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Several cell wall components in ripening pepino fruit have been quantitatively and qualitatively characterised, with the aim of identifying their contributions to the loss of tissue firmness. Pepinos were graded into nine groups based on progressive, characteristic skin colour changes, previously shown to correspond with decreasing fruit firmness. While fruit softening began when the pepinos were still green but with newly acquired purple stripes, the first significant quantitative signs of cell wall modification (total pectin and hemicellulose content declining and CDTA‐soluble pectin content increasing, on a fresh weight basis) were detectable later in ripening, when the fruit began to acquire yellow skin pigmentation. Gel fractionation studies demonstrated that there were increased levels of low‐molecular‐weight pectin and xyloglucan during pepino ripening. The change in molecular weight distribution of CDTA‐soluble pectin occurred as fruit started to acquire yellow pigmentation, while xyloglucan polymers were modified at an earlier stage that coincided with the initial loss of firmness. © 1997 SCI.

Plasma membrane‐enriched samples were extracted from pepino fruit (cv. El Camino) by phase partitioning. H+‐ATPase (EC 3.6.1.35) activity in these samples increased during late fruit development (immediately before the onset of ripening) and western blotting confirmed there was an increase in enzyme abundance at this time. H+‐ATPase activity decreased during early ripening and then increased again in the final phase of ripening. Immunolocalisation showed the plasma membrane H+‐ATPase was most abundant in the outer cell layers of the fruit, which are considered to have a major role in determining fruit texture. Fruit softening was not accelerated by harvest and there was no stimulation of H+‐ATPase activity by harvest. An in vitro tensile test using fruit rings showed tissue softening proceeded faster at low apoplastic pH (4.5) than at pH 6.5; and tissue buffered at pH 6.5 softened less than unbuffered rings. Erythrosin B, an inhibitor of the plasma membrane H+‐ATPase, also retarded softening in vitro. These data suggest that plasma membrane H+‐ATPase activity may contribute to the onset of pepino softening through a reduction in apoplastic pH.

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