Pineapple fruits of susceptible Cayenne variety exposed
to 10 and 25 degree exhibited electrolyte membrane
leakages unlike resistant MD2 (fig 2). Thus pineapple
fruit resistance to cold stress correlated well with
electrolyte leakage as it has been shown previously (2).
Our data support the findings that fruit and plant
varieties exhibited various resistance to membrane
damage (related to membrane permeability), the
resistant variety, greater membrane stability under
stress. Results concerning POX activity suggested that
this enzyme is involved in enzymatic browning (Fig 3)
and conversely correlated with cold stress : MD2 variety
(resistant) showing the lower values as compared to
Cayenne (susceptible).
Cold stress induced an increase in PAL and POD activity
in both pineapple varieties (MD2 and Cayenne) during
continuous low temperature storage (F), being higher in
C variety than in MD2. Rising the temperature after cold
stress lowered PAL and POD activities (fig 4 and 5).
According to the results : membrane permeability, POX,
PAL and POD activities could be useful parameters to
screen varieties for cold stress tolerance.
Previous studies (3) showed that endoproteolytic
activities were involved in drought stress response of
plants. Similarly the results obtained showed that cold
stress induced a decrease in protease gene expression,
particularly in the resistant variety (MD2). Therefore, it
seemed that endoprotease activity is likely to play a
role in chilling susceptibility. Experiments are in
progress on proteolytic enzyme regulation during
pineapple enzymatic browning.