RHA Newsletter 2(7): 16-18 (1970)
TEMPERATURE REVERSAL OF AFTER-RIPENING OF ROSE SEEDS
By Peter Semeniuk and Robert N. Stewart
The following is reprinted from Proceedings of the American Society for Horticultural Science, Vol. 80, 1962,
Irregular seed germination has commonly been a handicap in rose breeding. Crocker (3) described many causes of delayed germination of various kinds of seed, including mechanical barriers to water absorption and gas exchange such as impermeable seed-coats and physiological barriers within the seed necessitating after-ripening. The absorption of water by seed initiates the series of physical and chemical processes which, in the absence of any limiting factors, results in the emergence of the embryo from the seed. Working with the dormant embryo in apple seed, Abbott (1) showed that after-ripening can be a reversible process and that dormancy can be reimposed by subjecting incompletely after-ripened seed to a high temperature. Reversal was alternated up to 6 times with no apparent damage or loss of viability. Davis (5) found that secondary dormancy is induced in completely after-ripened Ambrosia embryos by exposure to high temperatures where gaseous exchange is restricted. Such a secondary dormancy could be overcome by low temperature-after-ripening and then re-imposed. Davis (6) reported that the embryos in mature seed of Xanthium are not dormant. When seed are moistened and exposed to high temperatures and oxygen is restricted, the embryos go into a secondary dormancy and must be after-ripened at low temperature before they will germinate. Flemion (7) with Sorbus, Lasheen and Blackhurst (9) with blackberry, Luckwill (10) and Abbott (1) with apple, found an influence of the seed coat on dormancy in that whole seed required a longer after-ripening period than did excised embryos (embryos that have been removed from the seed coat). Flemion (7) also induced secondary dormancy by exposure of partially after-ripened seed to high temperatures so that the seed required a second low temperature after-ripening period which was usually longer than the original stratification period. However, total germination after warm periods was never as high as that of controls.
Several workers (2,8,14) concluded that the dormancy of rose seed is conditioned by inhibiting effects of the seed coat since excised embryos were capable of immediate germination and growth. Asen and Larson (2) stated that the impervious nature of the inner seed coat is thought to play a major role in delaying germination and probably in failure of germination by preventing moisture imbibition by the embryo. Wherever dormancy of rose seed is controlled, germination is affected by after-ripening conditions. While Von Abrams (14) reported that rose seed from fruits maturing at high temperatures do not invariably require low temperature after-ripening before germination, many other reports indicate its usefulness (4,11,12,13). The generally recommended procedure for germination of rose seed is to after-ripen at about 40°F and then bring it to a germinating temperature of about 70°. The alternation of temperatures may be repeated several times and some breeders have continued alteration for as long as 7 years. Because many kinds of rose seed still do not germinate satisfactorily after the treatments so far applied, the present study was undertaken to determine the effect of interruption of the after-ripening period before the cold requirement is completely satisfied.
Seeds of 7 species and cultivars of roses were after-ripened by continuous exposure to 40°F for 30, 60, 90, 120, 150, 180 days. The seeds were then brought to a greenhouse at 60°-65° and germination recorded after 15 days. Seeds of all 7 roses were also exposed to 30 day periods at 40° alternated with 15 day periods in the greenhouse at 60°-65° up to a total of 180 days at 40°. Cumulative germination percentages after each 30 day ripening period were recorded.
Germination of R. Wichuraiana and R. Setigera cv. Serena was the same after continuous or interrupted after-ripening. For the other 5 roses, R. blanda, R. X reversa, R. bracteata, R. multiflora cv. Cathayensis and R. setigera cv. Beltsville, germination after interrupted after-ripening was about half that following continuous after-ripening. The after-ripening process was stopped after 3 interruptions as there was no additional germination with further alternation of temperatures. The interruptions apparently reversed the after-ripening process and put the embryos into secondary dormancy.