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The effects of heat shock on the chilling tolerance of mung bean [Vigna radiata (L.) Wilczek] seedling tissue were studied by using two measurements of chilling injury: increased 1‐aminocyclopropane‐1‐carboxylic acid (ACC) oxidase activity and solute leakage. ACC oxidase activity (measured as ACC‐induced ethylene production) of freshly excised mung bean hypocotyl segments was highly dependent on the temperature at which the seedlings were grown. However, this highly temperature‐dependent level of ACC oxidase activity was probably a wound response since it was almost entirely eliminated by incubating the excised segments at 20°C for 3 h. In contrast, heating of excised segments to 40°C for up to 4 h resulted in a time‐dependent increase in ACC oxidase activity which was sensitive to cycloheximide, indicating rapid protein synthesis during the heat treatment. ACC oxidase activity fell sharply during subsequent chilling at 2. 5°C. After 3 days of chilling, all treated segments, regardless of their initial ACC oxidase activity, showed a decline to the same low activity level and ACC oxidase activity continued to fall slowly for up to 9 days at 2. 5°C. Hypocotyl segments excised from seedlings held at 15°C showed no change in solute leakage, but leakage increased rapidly when seedlings were either chilled at 2. 5°C or heated to 32°C (just below the heat shock temperature). Chill‐induced leakage from non‐heat‐shocked segments increased steadily with chilling duration and was unaffected by cycloheximide concentration up to day 6. Within the elevated rate of leakage on day 9, however, leakage was lower from segments exposed to 10 and 50 μM cycloheximide. Solute leakage was markedly reduced for up to 9 days when segments were heat shocked at 40°C for 3 or 4 h with or without 10 M cycloheximide, but the presence of 50 μM cycloheximide caused an initial doubling of solute leakage and a 3‐fold increase after 3 days of chilling. Cycloheximide prevented formation of heat shock protection against chilling from the start at 50 μM and after 9 days at 10 μM. These results indicate that the protection afforded by heat shock against chilling damage is quantitative and probably involves protein synthesis.

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Ammonia, a product of protein catabolism associated with senescence of leafy green vegetables, is toxic to plant cells if not efficiently re-incorporated by glutamate synthetase. In experiments on various intact and fresh-cut leafy and floral vegetables (broccoli, cauliflower, romaine lettuce, spinach and sugar snap peas) at 5-7.5°C, tissue ammonia concentrations were measured as a possible indicator of stressful modified atmospheres and compared with the time course of changes in concentrations of the fermentative volatiles ethanol and acetaldehyde. In spinach, increases in ammonia were observed with high CO 2 atmospheres and were associated with darkening of leaf tissues. In cauliflower florets, increases in ammonia and tissue discoloration were associated with the increases in fermentative volatiles in 15% CO 2 atmospheres, but minimal changes in ammonia and the fermentative volatiles occurred in air or non-damaging low O 2 atmospheres with 7.5% CO 2 . Increases in ammonia concentrations in sugar snap peas were also associated with increases in fermentative volatiles in 12 and 18% CO 2 atmospheres with 3% O 2 ; these conditions resulted in browning discoloration. The same CA conditions resulted in no damage and only small increases in ammonia and fermentative volatiles in broccoli florets. In cut romaine lettuce, atmospheres with 9% but not 6% CO 2 resulted in notable increases in ammonia, but ammonia was not much affected by differences in O 2 concentrations (0.2, 1.0, or 21%). Increases in fermentative volatiles only occurred in cut lettuce stored in 0.2% O 2 alone or in combination with high CO 2 . These results indicate that ammonia may be a useful indicator of stress in modified atmosphere storage of leafy tissues. In cauliflower and spinach, atmosphere conditions that resulted in increases in ammonia also resulted in increases in fermentative volatiles. In romaine lettuce, ammonia concentrations indicated stressful atmospheres that were not associated with increases in fermentative volatiles. In other vegetables (sugar snap peas), increases in ammonia and fermentative volatiles had different time courses.