Abstract
Candidatus Liberibacter asiaticus (CLas), which causes huanglongbing (HLB) in citrus trees, has a great impact on tree root health, fruit development, and juice quality. HLB-affected trees have a fibrous root density loss of ∼30% to 80%, resulting in the limited capacity of citrus trees to uptake nutrients. Therefore, this study was conducted for 3 years to 1) assess the temporal changes in root density as a result of varied fertilization, 2) determine dynamics of HLB with regard to root growth and distribution as a result of varied fertilization for Valencia orange trees, and 3) evaluate the impact of varied fertilization rate and method of fertilization on fruit yield for HLB-affected trees. Macronutrients and micronutrients were applied at varying fertilization rates (0×, 1×, 2×, and 4×, of University of Florida guidelines). Root scans were done using minirhizotrons at 0 to 19.1 cm, 19.1 to 40.7 cm, 38.2 to 59.8 cm, and 57.3 to 78.9 cm soil depths. Results obtained from the study showed that root growth and distribution were greater in 0 to 19.1 cm than 19.1 to 40.7 cm to 57.3 to 78.9 cm soil depths. Thus, root growth decreased (P < 0.0004) with increasing soil depth due to variation in nutrient availability for tree uptake. Increased nutrient availability at occurrence of physiological processes in citrus trees also influenced root growth and distribution, resulting in root growth flushes in the months of Nov to early Feb and Jul to early Aug. Fruit yield was significantly different between treatments in 2 of the 4 years of the study (P = 0.001 and P = 0.003), and largely ascribed to soil fertilization of micronutrients compared with foliar. Therefore, at higher fertilization rates, particularly via soil application, nutrient availability was increased, thus promoting root growth and distribution and fruit yield in HLB-affected orange trees.
Citrus production in Florida accounts for 51% of national production (US Department of Agriculture, National Agricultural Statistics Service 2023). Production in the past 18 years has plummeted largely due to a devastating disease called citrus greening aka huanglongbing (HLB) (Kwakye et al. 2022a). Some of the key effects of HLB on citrus trees include severe defoliation, root loss up to 70% to 80%, yield reduction and excessive fruit drop (Graham et al. 2013; Johnson et al. 2014, 2020, 2021; Kumar et al. 2018; Kadyampakeni et al. 2014). The causal pathogen for HLB is Candidatus Liberbacter asiaticus (CLas) and has a great impact on tree root health (Rossi et al. 2019). CLas blocks phloem tissues, disrupting the functionality of phloem in the transportation of sugars from sources to sinks (Graham 2013; Bendix and Lewis 2018; Wang et al. 2017; Welker et al. 2021). The mechanism for phloem plugging that occurs in HLB-affected plants is ascribed to the induction of phloem proteins (P-proteins) (Achor et al. 2020). These gel-forming proteins are shown to undergo a rearrangement in the sieve elements after injury or irradiation and play a role in plugging of sieve plates to maintain turgor pressure within the sieve tube after injury and during pathogen and pest infection, but their exact role in these processes is still unclear (Knoblauch and van Bel 1998; Knoblauch et al. 2001). The leaves act as sources due to production of sugars in the photosynthetic process, and roots are sinks due to a higher demand of sugars for growth and nutrient uptake (Johnson and Graham 2015). There is no proven management option for prevention of HLB-associated root loss yet (Johnson et al. 2021); however, it is suggested that for HLB-affected trees, root density can be improved by altering soil applications through modification of irrigation and fertilization practices to balance water and nutrient requirements of the trees (Johnson and Graham 2015). The citrus root system is important for anchorage and transport of water and nutrients from the soil to the aboveground components (Kramer and Boyer 1995; Morgan et al. 2007; Rossi et al. 2019). The root system consists of structural and fibrous roots. Structural roots are for anchoring the tree, and the fibrous roots are for water, carbohydrate, and nutrient uptake (Johnson et al. 2021; Rossi et al. 2019). In an HLB-endemic environment, as is the case for Florida, citrus fibrous root development has been confined to the top 15 cm soil layer, thereby limiting tree capacity to uptake water and nutrients at lower depths compared with pre-HLB years (Atta et al. 2020; Morgan et al. 2006; Shahzad et al. 2020). Flooded soils or poorly drained soils cause root death due to anaerobic conditions, resulting in reduced productivity. To overcome this challenge, growers have adopted grove soil management practices such as production on raised beds (Simpson et al. 2020). For example, on the Florida Flatwoods where soils are poorly drained, trees are usually planted on double-row raised beds, with a crown of ∼90 cm to 120 cm above the bottom of the furrow (Vincent et al. 2021). In contrast, the Central Florida Ridge soils are well drained, and additional drainage measures such as raised beds are not necessary (Vincent et al. 2021). One strategy being explored is the use of improved and balanced nutrition to overcome some of the devastating effects of HLB. Emerging results and research for more than 4 years have shown that use of elevated macronutrients and micronutrients applied in optimal ratios could reverse some of the negative effects of HLB, including root loss and yield reduction (Atta et al. 2020, 2021; Morgan et al. 2016; Kwakye et al. 2022a, 2022b; Uthman et al. 2020a; Zambon et al. 2019). In addition, higher fertilization rates help improve tree growth, fruit yield, and fruit quality (Obreza et al. 2020). Considering that Florida soils are sandy (>95% for citrus producing regions), splitting fertilizer application into smaller doses is recommended to avoid applying too much fertilizer at once. This can result in toxicity and cause nutrient leaching because of heavy rainfall or irrigation (Atta et al. 2021; Morgan et al. 2021; Obreza and Schumann 2010; Zekri et al. 2021). Currently, the Florida fertilization guidelines for citrus were developed before HLB, and other research efforts have found that elevated concentrations of some nutrients tend to be therapeutic for HLB-affected citrus trees (Kadyampakeni and Chinyukwi 2021; Morgan et al. 2016; Zambon et al. 2019) but such impacts might be variety- and site-specific and warrant extensive investigations. Thus, there is dire need to assess the impact of altered nutrient ratios and fertilization rates to reverse the detrimental effects of HLB on tree growth (particularly on the root system) and fruit yield. The objectives of this research were to 1) assess the temporal changes in root density as a result of varied fertilization, 2) determine dynamics of HLB with regard to root growth and distribution as a result of varied fertilization for Valencia orange trees, and 3) evaluate the impact of varied fertilization and method of fertilization on fruit yield for HLB-affected trees. The underlying hypothesis was that elevated macronutrients (applied to the soil) and micronutrients (applied to the foliage or soil) would rehabilitate and restore root health in an endemic HLB world and improve overall tree performance over time.
Materials and Methods
Site description and experimental design.
The study was conducted on the Central Ridge region at the University of Florida Institute of Food and Agricultural Science (UF/IFAS) Citrus Research and Education Center (CREC), Lake Alfred, FL (28°06′28.6″N, 81°41′07.8″W) and on a citrus grove of the southwest Flatwoods near Clewiston, FL (26°44′20.851″N, 81°4′54.568″W). The two sites have ‘Valencia’ orange trees (Citrus sinensis L. Osbeck) on Swingle citrumelo (Citrus paradisi Macf. × P. trifoliata L. Raf.) rootstock planted in 2012 on the Ridge site and 2013 in Clewiston. The Ridge site are Entisols classified as hyperthermic, uncoated lamellic quartzipsamments family (US Department of Agriculture, Natural Resources Conservation Service 2019). These soils are excessively drained and formed from eolian deposits and sandy marine deposits (Table 1). The slope of the Ridge soils is 0% to 5% (US Department of Agriculture, Natural Resources Conservation Service 2019). Soils at the southwest Flatwoods site are Entisols of the siliceous, hyperthermic family of Mollic Psammaquents (Table 1). The soils are poorly drained, rapidly permeable soils that are formed in sandy marine sediment underlain by limestone. The slope of these soils is 0% to 2% (US Department of Agriculture, Soil Conservation Service 1990). The trees were planted 1359 and 716 trees/ha at the Ridge and the Flatwoods sites, respectively.
Initial soil physical and chemical characteristics at 0–15-cm depth of the Central Florida Ridge and the southwest Florida Flatwoods sites (adapted from Kwakye et al. 2023).
The experimental design for this study was a randomized complete block factorial design with an evaluation of macronutrients K and Ca at 1) 247 kg⋅ha−1 K and 45 kg⋅ha−1 Ca (1 × macronutrients), and 2) 494 kg⋅ha−1 K and 90 kg⋅ha−1 Ca (2× macronutrients) and micronutrients (Zn, Mn, and Fe) at 1) 5.6 kg⋅ha−1 (1 × micronutrients), 2)11.2 kg⋅ha−1 (2× micronutrients), and 3) 22.4 kg⋅ha−1 (4× of micronutrients) and micronutrient B at 1) 1.12, 2.24, and 4.48 kg⋅ha−1 for 1, 2, and 4× rates of the current UF/IFAS fertilization guidelines (Morgan and Kadyampakeni 2020; Obreza and Morgan 2008). The fertilizers applied on the trees were potassium sulfate, calcium sulfate, iron (II) sulfate, manganese sulfate, boric acid, and zinc sulfate. Macronutrients were soil-applied three times a year and micronutrients were soil- or foliar-applied three times per year. Each plot had 10 trees where the middle eight trees were the experimental unit. All treatments were replicated six times. The treatments are described in Table 2.
Treatment description of the study.
Root sampling methods.
Root samples were collected using a soil core sampler (2.5 cm in diameter) up to 15-cm depth from six trees in each plot five times during the study. At each sampling, the samples collected were mixed to form one composite sample per plot and dried at 65 °C for 3 d and weighed to determine the root mass per unit volume of soil as described by Kadyampakeni et al. (2014). Monthly root growth assessments using methods described by Han et al. (2016) were done using minirhizotrons installed in each plot. Installation of the minirhizotrons was at 50° angle up to a 1-m depth and root scans to estimate root growth and dieback were completed using the CID-600 root imager (CI-600 Root Growth Monitoring System, Fa. CID, Camas, WA, USA). Root scans were taken at four different depths (windows), which were as follows: 0 to 19.1 cm (window 1), 19.1 to 40.7 cm (window 2), 38.2 to 59.8 cm (window 3), and 57.3 to 78.9 cm (window 4).
Fruit yield measurements and juice quality analysis.
We determined fruit yield per plot from the experimental unit for each harvest every year in this study by picking fruit from the middle eight of the 10 trees in a plot to eliminate border tree effect. At the central Florida Ridge site, fruits were harvested in Apr 2019, 2020, 2021, and 2022, and at the southwest Florida Flatwoods site, fruits were harvested in Mar 2019 and 2021 and Apr 2022 (the 2020 harvest was skipped because of COVID-19 restrictions).
During harvest, ∼11 kg of round fruit, which is historically between 30 and 70 fruits depending on HLB severity, was randomly selected for juice analysis. For this analysis, total soluble solids (TSS) that estimate the sugar content of the fruit is measured. The TSS was estimated using a digital refractometer (ATAGO, PAL-1 BLT/i, Atago, Japan), at room temperature, and expressed as oBrix (Atta et al. 2023).
Disease ratings and management.
Disease rating was done using the following scale: 1) no suspect symptoms, 2) symptoms, 3) leaf drop present, 4) most of the quadrant has leaf drop, and 5) significant dieback. Root and canopy health were assessed monthly using minirhizotron scans, and semiannually with root collections for nutrient, fibrous root density, and ‘Candidatus Liberibacter asiaticus’ (Las) titer. Visual HLB disease ratings were taken every 6 months along with leaf samples for Las titer and activity [quantitative polymerase chain reaction (qPCR) of DNA and RNA] and nutrient content (Li et al. 2006). All practices for controlling other factors such as the Asian citrus psyllid and root pathogens such as Phytophthora were observed as part of grove management.
All trees were found to have CLas in 2019 at the start of the project with titers varying between 26 and 30, showing that the trees were affected by HLB and there were no differences in bacteria titer from the start to the end of the project. HLB severity did not change through the study and canopy size remained comparable from the start to the end of the project with HLB tree ratings showing moderate severity and no differences between treatments (Chinyukwi et al. in press).
Data analysis.
Data for root density were analyzed using one-way analysis of variance (ANOVA) and means were separated using Tukey’s honestly significant difference (HSD) test at P = 0.05. Monthly root scans were analyzed using Sigmaplot in visual graphical displays to determine any trends at 0 to 19.1 cm, 19.1 to 40.7 cm, 38.2 to 59.8 cm, and 57.3 to 78.9 cm. A two-way ANOVA using a repeated measures procedure in PROC MIXED as implemented in SAS (SAS Institute Inc. 2018) was used for fruit yield and juice quality analysis. Fertilizer treatment was considered as a fixed factor and year was considered random. The repeated measures aspect was then modeled through an R-side model using the REPEATED statement in the previously named procedure, either the unstructured (UN) or a first-order autoregressive structure with heterogeneous variances [ARH (1)] provided the best fit based on Akaike’s information criterion corrected for small sample size (AICC). Visual inspection of residuals (Kozak and Piepho 2018) indicated no violations of the underlying assumptions. Means within a treatment × year × response combination followed by the same letter are not significantly different according to Tukey’s HSD test at P = 0.05.
Results
Root growth and distribution.
At the Flatwoods site (Figs. 1 and 2), there was an increase in root growth from Nov 2019 until Feb 2020 (fall/winter season). Root growth decreased sharply during the beginning of the spring season (Mar 2020). Root growth continued to decrease for most treatments (Summer season). By Jul 2020, root growth for all treatments had decreased. As the fall season approached, root growth increased for all treatments in comparison with the previous season; however, at the end of the study (winter season), root growth had decreased again, and treatment 5 had the greatest root growth. Similarly, root dieback (Fig. 2) was low during the fall/winter season, and root dieback increased sharply during spring season. Toward the end of the summer season, root dieback had decreased, and this trend continued until winter season (Feb–Mar 2021) when root dieback began to increase again.
Root growth at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Flatwoods site, Clewiston, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2x MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI) and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root growth at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Flatwoods site, Clewiston, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2x MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI) and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root growth at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Flatwoods site, Clewiston, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2x MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI) and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root dieback at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Flatwoods site, Clewiston, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2× MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2x MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI), and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root dieback at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Flatwoods site, Clewiston, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2× MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2x MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI), and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root dieback at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Flatwoods site, Clewiston, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2× MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2x MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI), and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
At the Ridge site (Figs. 3 and 4), root growth decreased sharply from Nov 2019 to Jan 2020. Root growth began to increase during winter season until the end of spring season (Apr 2020) when decrease in root growth was observed. Root growth at the Ridge site increased for all treatments during the summer of 2020. However, root growth decreased once fall season had started. Similarly, root dieback at the Ridge site increased from Nov 2019 to Dec 2019. A sharp decrease in root dieback was observed during the winter season. Nonetheless, once spring season began, root dieback increased. Treatment 7 had the greatest root dieback. Another increase in root dieback was observed during the summer season of 2020. At the beginning of fall season 2020, root dieback began to decrease. Treatment 5 had the greatest root dieback and treatment 4 had the least dieback.
Root growth at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Ridge site, Lake Alfred, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2× MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI) and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI) and 4.48 (4× MI) kg·ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root growth at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Ridge site, Lake Alfred, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2× MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI) and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI) and 4.48 (4× MI) kg·ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root growth at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Ridge site, Lake Alfred, FL. Treatments are as follows: 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2× MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI) and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI) and 4.48 (4× MI) kg·ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root dieback at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Ridge site, Lake Alfred, FL. Treatments are as follows 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2x MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI), and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root dieback at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Ridge site, Lake Alfred, FL. Treatments are as follows 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2x MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI), and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Root dieback at 0–19.1 cm, 19.1–40.7 cm, 38.2–59.8 cm, and 57.3–78.9 cm depths at the Ridge site, Lake Alfred, FL. Treatments are as follows 1-SF, 2-SF+1× MA+1× MI soil, 3-SF+1× MA+2x MI soil, 4-SF+1× MA+4× MI soil, 5-SF+2× MA+1× MI soil, 6-SF+2× MA+2× MI soil, 7-SF+2× MA+4× MI soil, 11-SF+2× MA+1× MI foliar, 12-SF+2× MA+2× MI foliar, 13-SF+2× MA+4× MI foliar. Treatments are a standard fertilization (SF) (via fertigation) by University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) recommendation that included N, P, S, Mo, Cu No extra K, Mg, Ca, Mn, Fe, B, and Zn (SF). A 1× MA refers to single macronutrients of rates 45 kg⋅ha−1 of calcium (Ca) and magnesium (Mg) and 247 kg⋅ha−1 of potassium (K), and 2MA refers to double of each. A 1× MI refers to single micronutrients of rate 5.6 (1× MI), 11.2 (2× MI), and 22.4 (4× MI) kg⋅ha−1 of Fe, Zn, and Mn per year and B of 1.12 (1× MI), 2.24 (2× MI), and 4.48 (4× MI) kg⋅ha−1 per year. Error bars denote standard deviation of six replications.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Live roots at the Flatwoods site are mainly distributed in the first window (0–19.1 cm) followed by the second window (19.1–40.7 cm) (Supplemental Figs. 1 and 2). Initially, there were fewer live roots in the third (38.2–59.8 cm) and fourth (57.3–78.9 cm) windows. However, there was an increase in distribution of roots in the third (38.2–59.8 cm) and fourth (57.3–78.9 cm) windows at the Flatwoods site. Similarly, at the Ridge site, there is a greater distribution of alive roots in the first window (0–19.1 cm). However, over time, the distribution of live roots in the first two windows (0–19.1 cm and 19.1–40.7 cm) decreased for almost all treatments (Supplemental Figs. 3 and 4). On the other hand, there was an increase in live roots in the third (38.2–59.8 cm) and fourth (57.3–78.9 cm) windows. A comparison of the two sites shows that the Flatwoods site had a greater distribution of roots in the third (38.2–59.8 cm) and fourth (57.3–78.9 cm) windows than the Ridge site (Figs. 1–4, Supplemental Figs. 5–8). These trends appear to be consistent with the pre-HLB observations of Morgan et al. (2007) who reported that trees grown on Swingle citrumelo [Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock had significantly greater root density in the top 0.15 m than trees on Carrizo citrange (C. sinensis × P. trifoliata). Conversely, they reported that another rootstock Carrizo citrange had greater root density from 0.15 to 0.75 m below the soil surface, suggesting that trends might be contingent on rootstock as well. Root density was significantly greater for ‘Hamlin’ orange trees grown on Swingle citrumelo rootstock at distances less than 0.75 m from the tree trunk compared with those on Carrizo citrange. Fibrous roots of young citrus trees developed a dense root mat above soil depths of 0.3 m that expanded both radially and with depth with time as trees grow and canopy volume increased (Morgan et al. 2007).
Temporal root density distribution patterns.
Root density expressed as dry mass of root per unit volume of soil was different (P = 0.0431) at the Flatwoods site (Table 3) in Mar 2019 (although this was not expected because this was the start of the research) and at the end of Sep 2021 (P = 0.0148) but no differences were observed in Jan and Sep 2020, and Feb 2021. In contrast, root density distribution of HLB-affected trees at the Ridge site (Table 4) was similar among treatments in May 2019 (at the start), Mar 2020, and Oct 2021, but varied between treatments in Jul and Feb 2021. Overall, high root density (0.28–0.5 g⋅cm−3) was observed between Jan and Mar, probably because of commencement of root flush patterns and then started to decrease in Sep (<0.26 g⋅cm−3). At the Ridge site, high root densities (0.24–0.41 g⋅cm−3) were typically observed in May and Mar 2020, probably because of start of spring root flush but lower densities (<0.24 g⋅cm−3) were observed in Jul 2020 and Feb and Oct 2021.
Root density distribution of huanglongbing-affected citrus trees as a function of fertilization method and rate on Florida Flatwoods soils in Clewiston, FL. Error denotes standard deviations of six replications.
Root density distribution of huanglongbing-affected citrus trees as a function of fertilization method and rate on Florida Ridge soils in Lake Alfred, FL. Error bars denote standard deviations of six replications.
Fruit yield and juice quality patterns over time.
In 2019 and 2022, there was no significant difference (P > 0.05) in fruit yield at both experimental sites (Flatwoods and Central Ridge, Fig. 5). Because 2019 was just the beginning of the study, it was not expected to observe differences between the control (standard method − treatment 1) and all other treatments. However, in 2020 and 2021, there were significant differences between the treatments (P = 0.0001 and P = 0.003), respectively. For example, in 2020 at the Central Ridge site, the treatment number four that had the standard fertilization + 1× macro + 4× micronutrient (soil-applied) had the greatest yield (24.7 ton⋅ha−1), which was at least 24% greater than treatment 8 through 13 (with foliar micronutrients application). The latter trend tended to be true for 2021 at the Central Ridge. Generally, for 2020 and 2021 at the Central Ridge, supplemental fertilizers that were soil-applied tended to have greater yield than those that were foliar-applied. For all experimental years, fruit yield at the Central Ridge and Flatwoods sites had no observable differences between the standard fertilization rate (treatment 1) and all other treatments (2 through 13) that had supplemental fertilizers either via soil or foliar.
Fruit yield (ton⋅ha−1) of ‘Valencia’ orange (Citrus sinensis) trees at the Flatwoods and Central Ridge sites as a function of variable fertilizer rates. Treatment 1 (Standard rate-control); treatments 2 to 4 are standard rate + 1× macro + 1×-, 2×-, and 4×-micronutrient (soil-applied); treatments 5 to 7 are standard rate + 2× macro + 1×-, 2×-, and 4×-micronutrient (soil-applied); treatments 8 to 10 are standard rate + 1× macro + 1×-, 2×-, and 4×-micronutrient (foliar-applied); and treatments 11 to 13 are standard rate + 2× macro + 1×-, 2×-, and 4×-micronutrient (foliar-applied). Error bars with the same letter are not significantly different according to Tukey’s honestly significant difference test at P = 0.05.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Fruit yield (ton⋅ha−1) of ‘Valencia’ orange (Citrus sinensis) trees at the Flatwoods and Central Ridge sites as a function of variable fertilizer rates. Treatment 1 (Standard rate-control); treatments 2 to 4 are standard rate + 1× macro + 1×-, 2×-, and 4×-micronutrient (soil-applied); treatments 5 to 7 are standard rate + 2× macro + 1×-, 2×-, and 4×-micronutrient (soil-applied); treatments 8 to 10 are standard rate + 1× macro + 1×-, 2×-, and 4×-micronutrient (foliar-applied); and treatments 11 to 13 are standard rate + 2× macro + 1×-, 2×-, and 4×-micronutrient (foliar-applied). Error bars with the same letter are not significantly different according to Tukey’s honestly significant difference test at P = 0.05.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
Fruit yield (ton⋅ha−1) of ‘Valencia’ orange (Citrus sinensis) trees at the Flatwoods and Central Ridge sites as a function of variable fertilizer rates. Treatment 1 (Standard rate-control); treatments 2 to 4 are standard rate + 1× macro + 1×-, 2×-, and 4×-micronutrient (soil-applied); treatments 5 to 7 are standard rate + 2× macro + 1×-, 2×-, and 4×-micronutrient (soil-applied); treatments 8 to 10 are standard rate + 1× macro + 1×-, 2×-, and 4×-micronutrient (foliar-applied); and treatments 11 to 13 are standard rate + 2× macro + 1×-, 2×-, and 4×-micronutrient (foliar-applied). Error bars with the same letter are not significantly different according to Tukey’s honestly significant difference test at P = 0.05.
Citation: HortScience 58, 12; 10.21273/HORTSCI17372-23
TSSs ranged between 8 and 12 oBrix throughout the postharvest analysis from 2019 to 2022 at the Flatwoods and Central Ridge (Table 5). However, there was no significant difference between treatments for all years at both sites (Table 5) but higher TSSs were observed on the Ridge than on the Flatwoods site, meaning that such affects might be site-specific.
Total soluble solids (TSS) (Brixo) as a function of fertilization methods and rate on Florida Flatwoods soils and Central Ridge soils from 2019 to 2022. Data presented are the means ± standard deviations.
Pearson correlation of citrus yield with root growth, root dieback, and root density.
A significant negative correlation was observed between root growth and fruit yield for 2019 and 2021 at the Central Ridge (Table 6); however, at the Flatwoods site, a strong negative correlation was observed only in 2021 (Table 6). This helps to explain that root growth did not necessarily increase fruit yield for 2019 and 2021 at the Central Ridge, and 2021 at the Flatwoods sites (Table 6). Similarly, there was a significant negative correlation between root dieback and fruit yield on the Central Ridge, and the same trend was observed in 2021 on the Flatwoods.
Pearson correlation of citrus yield with root growth, root dieback, and root density as a function of increasing fertilizer application at the Central Ridge and Flatwoods sites.
Discussion
Dynamics of root growth and distribution.
Root growth is a complex physiological process that is affected by environmental factors as well as factors originating from the citrus tree itself (Castle 1978). Results of root growth from both the Ridge site and Flatwoods site showed that root growth occurs in flushes, peaking mostly in the months of Jan through May and decreasing in the latter part of the year. These flushes can be interlinked with the citrus tree physiological processes. The processes that could be at play include growth of new shoots, flower formation, and fruit formation (Iglesias et al. 2007). Florida fruit formation usually occurs from spring season until midwinter season. The nutrient expenditure is mostly on the formation of the fruits instead of root growth. Hence, a decrease in root growth at the Flatwoods and Ridge sites was observed during this period. Similarly, in spring season, flower formation occurs, the nutrients are taken from the roots to the aboveground organs (leaves and flowers); hence, nutrient availability is reduced in the roots in comparison with the leaves and flowers. Therefore, root growth is reduced at both sides, with root dieback also increasing during this season. These results confirmed earlier observations by Atta et al. (2020) who concluded that combined soil and/or foliar application of the micronutrients stimulated fibrous root density and improved root lifespan on HLB-affected sweet oranges by moderating the root-zone soil pH. Somehow, in contrast to the work of Atta et al. (2020), our study showed that the response on root growth was more pronounced on soil vs. foliar-applied micronutrient treatments with not much change as a result of elevated macronutrients.
Nutrient availability greatly affects root growth and distribution. Live roots are mainly distributed in the topmost soil layer (first window) because of increased nutrient availability. The nutrients can then be easily transported to the aboveground components by water. Consequently, promoting the growth of shoots, instead of roots. This ultimately leads to less root growth or lower root to shoot ratio (Syvertsen and Hanlon 2008). The lower root to shoot ratio can be used to explain the reduction in the distribution of alive roots at the Flatwoods site and Ridge site. Considering that the Flatwoods and Ridge sites both are composed of sandy soils, nutrient leaching from the first window to other windows could have resulted in more root growth in the third and fourth windows. However, the Flatwoods site has deeper roots as compared with the Ridge site and this can be attributed to the raised soil beds on which the citrus trees are planted. This creates an unsaturated soil volume that promotes root growth (Boman and Obreza 2018).
Fruit yield response as a result of variable rate fertilization.
The major differences in fruit yield were observed in 2 of the 4 years of the study. The differences were more pronounced at the Ridge site, where the soil is fairly well drained compared with the Flatwoods site. In 2020, for example, fruit yields in the soil-applied micronutrient treatments were ∼16% to 45% greater than foliar-applied micronutrients. The following year in 2021, the soil-applied micronutrient treatments were ∼7% to 18% greater than foliar-applied micronutrients This appears to have been the major impact of elevated micronutrients particularly in the soil by making them available in the root zone during peak nutrient demands. On the Flatwoods site, there was no significant difference between foliar and soil-applied micronutrients, suggesting that impacts for fertilization might have to be customized specifically for each site and not one size fits all. Our results are comparable with those of Atta et al. (2023), who reported increasing fruit yields on 13+-year-old trees because of elevated micronutrients (0× to 4× of current recommendations for Zn, B, and Mn) and increasing N, although they also found severe fruit drop (ranging from 13% to 37%) despite increased micronutrient levels. The results from our study appear to mirror those of Morgan et al. (2016) who studied varying foliar rates (0×, 1.5×, 3×, and 6×/year) of the current recommendations for Zn, B, and Mn. They reported that fruit yields were optimized at 3× current recommendations but decreased at the highest 6× rate. They concluded that at high rates of fertilization, the trees invested in excessive canopy growth at the expense of fruit yield. More recently, Uthman et al. (2020b) found that for 5- to 7-year-old trees, 1× or 2× the current recommendation for Zn along with an N rate of 224 kg⋅ha−1 was optimal.
Correlation between fruit yield with root growth and root dieback.
Fruit yield had a negative correlation with root growth at both sites for 2019 and 2021. This means that as the rate of fruit yield increased, root growth and root dieback decreased. This also suggests that increase in yield was dependent on very few healthy roots, as HLB negatively affects root growth (Graham et al. 2013; Johnson et al. 2014; Kadyampakeni et al. 2014). It is thus reasonable to assume that the HLB-affected citrus trees invest more in fruit formation at the expense of further root development below ground, although this might be more influenced by the time of the year and the time of fertilizer supply. However, fruit yields were improved with elevated micronutrients (2×–4×) but no impact on canopy size was observed (Chinyukwi 2021). Overall, bacteria titers stayed within the same levels as from the start to the end of the project.
In conclusion, root growth is affected by nutrient availability and citrus tree physiological processes, such as new shoot growth, flower formation, and fruit formation. In Florida citrus production, root growth is decreased during the spring and winter seasons when flower and fruit formation processes occur. However, with increased nutrient availability within the root zone, increased root growth is expected whereas at greater soil depth, root growth is decreased. However, factors such as nutrient leaching can affect root growth and distribution, with root growth being more pronounced in layers below the root zone. Moreover, it appears the influence of variable fertilizer is more pronounced depending on site and soil type. Fruit yield was significantly different between treatments in 2 of the 4 years of the study (P = 0.001 and P = 0.003), and largely ascribed to soil fertilization of micronutrients compared with foliar. Therefore, at higher fertilization rates, particularly via soil application, nutrient availability was increased, thus promoting root growth and distribution and fruit yield in HLB-affected orange trees. However, change in fertilization did not affect juice quality in both sites in all 3 to 4 years, but higher TSSs were observed on the Ridge than on the Flatwoods site. In future, economic analysis on the feasibility of these fertilization programs should be included in long-term studies.
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