Culture conditions and detachment of the fruit influence the effect of ethylene on the climacteric respiration of melon

The respiratory rise associated with ripening has been considered a defining feature of climacteric fruit. However, recent work has suggested that the respiration rate remains constant in melon fruit ripening whilst attached to the plant. To clarify the relationship between attachment and the respiratory climacteric, ethylene production and O₂ consumption were monitored during the ripening of melon fruits grown in France and Australia. All fruit allowed to ripen after being detached from the vine produced both respiratory and ethylene climacterics. However, although ethylene production increased in attached fruit, the concurrent rise in respiration was either eliminated or reduced in magnitude. A positive relationship was found between ethylene levels and respiration rate. However, for a given rate of ethylene production the stimulation of respiration was less in attached than detached fruit. The application of exogenous ethylene to melons antisensed for ACC oxidase stimulated O₂ consumption only if they were detached from the vine. These data also demonstrate that attachment to the plant inhibits the effects of ethylene on respiration. In addition, it was noticed that transgenic plants were better able to tolerate infection by powdery mildew than wild-type plants. As ethylene production was greater in attached fruit, it is further suggested that feedback inhibition of ethylene synthesis is reduced while fruit remain attached to the vine. It is concluded that the presence or absence of a respiratory climacteric in fruit attached to the vine is affected by environmental factors influencing plant development. These effects may be related to changes in gas permeance, turgor, or the supply of ‘plant factors’ translocated through the phloem. The results support the hypothesis that the respiratory climacteric is not an essential part of ripening, but an artefact caused by stress or detachment.