Physiology and firmness determination of ripening tomato fruit

Abstract

Tomato (Lycopersicon esculentum Mill.) genotypes varying in intrinsic firmness were examined to determine the quantitative relationships between polygalacturonase (EC 3.2.1.15) activity, firmness and other ripening parameters including rate (days from mature‐green to full red) and intensity (rate of ethylene production at climacteric peak) of ripening. Texture, respiration and ethylene production were monitored in the immature‐green through the red (ripe) stages of development. Polygalacturonase activity was measured by direct assay of salt‐extractable wall protein or by monitoring the release of pectins from isolated, enzymically active wall. In all fruit, polygalacturonase activity was highly correlated with pericarp softening, but only moderately correlated with softening of whole fruit (r = 0.920 and 0.757, respectively). Polygalacturonase activity was positively correlated with cell‐wall autolytic activity in pink (r = 0.969) and red (r = 0.900) fruit. Firmer genotypes exhibited lower rates of respiration and ethylene production during ripening. Polygalacturonase activity in isolates prepared from fruit at the climacteric peak was positively correlated with ethylene production and respiration, and negatively correlated with days to ripening (r = 0.929, 0.805, and ‐0.791, respectively). The data demonstrate the importance of selecting the appropriate method of firmness determination and are consistent with the hypothesis that pectin fragments released by polygalacturonase contribute to the production of autocatalytic (system II) ethylene.

Sensitivity of Tomatoes at Mature-green and Breaker Ripeness Stages to Internal Bruising

“…BR tomatoes were more sensitive to impacts than MG, as previously reported (Halsey, 1955;Hatton and Reeder, 1963;Sargent et al, 1989a). Compression testing has shown that BR tomatoes are measurably softer than MG fruit when deformation is applied along the equatorial plane (Ahrens et al, 1987;Ahrens and Huber, 1990;Kader et al, 1978). In this study, single drops of BR tomatoes from 10 cm caused IB in 15% to 73% of fruit, while two drops on a single location from this height caused 50% to 67% (Tables 1, 2, 3).…”

“…Fruit firmness, as measured by resistance to deformation, also varies with cultivar and most likely influences cultivar susceptibility to IB. 'Sunny' was determined to be moderately firm among five cultivars of fresh-market tomatoes, when measured at the IB-sensitive BR stage (Ahrens and Huber, 1990).…”

Positive and Negative Regulatory Regions Control the Spatial Distribution of Polygalacturonase Transcription in Tomato Fruit Pericarp

“…There is a rough correlation between the level of polygalacturonase activity and the degree of softening among different cultivars and mutants of tomato (Brady et ai., 1983;Speirs et al, 1989;Ahrens and Huber, 1990). However, experiments designed to directly address the role of polygalacturonase during ripening indicate that polygalacturonase activity alone is not sufficient for fruit softening (Giovannoni et al, 1989;Schuch et al, 1991).…”

Physical Properties of Fruit Firmness and Chemical Structure of Cell Walls during Fruit Softening

“…The force required to penetrate the fruit is recorded as a firmness index (Kojima et al 1991(Kojima et al , 1992. Flat probe is also used for compression of excised fruit (Arhens, and Huber 1990). Since the rate of travel is precisely controlled, the force data are more reliable and reproducible than those by a Magness-Taylor puncture tester.…”