Pawpaw [Asimina triloba (L.) Dunal] Fruit Ripening. I. Ethylene Biosynthesis and Production

Pawpaw fruit ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities, and tissue content of the ethylene precursor ACC and conjugate malonyl-ACC (MACC) were measured during postharvest ripening. Fruit were harvested near the advent of the ripening process and were ripened at room temperature. The fruit displayed increases in ethylene production and respiration rate during ripening with maxima for both 3 days after harvest. Mean ethylene maxima on a fresh weight basis were 4.7 and 7.6 μg·kg–1·h–1 and mean respiratory (CO2 production) maxima on a fresh weight basis were 220 and 239 mg·kg–1·h–1 in 1999 and 2001, respectively. The increase in ethylene evolution coincided with an increase in respiration and a rapid decline in fruit fi rmness. Internal and external fruit fi rmness declined in a parallel manner. The ethylene climacteric peak occurred after the greatest decline in fruit fi rmness, indicating that low levels of ethylene may be suffi cient to initiate the ripening process. The ethylene climacteric peak also coincided with the highest activities of both ACS and ACO as well as the maximum tissue ACC content. As ACC content increased, MACC content declined, suggesting a regulation of ethylene production via free ACC levels by malonylation of ACC. Thus, the climacteric development of ethylene production may be regulated by an increase of ACS activity and a decrease in ACC malonyltransferase activity, making more free ACC available for the production of ethylene by increased activity of ACO. The pawpaw belongs to the Annonaceae family, which includes cherimoya (Annona cherimola Mill.), sweetsop or sugar apple (Annona squamosa L.), custard apple (Annona reticulata L.), atemoya (Annona squamosa x A. cherimola), and soursop (Annona muricata L.). The genus Asimina Dunal is the only temperate zone member of this tropical family (Darrow, 1975; Kral, 1960). The few earlier studies of pawpaw ripening have described qualitative changes including an increase in soluble solids content, a rise in volatile production, and rapid loss of fi rmness (McGrath and Karahadian, 1994a, 1994b; Shiota, 1991). The shelf life of pawpaw fruit at ambient temperature does not exceed 5 d, although cold storage may delay ripening for at least 30 d (Archbold and Pomper, 2003). The rapid rate of pawpaw ripening and deterioration may have led to its lack of horticultural attention to date. Although pawpaw was initially confused in literature reviews with papaya and mistakenly characterized as a climacteric fruit many years ago, it does in fact exhibit respiratory and ethylene climacterics ≈3 d after harvest when held at ambient temperature (Archbold et al., 2003; Archbold and Pomper, 2003). The other Annonaceae fruit noted above are climacteric as well (Brown et al., 1988; Martinez et al., 1993; Paull, 1982; Tsay and Wu, 1989; Received for publication 25 May 2004. Accepted for publication 14 Jan. 2005. 1Former: Graduate Student, Dept. of Horticulture, Univ. of Kentucky, Lexington,