Polygalacturonase Activity Does Not Fully Explain Textural Differences of Melting Flesh versus Non-melting Flesh Peaches

摘要

Texture is the main distinction between melting-flesh (MF) and non-melting flesh (NMF) peach cultivars. MF peaches soften extensively toward the end of the ripening process while NMF peaches soften more slowly and lack the "melting" stage of fruit softening. Two pectolytic enzymes thought to be involved in peach softening are terminal cleaving exo-polygalacturonase (exo-PG) and random cleavingendo-PG. The decreased capacity of NMF peaches to degrade cell walls (i.e., soften) is thought to be relatedto a deletion of endo-PG gene or a truncation of the mRNA. Thus, NMF cultivars would be expected to possess lower endo-PG activity than MF cultivars. In this study, the extractable PG activity of two MF cultivars, 'Flordaprince' and 'TropicBeauty', and two NMF cultivars, 'UFSun' and 'Gulfking', during the climacteric ripening stage were determined. 'Flordaprince' possessed similar endo- and exo-PG activities as the two NMF cultivars while 'TropicBeauty' had the highest endo- and exo-PG activities of thefour cultivars. Surprisingly, the endo-PG activity of NMF 'Gulfking' was significantly higher than its exo-PG activity and was also higher than that of MF 'Flordaprince'. However, the higher endo-PG activity of 'Gulfking' was not reflected in its texture since it was almost five times firmer when ripe than 'Flordaprince'(10.77 N vs. 2.34 N), which implies that endo-PG activity does not fully explain the textural differences between MF and NMF peaches. Since cell wall disassembly presumably involves concerted and synergistic action of several different enzymes, other cell wall modifying enzymes may have a more crucial role than PG during peach fruit softening.