Frost hardiness in the juvenile and adult life phase of ivy (Hedera helix L.)

The common ivy (Hedera helix L.) remains juvenile at its northern, eastern and altitudinal distribution limits although juvenile parts are largely killed by severe frost spells. In order to explain this discrepancy we investigated the seasonal course of frost resistance in various organs of juvenile and adult parts of the same H. helix plants. Maximum frost resistance of leaves (LT(50) -25 degreesC) and axis (xylem parenchyma: LT(50) -29 degreesC; cambium: LT(50) -35 degreesC) was quite the same in juvenile and adult parts. Thus, H. helix is able to acquire full frost resistance in its juvenile phase. However, hardening of leaves was slower and dehardening of axis was faster in juvenile parts. Leaves of juvenile parts remain 2 to 4 K less resistant than those of adult parts until attaining the maximum resistance. This explains why mainly leaves of the juvenile parts were damaged following severe frost episodes with temperatures below -20 degreesC. The occurrence of H. helix in its juvenile phase at the frost-caused distribution limits may be explained as follows: Leaves of juvenile plants may occasionally be killed by severe frosts, but regeneration from dormant eyes enables survival. Loss of leaves may impede the change to the adult phase, but even if the plants became adult frost killing of rest buds (2 to 3 K less resistant than leaves) would induce rejuvenation.