Articles
IMPACT OF A CHRONIC EXPOSURE TO A HIGH TEMPERATURE STRESS IN TOMATO FRUIT RIPENING
Exposure for some days to temperatures of 30 to 40°C (depending upon cultivar) will block tomato ripening through thermoinhibition (Shewfelt, 1993). Although the process is reversible and ripening can be restored after lowering the temperature, a high temperature stress may permanently decrease quality by causing tissue damage and physiological disorders (Inaba and Crandall, 1988). The major biochemical events caused by heat stress are reduced ethylene production (Cheng et al., 1988) and polygalacturonase activity (Yoshida et al., 1984). Loss of certain neutral sugars (especially galactose) is also diminished by means of heat treatment (Mitcham and McDonald, 1992). The aim of our work was to follow the pattern of some ripening-related events (flesh firmness, pigment content, skin color change,
– and 
-galactosidase and endo--mannanase activities) in detached tomato fruit under both normal and heat-stress conditions.Detached mature-green fruits were incubated at 40 ± 0.2°C for 2 days and transferred to a 21 ± 1°C dark environment for further ripening.
Control fruit were stored in the 21 ± 1°C dark conditions from the beginning of the experiment.
Measurements were performed every two days during 18 days.
Exposure of tomato fruit to high temperatures disrupted softening, as indicated by fruit firmness evaluation.
The rates of skin color change, internal chlorophyll degradation and lycopene synthesis were also affected by the high temperature stress.
Tomatoes submitted to 40°C presented 36% cracking, and proved to be more prone to infection.
Preliminary ultrastructural studies showed that high temperature stress provoked a breakdown of membrane integrity with disruption of cellular organization.
Endo-a-mannanase activity was not detected in mature-green unheated controls but appeared at the breaker stage, rose rapidly between the breaker and the pink stage, and plateaued near the red-ripe stage.
In heat-treated tomatoes, endo--mannanase activity could be detected only 6 days after the fruit had been transferred to 21°C, and it increased slowly to reach 24% of that attained by the control group on the 18th day.
In control fruit, activity was higher for – than for – galactosidase, slightly increasing toward the red-ripe stage.
Both – and -galactosidase activity decreased in heat-treated tomatoes to a minimum of 22 and 20%, respectively, of the initial activity on the 4th day. -Galactosidase activity showed a low rate of recovery but the resumption of -galactosidase activity was almost complete after 16 days at 21°C. Results could be associated with an active role of these enzymes on the fruit ripening process.