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