Comparison of Ripening Processes in Intact Tomato Fruit and Excised Pericarp Discs

Abstract

Physiological processes characteristic of ripening in tissues of intact tomato fruit (Lycoperskon esculentum Mill.) were examined in excised pericarp discs. Pericarp discs were prepared from mature-green tomato fruit and stored in 24-well culture plates, in which individual discs could be monitored for color change, ethylene biosynthesis, and respiration, and selected for cell wall analysis. Within the context of these preparation and handling procedures, most whole fruit ripening processes were maintained in pericarp discs. Pericarp discs and matched intact fruit passed through the same skin color stages at similar rates, as expressed in the L*a*b* color space, changing from green (a* <-5) to red (a* > 15) in about 6 days. Individual tissues of the pericarp discs changed color in the same sequence seen in intact fruit (exocarp, endocarp, then vascular parenchyma). Discs from different areas changed in the same spatial sequence seen in intact fruit (bottom, middle, top). Pericarp discs exhibited climacteric increases in ethylene biosynthesis and CO2 production comparable with those seen in intact fruit, but these were more tightly linked to rate of color change, reaching a peak around a* = 5. Tomato pericarp discs decreased in firmness as color changed. Cell wall carbohydrate composition changed with color as in intact fruit: the quantity of water-soluble pectin eluted from the starch-free alcohol insoluble substances steadily increased and more tightly bound, water-insoluble, pectin decreased in inverse relationship. The cell wall content of the neutral sugars arabinose, rhamnose, and galactose steadily decreased as color changed. The extract-able activity of specific cell wall hydrolases changed as in intact fruit: polygalacturonase activity, not detectable in green discs (a* =-5), appeared as discs tumed yellow-red (a* = 5), and increased another eight-fold as discs became full red (a* value +20). Carboxymethyl-cellulase activity, low in extracts from green discs, increased about six-fold as discs changed from yellow (a* = 0) to red. Ripening in a climacteric fruit, such as tomato (Lycopersi-con esculentum Mill.), is a complex phenomenon. It encompasses a diverse set of physiological processes progressing differentially through a number of anatomically distinct tissues .

Cell Wall Metabolism in Ripening Fruit (IX. Synthesis of Pectic and Hemicellulosic Cell Wall Polymers in the Outer Pericarp of Mature Green Tomatoes (cv XMT-22)

“…There was no incorporation of tracer into 1,3-linked glucosyl residues in the cv XMT-22 discs (Tables I11 and IV). Whether other wound-induced responses continue beyond 48 h is unclear, but discs in culture display many of the physiological and biochemical changes described for ripening, intact tomato fruits, including severa1 aspects of cell wall metabolism (Campbell et al, 1990). Furthermore, the wall synthetic events we have detailed here are consistent with the pattern of incorporation of ['4C]Suc provided to intact, ripening tomatoes reported by Mitcham et al (1991).…”

“…In the CDTA / Na,CO,-soluble (combined pectic) fraction, the only significant difference between treatments was a higher Gal (about 35%) content in the in vivo control ( Table I). The lower Gal contents of the water-and CDTA/ Na,CO,-soluble fractions from the in vitro treatments undoubtedly reflect the ripening-related loss of wall galactan reported for all tomatoes studied, including cv Castlemart (Gross and Wallner, 1979;Campbell et al, 1990). Within tissue type, the outer discs had significantly more Rha than the inner discs.…”

“…A11 in vitro discs were incubated for 2 d to allow the dissipation of the disc excision (i.e. wound)-related responses we reported previously (Campbell et al, 1990;Greve and Labavitch, 1991). Then, the in vitro labeled discs were given [13C]G1c and incubated for an additional 2 d. The in vitro control discs were given water and incubated for an additional 2 d. There were two tissue types within each treatment: the "outer" 2 mm of pericarp, including the cuticle, epidermis, hypodermis, and some mesocarp; and the 2-mm portion immediately inward from that, consisting entirely of mesocarp, designated as "inner."…”

“…cv were obtained from a commercial wholesaler and were sorted to eliminate fruits with surface blemishes and those showing signs of color break. An excised pericarp disc system was utilized, based on the method of Campbell et al (1990). The experimental design consisted of three treatments: an in vivo control, an in vitro control, and an in vitro labeled group.…”

“…Our use of pericarp discs raises the concern that the [13C]G1c incorporation we have described is a consequence of the wounding caused by disc excision. We have described two aspects of wound metabolism, an increase in ethylene synthesis (Campbell et al, 1990), and incorporation' into callose-like glucan (Greve and Labavitch, 1991) by freshly cut discs from cv Castlemart tomatoes, but these responses are transient, ending within 36 h of disc preparation. We did not provide labeled Glc until48 h after excision.…”