P-type H+-ATPases activity, membrane integrity, and apoplastic pH during papaya fruit ripening

The P-type H+-ATPase activity and related apoplastic pH were analysed in papaya fruit (Carica papaya L. cv. Golden) at different stages of maturation. Postharvest ripening of papaya was characterized by ethylene emission, skin colouration, firmness, and titratable acidity (TA). The climacteric peak of ethylene production occurred on the 2nd day after harvest, along with a sharp decrease in ATP hydrolysis and a transient alkalization of the apoplast fluid. Such effects were consistent with progressive cell membrane deenergization occurring concomitantly with fruit softening and decrease of cell membrane integrity as manifest by electrolyte leakage. Nevertheless, it was possible to detect an expressive ATP-dependent H+ gradient 2 days after the climacteric suggesting the maintenance of cellular function even in the late stages of the fruit ripening. The data provide new insights into the physiology of ripening in papaya and how that P-type H+-ATPase activity can be used as a biochemical marker for postharvest analysis and management of climacteric fruit.