Proceedings American Society Hort Science 83: 825-828 (1963)
Induced secondary dormancy of rose embryos
P. Semeniuk, R. N. Stewart, J. Uhring

A PROBLEM often encountered in the growing of roses is poor germination of seed. Considerable research has been devoted to the factors responsible for the low germination. Sometimes an important reason for poor germination is dormancy of mature rose seed.

Several workers (2, 7, 11) concluded that the dormancy of rose seed is conditioned by inhibiting effects of the seed coats since excised embryos were capable of immediate germination and growth. Other workers (3, 8, 9, 11) reported that after-ripening of rose seed at 40°F was effective in increasing germination as compared with that of seed not after-ripened.

Not only can dormancy be overcome by following certain definite procedures, but it can also be induced under certain conditions. Davis (4, 5) found that secondary dormancy could be induced in some kinds of seed by exposures to low temperatures and in others by high temperatures. Abbott (1) showed that after-ripening can be a reversible process and that dormancy can be re-imposed by subjecting incompletely after-ripened seed to a high temperature. Flemion (6) 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 period. In a previous study Semeniuk and Stuart (9) showed that interruptions of the after-ripening process by high temperature apparently reversed the after-ripening process and put the embryos into secondary dormancy. The present study was undertaken to determine whether exposure of non-after-ripened rose seed to high temperature would induce secondary dormancy in embryos ready to grow.

MATERIALS AND METHODS

Mature, open-pollinated fruit of rose species Rosa setigera Michx. 'Serena' was harvested on December 7, 1963, after the first killing frost. The seeds were immediately removed from the fruit and washed. They were held 1 minute in a dilute detergent solution containing 0.5% sodium hypochlorite. Seeds which did not sink were discarded, since such seed generally have aborted or degenerated embryos.

Each treatment was replicated 6 times with 10 seeds or embryos per treatment. Seed was planted in 6-oz paper cups containing moist perlite. These cups were covered with polyethylene to reduce loss of moisture. All cups were examined and brought up to the original weight by the addition of water after each 30-day period. Embryos were excised from seed by the technique described by Asen and Larson (2) and placed on nutrient agar in glass vials.

The experimental temperature treatments (in the dark) were 40°F and 65°F. ± 1° obtained in small chambers equipped with automatic temperature controls. Cups of freshly harvested seed and vials of excised embryos were held at 65° for 90 days, switched to 40° for 90 additional days, and then returned to 65° for 30 days. Other lots were first held at 40° for 90 days and then changed to 65° for 30 days. Additional lots of seed were held at 65° for 90 days. The embryos were then excised and their ability to grow at 65° tested with and without after-ripening at 40°.

Seeds were recorded as germinated when the radicle and hypocotyl emerged from the seed coat and elongated and lifted the cotyledons off the agar medium. The seedlings were not kept until the epicotyl elongated, but in extensive previous experience with this and many other species, epicotyl dormancy was never found after root growth and hypocotyl elongation.

RESULTS

Seed held at 65°F did not germinate during the first 90 days or when given an additional 30 days at this temperature (Table 1). When after-ripened at 40° for 90 days immediately after harvest and then brought to 65° for 30 days, 56% of the seed germinated. Seed held for 90 days at 65° and then 90 days at 40° followed by 30 days at 65° gave 48% germination.

Embryos dissected from freshly harvested seed and held at 65°F gave 81% germination and grew rapidly within a few weeks. Many of the embryos held at 40° for 90 days showed considerable enlargement of the cotyledons but none were recorded as germinated. When transferred to 65°, 70% of these embryos germinated and grew rapidly. Embryos dissected from seed held at 65° for 90 days did not grow at 65° and the cotyledons did not enlarge. Embryos from similar seed given 90 days at 40° and then returned to 65° showed slight germination, 1.6%, and the cotyledons of many that did not germinate enlarged indicating that they were alive.

Table 1.—Germination of whole seeds and excised embryos of Rosa setigera
Michx. 'Serena' after post-harvest, moist storage at various temperatures.

Temperature regime after harvest Seeds
Percent germination
Embryos
Percent germination
1) 90 days at 65°F 0 81
2) 90 days at 40° + 30 days at 65° 56 70
3) Seed 90 days at 65°F + 30 days at 65° 0  

DISCUSSION

The results of these tests confirmed the after-ripening requirement of rose seed described by several previous workers (#, 8, 9, 10). Imposition of dormancy by the seed coat, as suggested by several workers (2, 7, 11), has been confirmed by the finding that embryos excised from freshly harvested seed germinated and grew without after-ripening.

Semeniuk and Stewart (9) reported that when after-ripening of rose seed was interrupted by periods of warm temperatures the total germination was decreased even though the cumulative total of time at after-ripening temperature exceeded the normal requirement. They suggested that a secondary dormancy has been imposed on the embryos and that the interrupted periods at the after-ripening temperature of 40°F were not enough to break this secondary dormancy. The present tests gave direct evidence that rose embryos became dormant as the result of exposure within the intact, moistened seed to a temperature of 65°F for 90 days. Embryos dissected from seed handled in this way showed no germination or enlargement of cotyledons after 30 days at 65°.

The secondary dormancy of embryos in the intact seed was broken by 90 days at 40°F to the extent that 48% germination was obtained/ However, the cotyledons of many embryos did enlarge considerably indicating some effect of the after-ripening since cotyledons of the control, dormant embryos did not enlarge.

This differential response to after-ripening of dormant embryos within intact seed and dormant embryos which were excised suggests some role of the seed coat in breaking the secondary dormancy.

Use of the same after-ripening treatment of 90 days at 40°F gave a higher percentage of germination by seed showing a primary dormancy imposed by the seed coat than by seed showing a secondary dormancy of the embryos. A secondary dormancy apparently requiring more after-ripening than the primary dormancy is similar to dormancy described by Abbott (1) and Flemion (6).

SUMMARY

Freshly harvested seed of Rosa setigera Michx. 'Serena' require after-ripening and do not germinate when kept moist for 90 or more days at 65° F. Embryos dissected from freshly harvested seed grew immediately. Embryos dissected from seed kept moist at 65° for 90 days after harvest would not grow. The secondary dormancy of such embryos was broken by after-ripening in intact seed at 40° for 90 days. The secondary dormancy of such embryos was not broken by after-ripening the dissected embryo on agar at 40° for 90 days.

LITERATURE CITED

  1. ABBOTT, D. L., 1955. Temperature and the dormancy of apple seeds. 14th Inter. Hort. Cong. 1955. Vol. 1. 746-753.
  2. ASÉN, S. and R. E. LARSON. 1951. Artificial culturing of rose embryos. Penn. State College Prog. Rept. No. 40.
  3. CROCKER, W. and LELA V. BARTON. 1931. After-ripening, germination, and storage of certain rosaceous seeds. Cont Boyce Thompson Inst. Vol. 36:385-404.
  4. DAVIS, W. E., 1930. Primary dormancy, after-ripening, and the development of secondary dormancy in embryos of Ambrosia trifida. Am. J. Bot. 17:58-76.
  5. ——————. 1930. The development of dormancy in seeds of cocklebur (Xanthium). Amer. J. Bot. 17: 77-87.
  6. FLEMION, FLORENCE, 1931. After-ripening, germination, and vitality of seeds of Sorbus aucuparia L. Cont. Boyce Thompson Inst. Vol. 3: 413-439.
  7. LAMMERTS, W. E., 1946. Use of embryo culture in rose breeding. Plants and Gardens 2: 111.
  8. ROWLEY, G. D. 1956. Germination in Rosa canina. Amer. Rose Annual 41: 70-73
  9. SEMENIUK, P. and R. N. STEWART. 1962. Temperature reversal of after-ripening of rose seeds. Proc. Amer. Soc. Hort . Sci. 80: 615-621.
  10. SHEPHERD, R. E. 1954. Media for rose seed germination. Amer. Rose Annual 39: 102.106.
  11. VON ABRAMS, G. J. and N. E. HAND. 1956. Seed dormancy in Rosa as a function of climate. Amer. J. Bot. 43: 7-12.