RELATIONSHIP BETWEEN SOFTENING AND THE POLYURONIDES IN RIPENING BANANA FRUIT

Banana fruits, Musa (AAA Group, Cavendish subgroup) 'Williams' were ripened in air at 20-degrees-C with ethylene. Pulp began softening between days 1 and 2 of ripening and reached a maximum by the fourth day. Total pulp cell wall uronic acid and uronic acid soluble in 40 mM EDTA, 50 mM acetate, pH 4.5, also began to decrease and increase respectively between days 1 and 2. By day 8, total uronic acid had decreased from 10.2 to 4.4 mg g-1 fresh weight, and had become entirely soluble in EDTA-buffer, while EDTA-buffer-soluble uronic acid had increased from 2.3 to 4.5 mg g-1. The molecular size distribution of the EDTA-buffer-soluble uronic acid was unchanged up to day 4, when there was a slight loss in the proportion of smaller species. The average molecular size of this uronic acid did not change significantly during 8 days of ripening (relative to dextrans, M(n) 36 kDa; M(w) 173 kDa). The large change in total content and extractability of cell wall polyuronides that correlated with softening was inconsistent with depolymerisation by endopolygalacturonase (EC 3.2.1.15), reduced cross-linking of polyuronides by calcium, or extraction artefacts.