Ultrastructure and Cytology of the Postharvest Avocado (Persea americana Mill) Fruit

نویسندگان

  • J. G. Luza
  • L. A. Lizana
چکیده

Basic postharvest work on 'Fuerte' avocado fruit cell wall anatomy and ultrastructure was carried out from fruit harvest to postclimacteric stage. Mesocarp samples were analyzed with light microscopy, scanning electron microscopy and transmission electron microscopy. Irregularity of the cell wall increased, microfibrils became disorganized, and some dissolution of the middle lamella was observed during ripening. However, the intercellular matrix remains almost unchanged without significant separation between the primary cell walls, as occurs in other fruits. This feature gives special consideration for avocado postharvest handling. Mature avocado fruit start the ripening process following removal or abscission from the tree (Dallman et al., 1988). A characteristic of the ripening process, common to most fruits, is an increase in the activity of the cell wall degrading enzymes responsible for fruit softening. Dissolution of the ordered arrangement of cell wall and middle lamella polysaccha-rides occurs (Dick and Labavitch, 1989). Periodic Acid Schiff's Reagent (PAS) has long been used for polysaccharides as has Calcofluor for staining cellulose (Hughes and McCully,1975). These methodologies in combination with transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies have the ability to extend previous cytological information and open new avenues for additional research. For a better understanding of gas diffusion and controlled atmosphere applications in postharvest handling, it is necessary to solve questions concerning changes in mesocarp cells during ripening. The present study reports the cytological and structural cell wall changes during ripening in avocado fruit. Materials and Methods Sixty recently harvested, 'Fuerte avocado fruits were used. They were sorted into two groups: the first group was analyzed the day after harvest, the remainder analyzed at a post-climacteric stage, using both ripe and overripe fruit. Softening was determined with Mangness-Taylor pressure tester using a 7.9 mm plunger. Samples for light microscopy were fixed in 4% glutaraldehyde, 0.2 M dipotassium phosphate, 0.1M citric acid monohydrate, and 4% sucrose at pH 7.0. Dehydrated samples were embedded in glycol methacrylate resin (DuPont-Sorvall; Wilmington, DE) (modified from O'Brien and McCully, 1981). All fluorescence observations were carried out on a Zeiss microscope equipped for epi-illumination with a HBO 50 mercury lamp. Calcofluor white MR2 (American Cyanimide Co.) was used to stain cellulose (Hughes and McCully, 1975). This stain emits a strong, pale blue fluorescence in the presence of 1,3 glucans. Periodic Acid Schiff's Reagent (PAS) was used to stain carbohydrates (Jensen, 1962). For specificity 2,4 dinitrophenyl hydrazine (DNPH) was used to block tissue aldehydes prior to staining (modified from O'Brien and McCully, 1981; van Duijin, 1961). SEM samples were submerged in freon for 1 minute and transferred to liquid nitrogen. Frozen samples were broken before the critical-point drying and were gold coated. TEM fixation was the same as LM, then washed, and post-fixed in 2% osmium tetroxide in buffer. The samples were embedded in Spurr's plastic (adapted from Dawes, 1979). Gold-colored sections were mounted on 300 mesh copper slim-bar grids (Ted Pella Inc.). The sections were stained in uranyl acetate and lead citrate solutions. Photographs and observations were made on a JEOL 100S TEM at 80kV.

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تاریخ انتشار 2011