Mango yield challenges of poor fruit setting and, high and early fruit drop rates are frequently associated with tissue Boron (B) and/or Zinc (Zn) deficiencies; however, how to mitigate the deficiencies remained largely unexplored. To explore whether foliar or soil B and Zn applications mitigate the deficiencies, we investigated their mitigating effects on leaf mineral contents and, yield and quality parameters of mango cv. Chaunsa (white). The experiment was comprised of seven treatments × four replications: control, individual and combined foliar applications of 0.5% Boric acid and 0.2% Zinc Sulfate (ZnSO4) solutions and, individual and combined soil applications of 75 g Borax and 200 g ZnSO4 per plant. The combined soil application of B and Zn significantly increased the leaf mineral B and Zn concentrations, fruit set, retention, yield, pulp recovery and total soluble solids (TSS) at ripening while it reduced the titratable acidity and early fruit shedding. Additionally, the treatment improved the fruit quality (taste, flavor, texture, aroma, acceptability). Therefore, we suggest that the combined soil application of B and Zn mitigates leaf mineral deficiencies and improves yield and quality of mango cv. Chaunsa (white) more efficiently than the other individual or combined foliar or soil treatments used in this study.
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One of the oldest and most popular fruits of tropical and sub-tropical regions, mango (Mangifera indica Linnaeus) is cultivated in more than 100 countries (Masroor et al. 2016), making it the second largest cultivated tropical and subtropical fruit and 6th major fruit crop around the world (Hegde and Venkatesh 2007). The fruit likely has a great cropping potential in Mediterranean region, e.g., Eastern Turkey, under the climate change scenario (Lelieveld et al. 2012). Therefore, mango is one of the important food sources for the consistently growing world population (FAO 2014); however, the mango orchards are facing micronutrient deficiencies, physiological stresses, and fruit yield and quality challenges that are being reflected in decreased production and exports. However, a little attention is paid to investigating as to how the micronutrient deficiencies can be mitigated under a comprehensive set of growth conditions to improve yield and quality and increase production and export.
Overall, many of the mango growing soils in Asia are calcareous in nature, mostly intercropped and receive much lesser than recommended doses of fertilizers (Masroor et al. 2016). Additionally, the soils are largely deficient in Zinc (Zn) and Boron (B) (e.g., Alloway 2009). Boron, an essential micronutrient plays a critical role in the growth and enlargement of reproductive cells, initiation of flowering and translocation of sugars (Hu and Brown 1994). Its deficiency is mostly transient and occurs during flowering and seed setting (Brown and Shelp 1997). Prolonged deficiency results in premature shedding of flower or fruit, suggesting a greater demand of B during floral or fruit development (Dell and Huang 1997). Zinc is also a known essential micronutrient that is required for metabolic processes and, enzymatic and redox reactions occurring in plant cells. Being a structural component of indoleacetic acid, Zn is directly involved in many plant growth processes, e.g., synthesis of certain amino-acids/proteins (Hegde and Venkatesh 2007). Zinc deficiency is very common in mango orchards due to calcareous nature of soils that do not support the micronutrient uptake (Zia et al. 2006). Mango is inherently prone to Zn and B deficiencies due to its partial respective ability to extract from the soil in sufficient amount (Sharma and Singh 2009). In fact, most mango soils may have imbalanced nutrient concentrations due to the exhaustive removal of selective nutrients by intercropped plants augmented by partial or no soil replenishment which result in plant deficiency of these micronutrients.
While uptake of the soil-applied micronutrients is largely reported to be extremely low in mango orchards (Nisar and Rashid 2003), foliar application of Zn or B is commonly observed to promote flowering, stabilize fruit setting, and significantly increase the productivity of a variety of orchards (Nyomora, Brown, and Krueger 1999). Foliar application of nutrients as compared to soil application results in rapid increase in plant use efficiency through quick and effective availability to plants (Bahadur, Malhi, and Singh 1998, Ling and Silberbush 2002). Specifically, the boric acid foliar application to mango cv. Langra improved flushes, inflorescence, fruit setting percentage and biochemical characteristics (Rajput, Singh, and Mishra 1976). Likewise, the foliar use of Zn resulted in the increase in fruit yield, fruit sugar level, and ascorbic acid content (Singh and Rajput 1976). A combined foliar application of Zn and B results in increases in fruit number, quality, pulp weight, total soluble solids (TSS) and ascorbic acid and sugar contents, and a decrease in stone weight (Anees et al. 2011).
To better understand how the mango orchards, respond to soil and foliar applications of Zn and B individually and combined, we conducted a field-based experiment (using a cultivar Chaunsa (white) in one of the densest mango growing belts. Therefore, our specific objectives were to assess the effect of foliar and soil-applied Zn and B on:
i. Leaf mineral Zn and B contents
ii. Fruit retention, yield, and quality
Since reduced fertilizer application especially Zn and B, and poor management practices are mainly responsible for declining yields and poor fruit quality (e.g., Richards 2009); therefore, we hypothesized that the foliar and soil-applied Zn and B minerals would positively impact the leaf mineral contents with improvement in fruit retention, yield, and quality characteristics.
2. Materials and methods
2.1. Study area
This research was conducted at an experimental mango orchard (30° 09′ N, 71° 26′ E; elevation 410 m asl.) located near the Mango Research Institute, Multan, over three growing seasons (2013, 2014, 2015). According to climate data obtained from a meteorological station installed three km to south-west at Central Cotton Research Institute, Multan, the mean annual minimum and maximum air temperatures were 10.3 ºC and 35.8 ºC, respectively, with a mean annual precipitation of 82 mm, during the study years (2013-2015).
2.2. Experimental Design
Twenty-year-old, 28 Chaunsa (white) mango trees, approximately equal in size and uniform in vigor were selected. The trees were laid out in a Randomized Complete Block Design (RCBD; 7 treatments × 4 replications) to study the effects of foliar and soil B and Zn applications on leaf mineral contents, and fruit retention, yield, and quality parameters. All experimental trees were subjected to the standard orchard management practices of irrigation, pruning, and weed eradication (Zia et al. 2006), and a balanced basal dose of 1000 g each of mineral N, P and K from Urea (CO (NH2)2), Single Super Phosphate (Ca (H2PO4)2 and Potassium Sulfate (K2 SO4) sources, respectively. A Full Phosphorus (1000 g) and one-half each of the Nitrogen (500 g) and Potassium (500 g) nutrients were applied to the soil under the canopy after fruit harvest (end of July 2012) and the remaining one-halves of N and K were applied before flowering (start of February 2013). At the pre-flowering stage, foliar sprays or soil applications of Zn and B were also completed every year as per the following treatments:
T2) Foliar spray: Boric Acid (H3BO3, 40 g @ 0.2%) + Zinc Sulfate (ZnSO4, 300 g @ 0.5%)
T3) Foliar spray: Boric Acid (H3BO3, 40 g @ 0.2%)
T4) Foliar spray: Zinc Sulfate (ZnSO4, 300 g @ 0.5%)
T5) Soil application: Borax (Na2B4O7.10H2O, 75 g) + Zinc Sulfate (ZnSO4, 200 g)
T6) Soil application: Borax (Na2B4O7.10H2O, 75 g)
T7) Soil application: Zinc Sulfate (ZnSO4, 200 g)
2.3.1. Soil and tissue analyses
Three randomly taken soil samples from under each tree canopy were homogenized into a composite sample before the start of the experiment (during June 2012) to determine to mean experimental soil characteristics of the orchard. The pH and electrical conductivity (ECe) of the soil were determined using saturated soil paste and soil extract methods. Available P and K were determined using Olsen’s P and flame photometer methods, respectively. The textural class was determined using Bouyoucos method, while the soil organic matter content was determined following Ryan, Estefan, and Rashid (2007). The Zn was evaluated by DTPA extraction method using Atomic Absorption Spectrometer (AASPM – Shimadzu 7000), while B was first extracted with Hydrochloric acid (HCl) and quantified spectrophotometrically.
During each of the three growing seasons, 6-7 months-old leaves from the middle of non-fruiting shoots were sampled from all heights and directions to determine leaf mineral N, P, K, Zn and B contents’ determinations following (Chadha, Samra, and Thakur 1980). The leaf samples were oven dried at 70°C in a convection oven until constant weight was obtained, ground in a Wiley Micro Mill to pass through a 40-mesh screen, and stored in labeled plastic bags. We used a yellow-color method with a tri-acid-digestion technique to determine total P, and flame photometric method to determine K. The aliquot was also used for B and Zn determinations using AASPM – Shimadzu 7000) respectively. Kjeldahl distillation method was used for total N determination form plant tissue.
2.3.2. Fruit retention, yield, and quality assessments
During each of the three fruit setting seasons, we quantified the fruit retention percentage form a randomly selected/marked area of 1.0 m2 at each side of a randomly selected experimental tree in a treatment on monthly basis. At the fruit harvest (end of July), the marked areas were sampled to obtain an average fruit weight, and an average yield in kg per plant was calculated by multiplying the average fruit weight with a total number of fruits. A top-loading, three decimal balance was used to measure fruit weight (g) after harvest (before and after ripening). Fruit volume (cm3) was measured using Archimedes principle (water displacement method) following Lang and Thorpe (1989).
2.3.3. Organoleptic and chemical assessments of fruit
Sensory evaluation of ripe mangoes was carried out by a panel of ten persons consisted of the technical staff of mango research institute, Multan. Taste of fruit, flavor, peel and flesh color, texture, aroma and overall acceptability were tested by using an 8-point hedonic scale. The titratable acidity of fresh mangoes for citric acid content (%) was measured using a standardized formula by titrating the sample juice to pH 8.2 with 0.1 N sodium hydroxide (NaOH) as described by Rangana (1979). The Total Soluble Solids (TSS) in fresh mango juice was measured by using a Medline Scientific Ltd digital hand refractometer model SELECT045.
2.4. Data analyses
All data were statistically analyzed using Statistix® v 8.1 software. A repeated measure analysis of variance (ANOVA) was used to analyze the effects of foliar and soil B and Zn treatments on leaf mineral contents, fruit retention, fruit yield, and fruit quality parameters. Since same parameters were quantified for study years (2013-2015); therefore, the year was taken as fixed as well as a repeated measure following (Munir et al. 2017). The difference between treatment means was compared by the least significant difference (LSD) at 5% probability level.
3. Results and discussion
The study orchard’s soil was loam in texture, and low in organic matter with pH 8.42. Nutritionally, the soil was deficient in available P and Zn and adequate in K and B concentrations (Table 1). No indications of salinity were found.
3.1. Yield and quality parameters
No significant between-years changes in fruit yield and quality parameters were found; therefore, only 3-yr means are provided in Table 2. Application of foliar and soil B and Zn resulted in overall significant increases in fruit quality (pulp recovery, TSS, Acidity) and yield (weight, volume) parameters as compared to control (Table 2). The significant increases in fruit weight (320 g), volume (308 cm3), pulp recovery percentage (62%) and total soluble solids (17.8), and a significant decrease in titratable acidity (0.32%) was recorded in response to T5. Similarly, T7 resulted in significant increases in fruit weight (274 g), volume (279 cm3), pulp recovery percentage (59%) and titratable acidity (0.35). The T4 (foliar application; ZnSO4 0.5%) resulted in the second highest TSS contents as compared to T5 and other treatments. Bahadur, Malhi, and Singh (1998) reported the similar findings of enhanced yield and better fruit quality (TSS, acidity, aroma, flesh color and taste) in response to the soil application of Zn as compared to control. Conversely, Masroor et al. (2016) reported the similar responses from two foliar applications of Zn in November and March in contrast to our single foliar application at flowering. Supportive responses of yield and quality parameters of Strawberry and pomegranate fruits were reported by Abdollahi, Eshghi, and Tafazoli (2010) and by Khorsandi, Yazdi, and Vazifehshenas (2009), respectively. Both investigations found that the soil application had overall better results than those in response to foliar application of micronutrients. The positive responses of mango fruit quality and yield to the combined application of Zn and B may be due to improvements in concentrations of sugars, vitamins and some physiological parameters (Hegde and Venkatesh 2007).
3.2. Leaf mineral contents
We noticed significant increases in leaf B and Zn contents in the 3rd year of study in response to the year-over-year applications of soil B and Zn (T5). Across all years, the soil application of micronutrients resulted in significantly higher concentrations in leaves, as compared to control (Table 3) as reported by Khan et al. (2012) for their research on sweet orange leaf concentrations of micronutrients. The T5 and T7 showed the highest concentration of Zn in all years as compared to the other treatments. Similarly, T5 and T6 were the highest in increasing the leaf concentration of B as compared to other treatments in all years (Table 3). Based on overall leaf mineral B and Zn contents, our research supports the findings of Zia et al. (2006) who suggested that the soil applications are better than the foliar applications of these nutrients in enhancing leaf mineral micronutrients.
No significant between-years changes in leaf mineral N, P and K were found; therefore, only 3-yr means are provided in Table 4. Overall significant increases in leaf N, P and K contents were observed in response to the Zn and B applications as compared to control. The T5 resulted in the highest concentration of NPK (1.06%, 0.19%, 0.57%) respectively, compared to all other soil or foliar applications of micronutrients; however, all micronutrient applications improved the mineral contents in mango leaves in general. We also found that the optimum concentrations of leaf mineral contents improved fruit quality and mango crop yield (Table 3, 4; Figure 1, 3). The balanced application of fertilizers with Zn and B ensures optimum nutrient concentrations in leaves which may lead to better quality and sustainable increase in mango production. South-Asian soils, typically under orchards, are Zn and B deficient (e.g., Zia et al. 2006) and may lead to reduced uptake of N and K by plants. No antagonistic impacts of soil Zn and P are reported upon each other’s uptake (when applied at different times); however, Razzaq et al. (2013) found that the P depressed the uptake of Zn when the two nutrients were applied in combination to the soil.
3.3. Fruit retention percentage
We present responses of fruit retention and yield to foliar and soil B and Zn applications separately for all years as well as for 3-yr means (Figure 1). Since no significant between-years differences for treatments were found; therefore, significance letters are associated with 3-yr means only. A significantly higher fruit shedding percentage and significantly lower fruit yield were noticed in control as compared to those at all treated plots. Significant and the highest fruit retention percentage (0.88%) and fruit yield (112 kg plant-1) were found in response to T5. The second highest yield (106 kg plant-1) was observed in T6; it indicates that the foliar application of Zn and B also resulted in yield increase but lesser than that in response to the soil application. The plant Zn concentration is found to be directly related to fruit drop as it is involved in the synthesis of tryptophan; therefore, greater the concentration of Zn in shoot and twigs, more will be the synthesized tryptophan or auxin (Indole Acetic Acid), potentially leading to a reduction in fruit drop (Singh, Malik, and Davenport 2010, Ahmed et al. 2012). Likewise, B application increases fruit setting and yield due to physiological changes or improvement in reproductive development (Chaplin, Stebbins, and Westwood 1977). Conversely, B deficiency causes rupture of internal and external tissues which may result in fruit drop (Westwood and Stevens 1979). Application of B and Zn may improve the biochemistry of fruit, and lead to enhanced number of fruit set per panicle and fruit retention percentage, resulting in sustainable mango yield (Alloway 2009); however, more research work is suggested to formulate the best management conditions and practices for sustainable increases in fruit quality and yield to be able to increase exports and contribute to meet potentially growing food security challenges.
3.4. Relationships – fruit yield and quality parameters
The fruit yield had significant and positive correlations with number of fruit set per panicle (R2 = 0.61; p = 0.039; Figure 2A) and fruit retention percentage (R2 = 0.91; p < 0.001; Figure 2B); the fruit retention was in turn related with the number of fruit set per panicle (R2 = 0.64; p = 0.031; Figure 2A). The highest leaf mineral B, Zn, N, P and K concentrations (Table 3 and Table 4), and the highest number of fruit-set per panicle, fruit retention percentage and fruit yield (Figure 1) were recorded in response to the combined soil application of B and Zn in T5. More and thorough investigations including all possible combinations of foliar and soil applications need to be conducted to formulate best management practices for enhanced yields of better or export quality. We also present the correlation between individual and clustered ranks assigned by 10 different testers based on sensory evaluation technique and Hedonic scale ranking (1-8) of various organoleptic parameters: taste, flavor, texture, aroma, and acceptability (Figure 3). Overall, significant Spearman rank correlation (R2 = 0.68; p < 0.05 in all cases) was found between the rankings of the mango fruit parameters. Significant differences were also found between individual parameter ranks among all treatments, with the highest ranks in response to T6 and the lowest overall ranks to T4. In contrast to our findings, Masroor et al. (2016) reported the highest acceptability and fruit quality in response to soil Zn application. Conclusions 1. Soil application of B and Zn combination significantly increased the leaf mineral B and Zn contents, number of fruit set per panicle, fruit retention percentage, and fruit yield (volume, weight), pulp recovery and TSS at harvest or after ripening, whereas it reduced the titratable acidity and early fruit shedding. 2. The combined application significantly improved the fruit quality parameters of taste, flavor, texture, aroma, and the overall acceptability. Therefore, the combined soil application of B and Zn mitigates B and Zn deficiency and improves fruit yield and quality of mango cv. Chaunsa (white) more efficiently than the other individual or combined foliar or soil treatments used in this study under the specific experimental conditions.