultivation in regions with specific edaphoclimatic conditions: altitudes between 1 500-4 700 m, temperatures ranging from 3-17 °C, acidic soils (pH 4-5) and electrical conductivity (EC) between 0.25 and 1.5 dS m-1 (Meléndez-Jácome et al., 2021).
As a calcifuge species, blueberry shows a preference for ammonium (NH4+); however, several studies report that nitrate (NO3-) can enhance nitrogen assimilation and improve physiological and agronomic traits (Alt et al., 2017; Anwar et al., 2024). While NH4+ can stimulate early growth, it also increases substrate EC, potentially inducing salt stress and limiting plant development (Machado et al., 2014; Hirzel et al., 2024).
In contrast, NO3- fertilization has been shown to promote photosynthesis, accumulation of antioxidant compounds, and stress tolerance, particularly under saline conditions (Leal-Ayala et al., 2021; Cárdenas-Navarro et al., 2024). Since salt stress disrupts key physiological processes but can also induce beneficial antioxidant synthesis (Krishna et al., 2023), it is crucial to evaluate nutritional strategies that mitigate its effects. Therefore, the objective of this study was to assess the impact of different NO3-/NH4+ ratios, combined with NaCl, on blueberry growth and yield, aiming to optimize nitrogen management under salinity stress.
The experiment was conducted under low-tech greenhouse conditions in Ascensión, Aramberri, and Nuevo León, Mexico (24° 20’ 14.96’’ N, 99°56’ 9.5’’ W, 1961 masl, mean annual temperature 15.6 °C). Blueberry plants (Vaccinium corymbosum L.), variety Biloxi, propagated in vitro and acquired in 1 L pots, were transplanted on October 2, 2023, into 50 × 50 cm plastic bags filled with 20 L of prewashed, medium-texture coconut fiber. During acclimatization, plants received a standard nutrient solution (SN1) composed of K2SO4, KH2PO4, Ca (NO3)2 4H2O, MgSO4 7H2O, (NH4)2SO4 and micronutrients, following Frías-Ortega et al. (2020).
A completely randomized design was used with nine treatments: eight from a 3 × 3 factorial combination of nitrogen source (NO3- or NH4+), concentration (75% or 100%) and salinity (0 or 30 mM NaCl) and 50/50 NO3-/NH4+ (control). Treatments were: T1: NH4+ 100%; T2: NH4+ 100% + NaCl; T3: NH4+ 75%; T4: NH4+ 75% + NaCl; T5: NO3- 100%: T6: NO3- 100% + NaCl; T7: NO3- 75%; T8: NO3- 75% + NaCl; T9: 50% NH4+ / 50% NO3-. Each treatment had 10 replicates (one plant per pot), arranged in a 48 m² area, with rows oriented north-south, spaced 0.3 m between pots and 0.8 m between rows. Anti-aphid mesh was used to prevent soil contact.
Treatments began on April 1, 2024. Irrigation was manual and daily, using treatment-specific nutrient solutions (Table 1), with 6-20% drainage. Water characteristics: EC 0.51 dS m-1, pH 6.95, containing Ca2+, Mg2+, Na+, HCO3-, Cl-, and SO42-.
Fertilizers included were: A) K2SO4; B) Mg (NO3)2 6H2O; C) (NH4)2 SO4; D) KH2PO4; E) KNO3; F) Ca (NO3)2 4H2O; G) CaSO4 2H2O; H) MgSO4 7H2O; I) (NH4)2 SO4 and J) micronutrients (Table 1). For salinity treatments (T2, T4, T6 and T8), irrigation once weekly included 30 mM NaCl only.
Every 14 days for 182 days, two tagged stems per plant were measured for length (cm), diameter (mm), number of leaves, secondary shoots, and buds. Flower count included fully developed white corollas. Fruits >5 mm in diameter were counted. On March 4, 2025, shoot biomass was collected by cutting stems 30 cm above the crown, chopped into 3-5 cm pieces, weighed fresh, and dried at 28.3 °C until constant weight.
Harvest began 180 days after treatment initiation. For each harvest, a random fruit was measured for polar and equatorial diameter (mm). All fruits with 8-13 mm diameter was considered for yield analysis, excluding outliers (Cortés-Rojas et al., 2016). Total fruit number and accumulated yield (g) per plant were recorded. Juice was extracted (0.5 ml), and soluble solids content (°Brix) and juice temperature were measured using a digital refractometer (HANNA HI96801).
Plants fertilized with NH4+ exhibited significantly higher values across all measured variables compared to those treated with NO3-: stem length (18.1%), diameter (21.3%), number of leaves (41.7%), secondary shoots (70.6%), total shoots (42.5%), flowers (316%) and fruits (193.5%). Biomass also increased by 121.2% (209.4 g vs 94.7 g with NO3- (Table 2). These findings align with reports showing NH4+ promotes root development and nutrient absorption (Arias et al., 2024), although proper pH management is critical for effective assimilation (Jiang et al., 2019).
[i] SL= stem length (cm); SD= stem diameter (mm); NL= number of leaves; SS= secondary shoots; TS= total shoots; Fo= number of flowers; Fr= number of fruits; DM= dry matter; N type (N); concentration (C) and NaCl (S, mM). Different letters in the means per column in each factor indicate significant differences (Tukey, p≤ 0.05).
In contrast, the lower efficiency of NO3- may stem from its higher energetic demands for reduction (Ali, 2020; Berger et al., 2020). Nonetheless, Alt et al. (2017) observed that blueberry plants can adapt to NO3- and activate nitrate reductase, highlighting their metabolic plasticity. Increasing nitrogen concentration to 100% further enhanced shoot number (19.5%), flowers (20.1%), and fruits (43.4%) compared to 75% N (Table 2). These results support the role of nitrogen in synthesizing amino acids, enzymes, and hormones essential for floral development (Santiago and Sharkey, 2019; Cárdenas-Navarro et al., 2024). Notably, fruit production rose by 43.3% in plants with 100% N vs 75% (Table 2) and 87.4% in harvested fruits (Table 3).
Salinity at 30 mM NaCl reduced secondary shoots (-30.4%), flowers (-28.8%) and fruits (-28.9%), while vegetative growth (SL, SD, NL, TS) remained unaffected (Table 2). These effects are attributed to Na+ displacing K+, which inhibits flowering-related enzymes (Wu, 2018; Atta et al., 2023). The findings align with Molnar et al. (2024), who documented shoot growth suppression under salt stress in vitro. Significant interactions were observed between N source × concentration (N × C) for most agronomic traits (SL, SD, NL, TS, Fo, Fr, DM (Table 2). NH4+ at 100% showed the strongest response. At 75%, differences between N sources diminished, but NH4+ still outperformed NO3- in terms of shoots, flowers, and dry matter. This suggests a concentration-dependent ionic modulation (Cárdenas-Navarro et al., 2024).
Under salinity, NH4+ fertilization further reduced flowering, while NO3- maintained production, likely due to NH4+ induced apoplastic acidification that exacerbates Na+ influx and limits K+ and Ca2+ uptake (Shilpha et al., 2023). Hence, nitrogen form and concentration must be jointly optimized under saline conditions.
NH4+ significantly outperformed NO3- in total fruit number (231%) and cumulative yield (162.7%) compared to NO3- and 147.8% and 68% versus control, respectively (Table 3). Fruit sizes 9-12 predominated in both NH4+ and NO3- treatments, although NH4+ slightly increased the share (81.4% vs 78.5%). At 100% N, fruit count per plant increased by 87.2% vs 75% and 110.2% vs. control. Cumulative yield rose by 35.7% over control and 40.8% over 75% N (Table 3). Fruit sizes 9-12 also predominated at higher N. These results highlight the importance of N in chlorophyll production and photosynthesis, improving energy availability for reproductive development (Zhang et al., 2023; Yang et al., 2023). A saturation points around 206-222 kg N ha-1 has been reported (Fang et al., 2020a), validating 100% as optimal. Doyle et al. (2021) emphasized NH4+ efficiency in translocating carbohydrates without causing osmotic stress.
Salinity (30 mM NaCl) reduced total fruit production by 70.3% compared to non-saline conditions (116.9 vs 68.6 fruits plant-1) and yield dropped 53.1% (Table 3). Fruit sizes decreased, especially size 10 (-94.3%), indicating a restriction in parenchyma expansion due to osmotic stress (Denaxa et al., 2022). The N × C interaction was significant for fruit sizes 8-12, total fruit and yield. NH4+ at 100% achieved the highest values, while NO3- performed better at 75%, confirming the importance of optimizing both source and concentration. Neither C × S nor N × C × S interactions showed significant differences, except for size 8 (Table 3), indicating that salinity effects were largely independent of N source.
NH4+ and NO3- treatments showed no significant differences in fruit diameter, weight, firmness or soluble solids (Table 4), consistent with previous studies (Petridis et al., 2018; Anwar et al., 2024). These traits are primarily genetically controlled and linked to source-sink dynamics (Ferrão et al., 2018).
However, NH4+ increased juice temperature by 17.9%, likely due to enhanced respiration and sugar accumulation (Shilpha et al., 2023; Duan et al., 2023). Soluble sugars rose by 8.7% with NH4+ vs 0.5% with NO3-. At 100% N, fruit size and weight decreased (-7.1% ED, -5.5% PD, -16.3% weight), likely due to resource dilution among more fruits (Jorquera-Fontena et al., 2018; Doyle et al., 2021). Soluble solids remained stable, indicating homeostatic sugar transport (Sellami et al., 2019). Juice temperature increased by 12.2% (100%) and 10.6% (75%).
Salinity (30 mM NaCl) caused a significant reduction in ED, PD, and weight (-8%, -6%, -18.6% (Table 4). Soluble solids increased slightly (7.9%) as a stress response, and juice temperature rose by 14%. The N × S interaction was significant for ED, PD, firmness, and TSS. The combined N × C × S interaction only affected juice temperature, indicating that thermal accumulation is particularly sensitive to nutrient-salinity interactions (Table 4). Moderate N supplies enhance NO3- transport, GS/GOGAT activity and osmoprotection (Nazir et al., 2023; Farvardin et al., 2020). Additionally, apoplastic redox signaling explains reduced flowering under high NH4+ and salinity (Kesawat et al., 2023).
Nitrogen source and concentration, in interaction with salinity, significantly influenced the vegetative and reproductive performance of blueberry plants. Ammonium fertilization consistently promoted greater shoot vigor, flowering, and fruit set compared to nitrate, particularly at higher nitrogen concentrations. In contrast, saline conditions markedly reduced reproductive development, while exerting minimal effects on vegetative growth. These findings highlight the critical importance of optimizing nitrogen form and dosage to enhance productivity and mitigate the adverse effects of salinity. The significant interactions among nitrogen source, concentration, and salinity stress reinforce the need for integrated nutrient management strategies tailored to saline environments in blueberry cultivation.
To the research support staff of the Antonio Narro Autonomous Agrarian University (UAAAN) and to the CONACYT graduate scholarship.
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