The study of heat stress in tomato fruits by NMR microimaging

The ripening of the tomato fruit was delayed for several days (average 5 days) by a 1-day heat treatment at 42°C. Ethylene production increased during the first 3 h, but, after 6 h inhibition was almost total in tomato fruit incubated at 42°C. However, recovery of ethylene production was rapid if fruits were returned to a temperature of 25°C after heating. In NMR microimaging, three imaging pulse sequences with different repetition and echo times at 42°C were used to obtain the proton density (TR = 6000 ms, TE = 15 ms), the T1 weighted image (TR = 1000 ms, TE = 15 ms) and the T2-weighted image (TR = 6000 ms, TE = 120 ms). After 12 h heating, the water in locular tissues began to show shorter T1 and T2 values. Though the tomatos were returned to 25°C and preserved one more day, the water having a shorter T2 value in locular tissues, did not change. These results show that tomato fruit do not fully recover from heating even after one day, although ethylene production is recovered almost immediately. For this reason, we suggest that some denaturation event inside the tomato, which goes on after the end of heating, is the cause of the delay in tomato ripening.