Apple and tomato fruits were analysed for changes in sensitivity to et
hylene during the later stages of their development. Dose-response stu
dies were conducted by measuring in vivo activity of ACC-oxidase and C
O2-production after treating the fruit with increasing concentrations
of ethylene. ACC-oxidase activity of apple fruits was increasingly sti
mulated by ethylene as fruit development progressed. Ethylene-stimulat
ed respiration (CO2-production) on the other hand was hardly affected
by the stage of fruit development. In contrast to apple fruits tomatoe
s do not seem to undergo a significant change in their ethylene sensit
ivity as far as the activity of ACC-oxidase and autocatalytic ethylene
production is concerned. The ethylene concentration required for half
maximal stimulation of ACC-oxidase activity and CO2-production was ca
lculated to be 0.8 and 1.3 mul . l-1, respectively, and was almost ide
ntical for apple and tomato fruits. The results are discussed in relat
ion to several hypotheses of receptor-dependent phytohormone sensitivi
ty.