Readiness to frost harden during the dehardening period measured in situ in leaves of Rhododendron ferrugineum L. at the alpine timberline

The readiness to frost harden during the dehardening period can be crucial to the frost survival of Rhododendron ferrugineum leaves. When already deha rdened plants occasionally fall snow free in late winter or spring still he avy night frosts can occur in the subalpine environment. R. ferrugineum shrubs were treated in situ using a newly developed field po rtable freezing chamber. They were exposed to controlled night frosts of cl ose to the lowest temperature sustained without frost damage (LT0) to deter mine the potential frost hardening response under otherwise completely natu ral conditions. There was a lag period of 3 days during which no significant increase of fr ost resistance was observed. After three days frost resistance increased su ddenly by 5.9 degrees C within 24 hours. High daytime leaf temperatures (+1 9 degrees C) combined with night frosts further retarded the rate of frost hardening. The in situ frost treatment generally yielded frost resistances (LT10) appr oaching the highest ever measured in that season. In early spring the in si tu frost hardening response was three times greater than early reports for detached twigs with a total hardiness gain between 7.8 degrees C and 9.2 de grees C. It is suggested that the limiting step in frost hardening in leaves of Rhod odendron ferrugineum in spring is not the extent of frost resistance achiev eable but rather the slow rate of frost hardening.