JARS v38n4 - Cold Acclimation of Plant Parts in an Evergreen and a Deciduous Azalea

Cold Acclimation of Plant Parts in an Evergreen and a Deciduous Azalea
Leslie A. Alexander and John R. Havis 2
Department of Plant and Soil Sciences
University of Massachusetts, Amherst, MA

Reprinted from Hort Science 15(1):89-90. 1980.

Cold hardiness of branch, lower stem and root parts of Rhododendron cvs. Mother's Day (evergreen) and Homebush (deciduous) were determined after artificial acclimation. Lower stems of the evergreen azalea acclimated more slowly than the upper branches. In the deciduous azalea the lower stems acclimated more rapidly than the upper branches initially, but both parts were equal in hardiness after 53 days of acclimation. The lower stem of the deciduous acclimated more rapidly than that of the evergreen azalea.

Freeze injury to lower stem tissues, which may manifest itself as bark splitting, is a problem common to many evergreen azaleas. This lower stem injury often occurs in the autumn when severe frosts occur early in the season, and suggests that the plants have not fully acclimated. Injury of this type is not common in deciduous azaleas, which are generally more cold hardy than evergreen varieties.
A delay in the acclimation process in the lower stem could account for the injury being localized in that area. Sakai (2) studied cold hardiness in a variety of woody plants and found that the lower stem area was less hardy than the upper parts of many plants.
Steponkus and Lanphear (3) studying the light stimulation of cold acclimation in Hedera helix found that hardiness promoting substances moved through the phloem in an acropetal direction in the early stages of the acclimation process. As acclimation progressed, the promoters were found to move in a basipetal direction as well, and flow was then through the xylem as well as the phloem. Thus, the induction of cold hardiness proceeded in phases throughout the plant, with the apex of the plant the first area to harden.
The following study was undertaken to follow the progression of cold acclimation in an evergreen and a deciduous azalea species.
One year old plants from cuttings, of an evergreen azalea 'Mother's Day', a kaempferi hybrid, and a deciduous azalea 'Homebush', a Knap Hill hybrid, were grown in 15 cm diameter azalea pots containing a 1 peat: 1 sand mix (by volume). These plants were moved in mid-July, 1977, from an outdoor growing area into a growth chamber, where they received an 8 hr light period (11 klx intensity) and steadily declining temperature. The temperature schedule was as follows: 9° ± 1°C day, 5° ± 1° night for 2 weeks; 5° ± 1° day, 3° night for 4 weeks; and 5° ± 1° day, 1° night for 2 weeks. Plants were tested for cold hardiness at the beginning of the experiment and at the fourth and eighth weeks of the acclimation study. Samples from the branch, stem and root were used. Samples from the branch 3 cm in length were taken 3 cm from the tip. The lower stem sections were sampled from soil line up to 4 cm. Sections of woody roots 1 mm in diameter and 23 cm in length were excised. These roots were classified as secondary mature roots by Mityga and Lanphear (1). Four samples of each section from 3 plants were frozen at each of 4 test temperatures. The plant tissue was placed in a 50 ml plastic centrifuge tube, capped, and frozen in a circulating methanol bath, with a temperature reduction of 2.5 per hr. The tissue remained at the temperature to be tested for 1 hr and then was removed and allowed to thaw at 2. Injury was determined using the modified ninhydrin technique (4). Preliminary tests showed that the tissues were injured at release of 29-33% of the total ninhydrin-reactive compounds and were dead at 33% and higher. Release of 33% was used to represent the killing temperature. Extra tissue was frozen in separate tubes, incubated and examined for browning to confirm injury.

Figure 1
Fig. 1. Cold acclimation of branch, lower stem and root of 'Mother's Day'
(evergreen) and 'Homebush' (deciduous) azalea plants.

Upper branches of both the evergreen and the deciduous azaleas acclimated to a greater degree than the roots, and the roots of both acclimated linearly with time (Fig. 1). These results agree with those of a similar study of pyracantha branches and roots (5). The branches of the deciduous azalea exhibited an initial lag before rapid acclimation, which was also similar to pyracantha (5). The deciduous lower stem acclimated linearly with time, but more rapidly than the roots. After 25 and 53 days of acclimation, the lower stem of the deciduous was about 5°C hardier than that of the evergreen azalea.
The evergreen azalea exhibited a different pattern of acclimation. The upper branches of this plant acclimated rapidly the first 26 days, but the lower stem tended to lag and was similar to the roots in hardiness. This slow rate of acclimation of the lower stem could be caused by a quantitatively preferential movement of hardiness promoters in an acropetal direction. However, we did not see a marked sequence of hardening such as that reported for Hedera (3). This could possibly have occurred before the first test date.
These results suggest that deciduous and evergreen azaleas differ in rate of acclimation. Deciduous azaleas are generally more cold hardy than the evergreen species, and as the results from this study indicate, each section of the deciduous azalea was hardier than the corresponding sections of the evergreen plant at the termination of the experiment. The lower stems showed the largest differences. Of course, various cultivars of evergreen azaleas differ in acclimation rate and ultimate cold hardiness. However, the slow rate of acclimation of evergreen azaleas, especially the lower stems, may account for the frequent incidence of lower stem injury seen in these species.

Literature Cited
1.  Mityga, H.G. and F.O. Lanphear. 1971. Factors influencing the cold hardiness of Taxus cuspidata roots. J. Amer. Soc Hort. Sci. 96:83-86.
2.  Sakai, A. 1968. Frost damage on basal stems in young trees. Contr. Inst. Low Temp. Sci. B 15:1-14.
3.  Steponkus, P.L. and F.O. Lanphear. 1967. Light stimulation of cold acclimation: Production of a translocatable promoter. Plant Physiol. 42:1673-1679
4.  Wiest, S.C., G.L Good, and P.L. Steponkus. 1976. Evaluation of root viability following freezing by the release of ninhydrin-reactive compounds. HortScience 11:197-199.
5. _________and P.L. Steponkus. 1976. Acclimation of pyracantha tissues and differential thermal analysis of the freezing process. J. Amer. Soc. Hort. Sci. 101:273-277.