Effects of Daily Photoperiod/Nyctoperiod and Temperature on Autumn Development of Crown Buds and Dormancy, Freeze Tolerance, and Storage of Food Reserves in Latitudinal Ecotypes of Biennial

White Sweetclover

Leslie J. Klebesadel, Emeritus Professor of Agronomy

Agricultural and Forestry Experiment Station; Palmer, Alaska

Bulletin 95; November 1993 (22 pages)

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Summary

The investigation consisted of both an outdoor experiment and one conducted indoors in controlled-environment chambers. Objectives were to determine during late-summer/autumn the effects of (a) the changing daily light/dark (photoperiod/nyctoperiod) pattern, and (b) seasonally lowering temperatures on pre-winter behavior of sweetclover. Measurements were made of certain morphological characteristics and various facets of the winter-hardening process. Both experiments were conducted at the University of Alaska's Palmer Research Center (61.6°N) in southcentral Alaska.

Ecotypes

Three strains of biennial white sweetclover (Melilotus alba Desr.) ­ adapted to widely diverse latitudes ­ were compared for responses to treatments. The northernmost-adapted ecotype, 'Matanuska white,' is an introduced strain that has undergone adaptive modification through natural selection during many years at 61.6°N in Alaska's Matanuska Valley. The intermediate-latitude ecotype was the Canadian cultivar 'Arctic' adapted at 50° to 56°N; and the southernmost-adapted ecotype was the U.S. cultivar 'Spanish,' adapted at 35° to 50°N.

Outdoor Experiment

All three were broadcast-seeded in field plots 28 June and subjected to four different patterns of photoperiods/nyctoperiods after 25 August. Some plants were dug on 20 September and more on 12 October (the latter near the time of seasonal soil freeze-up) to measure treatment effects. Experimental photoperiod/ nyctoperiod patterns compared were (a) normally prevailing for 61.6°N (photoperiods decreasing gradually

from 15 to 10.5 hours), (b) normally shortening photoperiods but with nyctoperiods interrupted for 90 minutes near midpoint, (c) constant, long (15-hour) photoperiods, and (d) photoperiods gradually shortened artificially during the same calendar period (from 10.5 to 9 hours), simulating conditions prior to onset of winter conditions at more southern latitudes.

· Ecotypes differed under normal photoperiods and temperatures; subarctic-adapted Matanuska white produced the most and largest crown buds and developed highest level of freeze tolerance. Spanish, the most southern-adapted, produced the fewest and smallest crown buds, had lowest dry-matter concentration in overwintering tissues, and developed least freeze tolerance. Arctic, of intermediate latitudinal adaptation, was intermediate in all respects.

· Artificially shortened photoperiods caused the two introduced cultivars to develop the most and largest macroscopic crown buds, smaller mainstem diameters, highest levels of both dry-matter concentration and stored food reserves in overwintering tissues, and greatest tolerance to freeze stress.

· Interrupted nyctoperiods, at the other extreme, resulted in lowest dry-matter concentration in overwintering tissues and greatest injury from freeze stress in all three strains. Moreover, interrupted nyctoperiods caused poorest development of crown buds and lowest levels of stored reserves in Arctic and Spanish. These results confirm that the integrity and the duration of nyctoperiods are critical environmental stimuli for inducing plants to prepare adequately for winter.

· Crown-bud development and food-reserve storage of Matanuska white were generally less affected by photoperiod treatments than occurred with Arctic and Spanish.

Growth-Chamber Experiment

· With progressively shortened photoperiods (12 to 6 hours) in growth chambers, widely alternating but gradually lowering temperatures (daily minimum lowered from 40° to 32°F) resulted in vastly-increased freeze tolerance and greater food-reserve storage than were engendered by continuous warm temperatures.

· In contrast to the considerable influences of light/dark treatments on development of crown buds and dry-matter concentration in overwintering tissues in the outdoor experiment, lowering temperatures in the indoor experiment did not affect those characteristics in any of the three sweetclover strains.

General

· The most favorable growth-chamber treatment for high levels of stored food reserves and maximum development of freeze tolerance (strongly alternating but consistently and regularly lowering temperatures with shortening photoperiods) was markedly more effective with all ecotypes than the most favorable treatment in the outdoor experiment (shortened photoperiods). The considerable difference between the most favorable treatment in each of the two experiments in the levels of food-reserve storage and freeze tolerance achieved is believed attributable to less effective temperature stimulus in the outdoor experiment; randomly fluctuating temperatures with relatively warm minima occurred during much of the growth period outdoors prior to freeze-tolerance tests.

· The pre-winter dormancy detected in sweetclovers in this and other studies at this location was confirmed to be a response to shortening photoperiods/lengthening nyctoperiods, rather than lowering temperatures.

· These results confirm that adaptation to subarctic photoperiod/nyctoperiod pattern during the time of lowering temperatures in late summer and autumn is essential for attainment of optimum morphological and physiological changes associated with typical biennial habit of sweetclover and best winter survival at this latitude. Additionally, however, appropriately lowering temperatures also contribute vitally to high levels of food-reserve storage and to maximum development of freeze tolerance. Low temperatures alone apparently propel freeze-tolerance development to maximum levels that are achieved some weeks after the foliage-receptor of the photoperiod/nyctoperiod stimulus has been destroyed by killing frost.

· These findings also provide insights into the effects of natural selection over many years on Matanuska white­an introduced, naturalized sweetclover strain.

· Finally, these results provide a better understanding of why northern-adapted plants tend to survive winters well at this latitude and why those adapted at more southern latitudes generally are not adequately stimulated in this subarctic area to undergo fully the physiologic alterations that contribute to good winter survival.