changes in the vascular tissue of fresh hass avocados treated with cobalt 60
TRANSCRIPT
Radiation Physics and Chemistry 63 (2002) 375–377
Changes in the vascular tissue of fresh Hass avocadostreated with cobalt 60
Lourdes Arevaloa, Ma. Emilia Bustosb, Cresenciano Saucedoa
aColegio de Postgraduados, Edo. de M!exico, C.P. 56230 Montecillo, Mexicob Instituto Nacional de Investigaciones Nucleares. Col. Escand !on, M!exico, D.F. C.P: 11800, Mexico
Abstract
This research was based on fresh avocado fruit treated with gamma rays at quarantine doses and stored at room
temperature. The effects of irradiation were analyzed and measured by three different types of studies: histological,
biochemical and physiological. Histological studies were focused on the effect of Cobalt 60 gamma rays in the mesocarp
of avocado irradiated at three different doses; 150, 250, and 350Gy. Damage was observed principally in the
parenchyma tissue where the cell membrane was plazmolized and a red color was observed due to the development of
phenol compounds. Another important effect was an increase in the size of xylem and phloem cells in the vascular tissue
even at the minimum dose of 150Gy. The biochemical and the physiological studies were done on avocado fruit
irradiated at 100 and 150Gy. An increase in L-phenilalanine ammonialyase activity was observed and therefore, an
increase in the concentration of phenol compounds. These changes were not perceived by panelists in a sensorial test.
Irradiated fruits were accepted by panelists as well as control fruit as regards parameters of taste, internal color and
external color. These results demonstrate the feasibility of using irradiation to disinfest avocado fruit using a minimum
dose of 100Gy. r 2002 Published by Elsevier Science Ltd.
Keywords: Irradiation; Avocado; Disinfestation; Gamma rays; Quarantine; Vascular tissue
1. Introduction
Alternative quarantine treatments for fresh avocado
are needed because traditional treatment by fumigation
is becoming less acceptable due to health and environ-
mental problems. Cold treatments which require more
than 12 days at low temperatures affect the quality of
the fruit. These treatments provoke peel pitting, brown
spots and darkening of the vascular tissue (Bower and
Cutting, 1988).
The objective of this research was to determine the
biochemical, physiological and histological responses of
avocado fruit to irradiation treatment at quarantine
doses. Specifically, the research was undertaken to
explore the feasibility of irradiation as a quarantine
treatment for the seed weevils Conatranchelus persea,
Helipus lauri and Stenoma catenfier. These pests are
known to be associated with avocado in Mexico and are
significant for quarantine purposes. Studies reported by
APHIS (2000) indicate that low doses of irradiation may
be used to disinfest fresh fruit for these kinds of insects.
2. Methodology
Fresh Hass avocado fruits were harvested at com-
mercial maturity (30% dry matter, 245725 g) fromorchards in Uruapan Micoacan, M!exico. The study was
done in two lots; one histological study with avocados
irradiated at 100, 150 and 250Gy, and the second lot for
the biochemical and physiological response in fruit
irradiated at 100 and 150Gy. The irradiation source was
Cobalt 60 gamma rays, dose rate at 9.75Gy/min.
Dosimetry was performed using a Fricke dosimeter.
Fruit were stored for 11 days at room temperature. The
following parameters were measured in triplicate every
0969-806X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd.
PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 6 2 7 - 2
three days: respiratory rate; ethylene production; phe-
nylalanine ammonialyase activity; and total phenolic
concentration. Histological changes were observed with
an optical microscope and the sensorial evaluation was
done using a hedonic scale.
3. Results and discussion
3.1. Histological studies
Plasmolysis was observed in the parenchyma cells of
the mesocarp in treated fruit. The vascular tissue of
fruits treated at 250 and 350Gy showed hardness on the
sixth day after the treatment. In addition, the vascular
tissue cells were bigger compared with cells in the fruit
irradiated at 150Gy and the control. The vascular tissue
treated with doses of 150 and 250Gy showed an increase
in the number of cells, although the number of cells was
reduced dramatically at 350Gy because the parenchyma
cells of the vascular tissue were so dark that they were
difficult to count (see Table 1).
3.2. Respiration and ethylene concentration
A sharp increase in respiration and ethylene produc-
tion was observed in irradiated fruit compared to the
control fruit. Ethylene production in irradiated fruit
reached a peak near the third day and then started to
decrease. The control fruits increased their ethylene
production gradually with ripeness starting on the third
day and reaching the maximum concentration on the
ninth day. The increase of ethylene production in
irradiated fruits can be explained by the stimulation of
ACC syntase activity up to the third day, but afterwards
the reduction of ethylene can be in response to the
damage of irradiation on cell membrane integrity which
is basic for the normal biosynthesis of this hormone
(Larrigaudi!eere et al, 1991).
3.3. Biochemical activity
L-phenylalanina ammonialyase (PAL) is the key
enzyme in the metabolism of phenol compounds. This
enzyme can be induced in response to stress conditions
and its activation is considered to be a defensive
mechanism of plants against stress, including the
application of irradiation (Frylick et al., 1987). An
increase in the activity of the enzyme was observed in the
irradiated sample at the minimum dose of 100Gy. An
increase in the phenol compound was observed between
the third and seventh days (see Table 2). Macheix et al.
(1990) explained that the accumulation of phenols and
the increase of enzymatic activities can lead to the
polymerization of the phenols in the cell wall, a process
known as lignification, which probably happened to the
avocado fruit after the fifth day.
Table 1
Effect of the irradiation treatment on the mesocarp vascular tissue of ‘Hass’ avocado fruit stored at 201C
Area of mesocarp vascular tissue (mm2)* Number of cells in the mesocarp*
Treatment 3 days 7 days 11 days 3 days 7 days 11 days
Control 6367 b 2927 b F 87.0 b 48.0 c F150Gy 5865 b 9786 b 17,025 b 128.0 b 173.3 a 207.3 a
250Gy 23,268 a 22,826 a 16,051 b 216.7 a 141.3 b 127.7 a
350Gy 6535 b 20,350 a 34,361 a 92.7 b 75.3 c 177.7 a
Coefficient variation (CV%) 15.66 19.78 9.28 20.92 10.38 30.00
*Different letters denote significant differences within each column group at Pp0.05 (Tukey test).
Table 2
Effect of the irradiation treatment in PAL activity and total phenol content in ‘Hass’ avocado fruit stored at 201C
PAL activity (pkat/g DW)n Total phenol content (mg -1 DW)n
Treatment 3 days 7 days 11 days 3 days 7 days 11 days
Control 343.8 b 556.0 a 540.4 a 11.0 c 26.0 c 46.8 a
100Gy 803.2 a 623.6 a 604.3 a 16.2 b 47.5 a 40.4 a
150Gy 716.2 a 548.5 a 565.2 a 36.3 a 40.0 a 37.3 a
Coefficient variation (CV%) 8.18 7.40 15.05 5.8 11.8 10.28
nDifferent letters denote significant differences within each column group at Pp0.05 (Tukey test).
L. Arevalo et al. / Radiation Physics and Chemistry 63 (2002) 375–377376
4. Conclusions
Irradiation promotes cell growth, an increase in
the number of vascular tissue cells, stimulation of
ethylene biosynthesis, increased PAL activity and the
synthesis of the phenols. These changes in the cell
produce lignification through the polymerization of
phenol compounds. The increase in the number of
mesocarp cells in the irradiated fruit led to the
hypothesis that irradiation stimulated cell division
at those doses. Panelists accepted avocados irradiated
at 100Gy as well as the control fruit, leading to the
conclusion that this dose may be suitable as a quarantine
treatment for avocado.
References
Animal and Plant Health Inspection Service, USDA, 2000.
Irradiation as a phytosanitary treatment of imported fruits
and vegetables. Proposed rule. Fed. Reg. 65(103).
Bower, J.P., Cutting, J.G., 1988. Hortic. Rev. 10, 240–261.
Frylick, L., Dubery, I.A., Schabort, J.C., 1987. Biochemical
changes involved in stress response and ripening behavior of
irradiated mango fruit. Phytochemistry 26, 682–686.
Larrigaudi!ere, Latche, A., Pech, J.C., Triantaphylides, C., 1991.
Relationship between stress ethylene production by gamma
irradiation and ripening of cherry tomatoes. J. Amer. Soc.
Hortic. Sci. 116, 1001–1003.
Macheix, J., Fleuriet, A., Billot, J., 1990. Fruit Phenolics. CRC
Press, Gulph, Ont.
L. Arevalo et al. / Radiation Physics and Chemistry 63 (2002) 375–377 377