Supercooling and freezing in the main stem of Valencia orange trees

Abstract

Freeze avoidance (supercooling) and subsequent freezing of freeze-sensitive Citrus was determined in controlled environment tests on potted 1- to 2-year-old sweet oranges trees, Citrus sinensis (L.) Osbeck cv. Valencia on rough lemon ( Citrus jambhiri Lush.) rootstock. Cold hardening was done in controlled-environment, walk-in rooms with programmed light/temperature regimes. Trees were frozen in a separate room and freezing was monitored with multiple thermocouples attached to or inserted into different tissues. Apparent rate of ice propagation and apparent initial ice-nucleation temperature were significantly correlated with cold-hardened trees. Supercooling levels were also increased in cold-hardened trees. Leaves were not critical sites for catalyzing freezing in the main stem, and neither bark girdles nor partial stem severance with overlapping cross-sectional cuts slowed rapid ice propagation in the wood. Supercooling in the main stem was largely transient in temperature and stem location. Decreases in supercooling levels were induced with short periods of ice crystallization in trees during repeated freeze cycles. These supercooling levels were fully restored to original levels with longer periods of thaw. The initial perceived nucleation site was not necessarily a site where visible freeze kill developed, and direction of ice propagation was more characteristically from the lower to the upper stem in cold-hardened trees and vice versa in nonhardened trees.