HEAT-SHOCK INCREASES CHILLING TOLERANCE OF MUNG BEAN HYPOCOTYL TISSUE

The effects of heat shock on the chilling tolerance of mung bean [Vign a radiata (L.) Wilczek] seedling tissue were studied by using two meas urements of chilling injury: increased 1-aminocyclopropane-1-carboxyli c 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 wh ich the seedlings were grown. However, this highly temperature-depende nt level of ACC oxidase activity was probably a wound response since i t was almost entirely eliminated by incubating the excised segments at 20-degrees-C for 3 h. In contrast, heating of excised segments to 40- degrees-C for up to 4 h resulted in a time-dependent increase in ACC o xidase activity which was sensitive to cycloheximide, indicating rapid protein synthesis during the heat treatment. ACC oxidase activity fel l sharply during subsequent chilling at 2.5-degrees-C. After 3 days of chilling, all treated segments. regardless of their initial ACC oxida se activity, showed a decline to the same low activity level and ACC o xidase activity continued to fall slowly for up to 9 days at 2.5-degre es-C. Hypocotyl segments excised from seedlings held at 15-degrees-C s howed no change in solute leakage, but leakage increased rapidly when seedlings were either chilled at 2.5-degrees-C or heated to 32-degrees -C (just below the heat shock temperature). Chill-induced leakage from non-heat-shocked segments increased steadily with chilling duration a nd was unaffected by cycloheximide concentration up to day 6. Within t he elevated rate of leakage on day 9, however, leakage was lower from segments exposed to 10 and 50 muM cycloheximide. Solute leakage was ma rkedly reduced for up to 9 days when segments were heat shocked at 40- degrees-C for 3 or 4 h with or without 10 muM cycloheximide, but the p resence of 50 muM cycloheximide caused an initial doubling of solute l eakage and a 3-fold increase after 3 days of chilling. Cycloheximide p revented formation of heat shock protection against chilling from the start at 50 muM and after 9 days at 10 muM. These results indicate tha t the protection afforded by heat shock against chilling damage is qua ntitative and probably involves protein synthesis.