The foliar application at flowering. Supportive responses of yield

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.

Yield and quality parameters

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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).

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.

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.

Relationships – fruit yield and quality

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