Abstract
M.Sc.
The senescence of climacteric flowers such as carnations is accompanied by an increase
in ethylene synthesis during the later stages. This increase in ethylene synthesis is
preceded by an increase in the sensitivity of the flowers to ethylene. The increase in
ethylene sensitivity is accompanied by a concomitant increase in the levels of short-chain
saturated fatty acids (SCSFAs). Treatment of carnation flowers with SCSFA results in an
increase in ethylene sensitivity. It appears that these acids act by increasing membrane
fluidity, causing slight conformational changes in membrane associated proteins and
thereby increasing the ability of the tissue to bind ethylene to its membrane associated
receptor molecules. The levels of SCSFAs in senescing carnation petals is controlled by
the activity of the enzyme acetyl-coenzyme A carboxylase (ACCase). A decrease in the
activity of this enzyme results in an increase in the levels of the SCSFAs and vice versa.
During the senescence of carnation flowers, ACCase activity fluctuated from day to day.
This fluctuation can be correlated to the fluctuations in the ethylene sensitivity of the
flowers on a daily basis. In carnation petals, ACCase is located mainly in the plastids.
ACCase activity could be controlled via feedback inhibition by long-chain fatty acids
such as oleic acid. Treatment of carnation flowers with oleic acid resulted in a
concomitant inhibition of ACCase activity, an increase in SCSFA-levels and an increase
in ethylene sensitivity. Oleic acid is a competitive inhibitor of ACCase activity, and
changes in the levels of oleic acid will affect the activity of the enzyme. An increase in
oleic acid concentration resulted in a decrease in enzyme activity. However, in
carnations it appears that ACCase activity is not controlled via feedback inhibition by
long chain saturated fatty acids. The results of this study clearly show that ACCase
activity is controlled directly by the expression of at least the biotinylated (BCCP)
subunit of the enzyme. A decrease in the expression of the gene during the early stages
of senescence coincided with a decrease in ACCase activity and was accompanied by a
concomitant increase in ethylene sensitivity. These results indicate that the increase in
ethylene sensitivity caused by an increase in SCSFA levels is directly controlled by the
expression of the ACCase genes.