RAPID-FREEZE ACCLIMATION OF PONCIRUS-TRIFOLIATA SEEDLINGS EXPOSED TO 10-DEGREES-C AND LONG DAYS

Poncirus trifoliata (L.) Raf. seeds were germinated in perlite under i ntermittent mist at about 25 degrees C and natural daylight in a green house, Two-week-old seedlings were then transferred into a growth cham ber at 25 degrees C and 16-hour daylength for 1 week, Tissue samples w ere collected at 0, 6, 24, 168, and 504 hours after temperature equili bration at 10 degrees C. Freezing tolerance at -6.7 degrees C, as dete rmined by electrolyte leakage, and stem (leaves attached) water potent ial (Psi(x)), measured using a pressure chamber, was recorded for a su bset of seedlings for each time interval, Red coloration (apparently a nthocyanin) developed at the petiole leaflet junction and buds after 4 8 hours at 10 degrees C and gradually occurred throughout the leaves d uring further exposure. Complementary DNA clones for phenylalanine amm onia lyase (PAL), 4-coumarate : coA ligase (4CL), and chalcone synthas e (CHS) were used to probe RNA isolated from the leaves, No increase i n steady-state messenger RNA level was detected, Increases in freeze h ardiness occurred within 6 hours in the leaves, and continued for up t o 1 week, Water potential initially decreased from -0.6 to -2.0 MPa af ter 6 hours, then returned to -0.6 MPa after 1 week. Thus, Poncirus tr ifoliata seedlings freeze-acclimate significantly after only 6 hours a t 10 degrees C.