QBARS - v31n4 Flower Bud Hardiness of Rhododendron x 'Kostarianum'

Flower Bud Hardiness Of Rhododendron x 'Kostarianum'
Orville M. Lindstrom, Paul H. Li and Harold M. Pellett
Laboratory of Plant Hardiness
Department of Horticultural Science and Landscape Architecture
University of Minnesota St. Paul, Minnesota

Misc. Journal Series Article No. 1658 of the Minnesota Agricultural Experiment Station.
The research reported in this paper was funded in part by a grant from the American Rhododendron Society.

Plants must adapt methods to resist damage due to freezing, in order to survive winter's coldest temperature. Plants vary as to the amount of cold hardiness they possess. It is of interest then to understand the various mechanisms that plants use to resist the potentially damaging effects of subzero temperatures.
Some plant species can tolerate frozen water within the intercellular spaces of certain tissues. The frozen water, located in the intercellular spaces, has a lower vapor pressure than the liquid water inside of the cell. This causes the intracellular (inside) water to move out of the cell to the intercellular ice where it also freezes. This, in turn, dehydrates the cell and is called freezing-induced cell dehydration. Tissue survival in this case is dependent on its ability to withstand the stress of the freeze induced dehydration. Certain plants; such as the red-osier dogwood ( Cornus stolonifera ) adapt this mechanism and have been shown to be able to survive exposure to liquid nitrogen (-196° C) ¹ .
There are other survival mechanisms for plants exposed to subfreezing temperatures in nature. The freezing point depression of cellular solutions is one mechanism. The lower freezing temperature of this solution is due to solutes present in the cellular water. The more solutes that are present in the water solutions, the lower the freezing point will be depressed. This mechanism is used in conjunction with freeze induced dehydration. When water is pulled out of the cell, the remaining water thus has a higher solute concentration and therefore, a lower freezing point. The freezing point depression mechanism usually protects plants to no lower than -4° C (25° F). "Deep" supercooling is another mechanism of freezing avoidance. This is the phenomenon in which cell water remains in the liquid state at temperatures below the normal freezing point (0° C or 32° F). Water, however, cannot supercool indefinitely; the maximum level which has been observed is at approximately -40° C (-40° F) in some winter hardened woody plants. ² Plant parts that exhibit supercooling do not always achieve this uniform level. Some tissues may supercool only a few degrees while other tissues may supercool to near the -40° C level.
Most cultivated azaleas do not possess a great amount of frost hardiness. It is of interest then, to develop azalea cultivars that can withstand more cold and thus, extend their distribution to colder regions. In locations where winter temperatures dip below the freezing point, azaleas must adapt some cold protection mechanism(s) for winter survival. It has been suggested that the tolerance of extra cellular (outside) freezing together with the avoidance of intracellular freezing are the common mechanisms of freezing survival in plants ³ . Both mechanisms are thought to have been adapted by ( R. japonicum x R. molle ) x 'Kostarianum' as well as many other azalea cultivars ^{4, 5} .
Flower buds were chosen for our hardiness study since they have been reported as being the first tissues of azaleas to be injured by low temperatures ⁴ . The bud of R. x 'Kostarianum' uses more than one mechanism of freezing resistance. It has been found that the bud's scales and axis use the freezing-induced dehydration mechanism for survival. Although the water is frozen in these tissues, the tissues can survive this freezing. Each individual flower primordium, however R. x 'Kostarianum' contains 7-15 of them), uses supercooling as the mechanism of freezing avoidance for survival ⁴ . For this reason the overwintering flower primordia are the most susceptible parts of the bud to low temperatures. In our freezing tests the hardiness of the bud is therefore measured as the average of the freezing points of this weakest tissue.
R. x 'Kostarianum' flower primordia exhibit changing hardiness levels in relation to the time of year (Table 1). The cause(s) that enables the flower primordia to be more hardy in winter than summer is not known. It has been suggested that cool temperatures in the fall and warm temperatures in the spring may play an important role in hardening and de-hardening, respectively. Water content of the buds also shows a close correlation to cold hardiness changes. As the water content of the primordia decreases, the cold hardiness increases. ⁴

As shown in Table 1, all of the flower primordia do not freeze at the same temperature. Although the average freezing temperature of all the flower primordia in one bud is used to estimate cold hardiness; it is possible for a bud to be exposed to this average temperature and still have some of the flower primordia in the bud survive. For example, on January 2, 1976, R. x 'Kostarianum' was observed to have an average primordium killing temperature of -21 ° C. The individual primordia freezing points (survival temperatures) within the bud, however, ranged from -13° C to -25° C. Later that day a cold front moved into the Twin City area, and early on January 5 a minimum overnight temperature of approximately -25° C was recorded. Samples collected on January 5 showed the average hardiness of the flower primordia to be only -24° C. The range, however, then extended from -18° C to -28° C, thus enabling several of the more hardy flower primordia to survive in each bud.
It is evident from this investigation that R. x 'Kostarianum' does have potential resistance to freezing damage down to at least -28° C. Even if it is not possible to increase the maximum cold hardiness level, it may be possible to develop cultivars that can attain maximum cold hardiness earlier in the fall and retain these high levels longer in the spring. This would reduce the possibility of killing the flower buds since freezing usually causes major damage in early fall when flower buds have not been fully acclimated or in late spring when buds normally become less hardy. It is also evident, from the observation of R. x 'Kostarianum' during the cold spell, that it is desirable to select cultivars which can increase their cold hardiness levels at a faster rate to avoid injury due to rapid temperature changes.
In summary:
1) The flower buds of azalea seem to be their weakest organs in cold temperature survival.
2) The scales and axis of the whole flower bud are not harmed by frozen water in their tissues.
3) The water in the cells of the flower primordia supercools. The degree of supercooling, and thus hardiness,  varies with the time of the year.
4) The 7 to 15 flower primordia in a whole flower bud do not all supercool to the same degree, explaining in some cases the death of only a few primordia in a bud.
5) R x 'Kostarianum' can harden in response to approaching cold temperatures and thus may totally or partially resist injury due to freezing.

LITERATURE CITED
1. McLeester, R. C., C. J. Weiser, and T. C. Hall, 1968, Seasonal variations in freezing curves of stem sections of Corpus stolonifera Michx., Plant Cell Physiol. 9:807-819.
2. George, M. F., M. J. Burke, H. M. Pellett, and A. G. Johnson, 1974, Low temperature exotherms and woody plant distribution, HortScience 9:519-522.
3. Levitt, J., 1972, Responses of plants to environmental stress, New York: Academic Press, 697 pp.
4. Graham, P. R. and R. Mullin, 1976, The determination of lethal freezing temperatures in buds and stems of deciduous azalea by a freezing curve method, J. Am. Hort. Soc., 101:3-7.
5. George, M. F., M. J. Burke, and C. J. Weiser, 1974, Supercooling in over wintering azalea flower buds, Plant Physiol., 54:29-35.