Effect of postharvest silicon application on 'hass' avocado (Persea americana Mill.) fruit quality.
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Date
2010
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Abstract
The South African avocado industry is export-orientated with forty percent of total
production sold overseas. The avocado fruit is a highly perishable product with a
relatively high rate of respiration which results in the quick deterioration of fruit
quality. Good phytosanitary procedures are a necessity in ensuring good product
quality. Due to the threat of pests and diseases becoming resistant to the
conventional chemicals currently used to control them, there has been a great
need to diversify from their usage.
Silicon (Si), being the second most abundant element (28%) in the earth’s crust
after oxygen, is a major constituent of many soils and has been associated with
disease resistance in plants for a long time. It has been used in a number of crop
species to provide resistance against pathogenic agents. In some horticultural
crops Si has been found to offer protection against fungal infections by
strengthening cell walls, thus making it more difficult for the fungi to penetrate and
colonize the plant. The aim of this research was to investigate the effects of postharvest
silicon application on the quality of ‘Hass’ avocado fruit. The specific
objectives included investigating the effect of silicon on the ripening pattern as well
as the metabolic physiology of the avocado fruit.
Avocado fruit were obtained from two locations in the KZN Midlands (Everdon
Estate in Howick and Cooling Estate in Wartburg). Fruit were treated with different
forms of Si (potassium silicate (KSil), calcium silicate (CaSil), sodium silicate
(NaSil) and Nontox-silica® (NTS)) at concentrations ranging from 160 ppm to 2940
ppm. After dipping for 30 minutes in the silicon treatments, the fruit were stored at
-0.5°C, 1°C, 5°C or at room temperature (25°C). Energy dispersive x-ray (EDAX)
analysis was then conducted on the exocarp and mesocarp tissues to determine
the extent of silicon infiltration within each treatment. Firmness measurements,
ethylene evolution and CO2 production were recorded as fruit approached
ripening. The CO2 production of fruit that were stored at room temperature was
analysed daily until they had fully ripened, while fruit from cold storage were
removed weekly to measure respiration. Mesocarp tissue from each fruit was
extracted using a cork borer and subsequently freeze-dried and stored for
physiological analysis. The freeze-dried mesocarp tissue was then finely ground
and later analysed for sugar content, total anti-oxidant capacity (TAOC), total
phenolic (TP) content and phenylalanine ammonia lyase (PAL) activity using their
respective assays. Statistical analyses were carried out using GenStat® version
11 ANOVA. Treatment and storage temperature means were separated using
least significant differences (LSD) at 5% (P = 0.05). The experimental design in
this study was a split-plot design with the main effect being storage temperature
and the sub-effect being treatments. Each replication was represented by a single
fruit.
EDAX analysis revealed that Si passed through the exocarp into the mesocarp
tissue in fruit treated with high concentrations of silicon, i.e., KSil 2940 ppm.
Significant differences (P < 0.001) were observed in temperature means with
regards to firmness. Fruit treated with KSil and NTS only and stored at 5°C were
firmer than fruit stored at other temperatures. Fruits treated with Si in the form of
KSil 2940 produced the least amount of CO2, while non-treated fruits (Air) had the
highest respiration rate. Fruit stored at room temperature (25°C) produced
significantly higher amounts of CO2 and peaked much earlier than fruit stored at
other temperatures. Ethylene results showed that there were differences (P <
0.05) between temperature means with the highest net ethylene being produced
by fruit stored at 25°C. There were also significant differences amongst treatment
means (P < 0.001), with fruits treated with KSil 2940 ppm producing the least
ethylene.
There were significant differences (P < 0.001) in temperature means with regards
to the total phenolic concentration with fruits stored at 1°C having the highest TP
concentration (26.4 mg L-1 gallic acid). Fruit treated with KSil 2940 ppm had the
highest total phenolic concentration whilst the control fruit (Air and Water) had the
lowest. There were also differences (P < 0.05) in storage temperature means with
respect to the total antioxidant capacity. Fruit stored at -0.5°C had the highest
TAOC (52.53 μmol FeSO4.7H2O g-1 DW). There were no significant differences in
TAOC (P > 0.05) with regards to treatment means although fruit treated with KSil
2940 ppm and stored at -0.5°C showed the highest TAOC of 57.58 μmol
FeSO4.7H2O g-1 DW. With regards to the concentration of major sugars in
avocado, mannoheptulose and perseitol (mg g-1), no significant differences (P >
0.05) were observed in temperature means. However, fruit stored at -0.5°C had
the highest concentration of these C7 sugars compared with fruit stored at other
temperatures. There were significant differences in treatment means (P < 0.001)
showing that fruit treated with KSil 2940 ppm had the highest concentration of both
mannoheptulose (18.92 mg g-1) and perseitol (15.93 mg g-1) in the mesocarp
tissue.
Biochemical analyses showed differences (P < 0.05) in storage temperature
means with respect to PAL enzymatic activity. Fruit stored at 5°C had the highest
PAL activity (18.61 mmol cinnamic acid g-1 DW h-1) in the mesocarp tissue
compared with fruit stored at other temperatures. There were significant
differences in treatment means (P < 0.001) with regard to PAL activity. Fruit
treated with KSil 2940 ppm had the highest PAL activity (23.34 mmol cinnamic
acid g-1 DW h-1).
This research has demonstrated the beneficial effects, particularly applications of
2940 ppm Si in the form of KSil. This treatment successfully suppressed the
respiration rate of avocado fruit. Biochemical analyses of total antioxidants, total
phenolics and PAL activity in the mesocarp tissue have shown the usefulness of Si
in improving the fruit’s metabolic processes. The C7 sugars (D-mannoheptulose
and perseitol) also seem to be more prevalent in avocado fruit treated with Si
(particularly KSil 2940 ppm) than in non-treated fruit. This suggests that an
application of Si to avocado fruit can aid in the retention of vital antioxidants (C7
sugars).
Description
Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.
Keywords
Avocado., Avocado--Postharvest physiology., Silicon in agriculture., Avocado--Quality., Avocado--Ripening., Avocado--Storage., Avocado--Postharvest technology., Theses--Horticultural science.