The activities of various mitochondrial enzymes in the peel of apples picked before the climacteric peak in respiration (R), were followed throughout the development of the climacteric in storage at 12°C they showed an increase to the climacteric peak and beyond. In pulp tissue the peak values for mitochondrial activity occurred several days before the peak value of R. The soluble enzyme, malic enzyme (M.E.), reached a maximum in both peel and pulp at a point very close to the maximum value of R, while pyruvic carboxylase continued to rise past the climacteric peak.

In fruit picked during the development of the climacteric on the tree and stored at 12°C, mitochondrial activity tended to follow changes in R; in the pulp a falling rate of mitochondrial activity probably commenced somewhat before the commencement of a fall in R. M.E. activity increased rapidly on picking in the early stages of the climacteric but little further increase occurred either in peel or pulp when fruit was picked near the point of maximum activity on the tree. This maximum activity on the tree was reached well before the peak in R and corresponded in time with the maximum rate of R ever reached in detached fruit.

It is concluded that an increase in activity of M.E. and carboxylase is largely responsible for the climacteric rise in R up to the maximum rate attained in detached fruit, and that the excess respiration above this value ‘on’ the tree stems from other sources.