Morphological, Physiological, and Winterhardiness Comparisons

Among Latitudinal Ecotypes of Biennial Sweetclover (Melilotus species)

in Subarctic Alaska

Leslie J. Klebesadel, Emeritus Professor of Agronomy

Agricultural and Forestry Experiment Station; Palmer, Alaska

Bulletin 91; November 1992 (23 pages)

[ Entire Publication | Bulletins | AFES Publications ]

Summary

Objectives of this study were to compare, within two species of biennial sweetclover, several morphological and physiological characteristics of strains adapted to a wide range of latitudes and to relate those characteristics to winter survival and forage production in subarctic Alaska. All experiments were conducted at the University of Alaska's Matanuska Research Farm (61.6°N) near Palmer in southcentral Alaska.

Strains included in the most extensive comparisons were two long-resident, introduced roadside populations at 61.6°N in Alaska's Matanuska Valley; both were suspected of having undergone some degree of genetic modification toward subarctic adaptation as a result of natural-selection pressures during many generations in the Matanuska Valley. One of those was grown from seed gathered from a colony of biennial white sweetclover (Melilotus alba Desr.), called "Matanuska white" in this report, and the other was from a stand of biennial yellow sweetclover (M. officinalis [L.] Lam.), referred to as "Palmer yellow."

Also compared were two cultivars of intermediate latitudinal origin, adapted at 50° to 56°N in Canada; these were Arctic (white) and Erector (yellow). Two cultivars of still more southern adaptation (35° to 50°N in the conterminous United States) were Spanish (white) and Madrid (yellow). Hubam, a cultivar of annual white sweetclover (M. alba var. annua Coe) was included, also.

Others compared for winter hardiness only were biennial white cultivars Cumino and Polara from Saskatchewan and Denta from Wisconsin, and biennial yellow cultivars Yukon from Saskatchewan and Goldtop from Wisconsin. Additionally, experimental lines derived from certain of the above-mentioned strains, through seed harvest from surviving plants following one to several generational cycles of natural selection for winter survival, were also compared for

progress toward increased winter hardiness.

·All strains flowered profusely during the seeding year, a phenomenon not seen in biennial sweetclovers when grown at more southern latitudes. This and other nontypical behavioral responses of introduced biennial cultivars are attributable to growth under unaccustomed patterns of long daily mid-summer photoperiods/short nyctoperiods at subarctic latitudes, far north of the historic global areas of sweetclover culture.

· The southernmost-adapted biennials exhibited growth characteristics more like the annual cultivar Hubam than like biennials in the seedling year. They produced high forage yields from tall, large-diameter stems, formed few and small crown buds, stored low levels of food reserves, and developed inadequate levels of freeze tolerance; thus their winter survival was very poor.

· The cultivars Hubam, Spanish, and Madrid, that produce tall growth in the seeding year with early planting, and, by inference, newer low-coumarin biennial cultivars adapted at 30° to 50°N, should be considered useful for annual forage production in Alaska.

· Contrasted with recently introduced cultivars, the long-resident subarctic strains produced lower seeding-year forage yields from shorter plants with smaller-diameter stems, formed more and larger crown buds, stored high levels of food reserves, achieved high levels of freeze tolerance and dormancy, and survived winters well when cold stress was not extreme. Except for flowering, the two subarctic strains generally exhibited developmental and physiologic characteristics during the seedling growing season typical of biennial sweetclover when grown where it is adapted.

· The Canadian cultivars tended to be intermediate in

most characteristics between the southernmost-adapted and the subarctic strains. Arctic, and a related strain (AK-Syn.1) selected in Alaska from Arctic for enhanced winter survival, tended to survive better than the roadside strains during extremely cold winters, presumably because field-selection for winter hardiness selects for greater tolerance to certain winter-stress facets than occur in the snow-insulated roadside habitats where the subarctic strains evolved.

· Matanuska white initiated significant storage of food reserves earlier than the three other biennial white strains, Arctic, AK-Syn.1, and Spanish. Palmer yellow stored higher levels of food reserves than the Canadian cultivar Erector which in turn surpassed the more southern-adapted U.S. cultivar Madrid. The annual Hubam stored no measurable food reserves.

· The very slow expression of stored food reserves as etiolated growth by Matanuska white, and to a lesser extent by AK-Syn.1, is interpreted to indicate a state of dormancy that could be broken in two ways: (a) when autumn-dug Matanuska white plants were subjected to three freeze-thaw cycles before placement in the warm, dark chamber, most of total stored food reserves were released to grow to exhaustion in only 6 weeks in contrast to 18 weeks required without the freeze-thaw treatment, and (b) when winter-surviving plants of Matanuska white and AK-Syn.1 were dug in early May, dormancy was no longer evident.

· The somewhat inconsistent ranking of the most winter-hardy strains in percent winter survival from year to year suggests that the dominant stresses operative in killing of plants differ from winter to winter. For example, it is believed that the subarctic-adapted roadside strains that evolved locally over many years and possess conspicuous pre-winter dormancy survive winters best when the dominant stress is freeze-thaw temperature oscillations. In contrast, experimental lines derived during a shorter period of natural selection in more exposed field nurseries apparently survive winters better than the roadside strains when the dominant stress is low temperature.

· Selection for improved winter survival by harvesting seed from surviving second-year plants in stands of biennials severely decimated by winter kill showed promise on the modest scale pursued. The increase in winter hardiness through this avenue tended to be more rapid and effective in a relatively nonhardy cultivar than in more winter-hardy strains. This strategy should be expanded in the future with low-coumarin strains toward developing subarctic-adapted cultivars free of the disadvantages inherent in high-coumarin sweetclovers.

· These results provide a better understanding of the relationships of freeze-tolerance development, food-reserve storage, and dormancy to winter survival of biennial sweetclover in the Subarctic. They also demonstrate why only northern-adapted crop strains can be depended upon for satisfactory winter survival in northern agricultural areas.

· The obligate biennialism of both sweetclover species facilitates rapid generational cycling which in turn accelerates gene sorting through natural-selection pressures in a new environment. Natural selection over many generations in this subarctic environment apparently is adapting the introduced roadside populations to the unusual photoperiod/nyctoperiod regimen characteristic of this high latitude and thereby shifting them back toward their original biennial habit. This adaptive modification is occurring at a latitude where biennial sweetclovers newly introduced from more southern latitudes initially are ill-suited to function as biennials, due to the northern growing season's unique and unaccustomed diurnal photoperiod/nyctoperiod pattern.

· These documented instances of adaptive modification toward harmony with subarctic climatic influences are informative in appreciating the breadth and plasticity inherent within a species gene pool and how environmental influences can act through natural selection to shift the genetic constitution of a population toward more successful survival.