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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).

Experimental Design

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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:

T1) Control

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

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

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.

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.

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