For The Answer
Improving Seed Germination of Aquilegia chrysantha by
Tim D. Davis, Daksha Sankhla', N. Sankhla', A. Upadhyaya, J.M. Parsons', and S.W. George
Texas A&M University Research and Extension Center, 17360 Coit Road, Dallas, TX 75252-6599
Abstract. Seeds of Aquilegia chrysantha Gray were germinated under a variety of temperature regimes. Germination was nearly 90% under a day/night cycle of 25/20C, but was reduced to 40% under constant 25C or a 25/1OC day/night cycle. With days between 25 and 29C (night = 20C), germination percentage dropped gradually to 60 % with increasing temperature. With days >29C, germination declined dramatically such that no germination occurred at 31C. Neither kinetin (4.6 to 46 µm) nor ethephon (6.9 to 207 µm) was able to reverse the inhibitory effects of 33C days. Our results indicate that germination of A. chrysantha seed is sensitive to temperature and that germination >>-75% can be obtained under a 25 to 27C day/20C night regime. Chemical names used: 2chloroethylphosphonic acid (ethephon); 6-furfurylaminopurine (kinetin).
Aquilegia chrysantha, sometimes erroneously referred to as A. hinckleyana Gray (Lott, 1985), is a brilliant, yellow-flowered herbaceous perennial native to a very small area of southwestern Texas. The plant has excellent heat tolerance, particularly for an Aquilegia species, and considerable potential as a landscape plant (Welch, 1989). Aquilegia seeds often exhibit irregular germination patterns that complicate transplant production (Nau, 1989). Indeed, erratic seed germination is a serious obstacle to A. chrysantha commercial production. Although little is known about the seed biology of this species, germination appears to require relatively low temperatures. For example, Finnerty et al. (1992) obtained 80%e to 90% germination at constant 20C, but almost no germination at 26C. Temperatures near 20C may be difficult to maintain in greenhouses in the southern United States. Furthermore, A. chrysantha seeds should be planted in late spring or early summer to produce transplants of suitable size and quality for fall planting outdoors. Maintaining the low temperatures required for germination during this period is often impossible or, at best, costly.
Because A. chrysantha is native to a region with considerable diurnal temperature fluctuation during the normal germination period (i.e.,autumn), alternating temperatures might enhance seed germination. We hypothesized that the inhibitory effects of a high daytime temperature could be offset at least partially by cool nights. One objective of the research reported herein was to test this hypothesis. We also sought to determine the temperature range over which thermodormancy is induced. Finally, we tested whether two seed dormancy breaking agents, kinetin and ethephon (Rao et al., 1975; Sankhla and Sankhla, 1973), would be useful in overcoming high-temperature induced dormancy.
Aquilegia chrysantha seeds were collected in late Spring 1990 and 1991 (hereafter referred to as 1990 and 1991 seeds, respectively) from plants growing in a garden in San Antonio, Texas. A tetrazolium test (Hartmann et al., 1990) indicated that 95% of the seeds were viable. Three separate experiments were conducted. In each experiment, seeds were placed on moistened filter paper inside clear plastic Petri dishes (30 seeds per dish) and germinated in temperature-controlled chambers (±0.5C). An 8-h photoperiod was used, during which the photosynthetically active radiation level at the top of the Petri dishes was 50 to 100 µmol·m-2·s-1. The filter paper was remoistened every 3 to 4 days. There were three dishes per treatment, and each experiment was conducted at least twice. The number of germinated seeds was recorded daily for 1 month. A seed was considered germinated when the radicle became visible to the naked eye. Means and standard errors were calculated of reach treatment. Temperature response data were subjected to regression analysis.
In the first experiment, seeds were placed under the following four temperature regimes: 25C constant, 25C day/20C night, 25C day/ 15C night, and 25C day/IOC night. For the second experiment, seeds were kept at 25, 27, 29, or 31 C during the day, each with nights at 20C. In the final experiment, seeds were placed under a 33C day/20C night cycle on filter paper moistened with aqueous solutions of 0, 4.6, 23.2, or 46 µm kinetin or 0, 6.9, 34.5, 68.9, or 207 µm ethephon. The growth regulator dosage ranges were chosen based on our previous experience with other herbaceous species (Rao et al., 1975; Sankhla and Sankhla, 1973).
With the 25C day/20C night regime, germination was first observed after 9 to 11 days (Table 1). In contrast, seeds at the lowest night temperature (25/1 OC) required at least 2 weeks before any germination occurred. The 1990 seeds germinated more rapidly and required less time to reach 50% germination than the 1991 seeds; nonetheless, the response of the two seed lots to the temperature regimes was similar. The highest germination percentage after 1 month (≈87%) occurred in the 25/20C cycle. At constant 25C, germination was <<-34%. Thus, as with several non-Aquilegia species (Mayer and Poljakoff-Mayber, 1989;Thompson and Grime, 1983), alternating diurnal temperatures appear to promote germination of A. chrysantha. In any case, our results indicate that days at 25C coupled with nights at 20C can yield relatively high germination percentages. With nights at 15C rather than 20C, germination was reduced slightly, but was still much higher after 1 month than that observed at constant 25C or when the night temperature was l OC (Table 1).
As in the first experiment, the 1990 and 1991 seeds responded similarly to temperature, and germination was relatively high (80% to 90%) with the 25/20C cycle in the second experiment (Fig. 1). As the day temperature was increased to between 25 and 29C, germination gradually declined quadratically, and no germination occurred at 31C. Thus, A. chrysantha seeds exhibit a high-temperature induced thermodormancy similar to several other species (Mayer and Poljakoff-Mayber, 1989: Rao et al., 1975: Sankhla and Sankhla. 1973). Our data indicate that day temperatures must be ≤29C to obtain >50% germination. Further, the quadratic response indicates that germination will decline precipitously above 29C.
Neither kinetin nor ethephon, at any of the concentrations used, promoted germination under the 33C day/20C night regime (data not shown). Under these conditions, none of the seeds germinated. Thus, at the concentrations tested, kinetin and ethephon do not appear to be of much potential value for overcoming high-temperature-induced thermodormancy of A. chrysantha seeds. In contrast, these compounds have promoted germination of lettuce (Lactuca sativa L.) at high temperature (Rao et al., 1975: Sankhla and Sankhla, 1973; Sharpies, 1973).
Finnerty et al. (1992) reported that cool, moist stratification slightly improved germination of A. chrysantha under constant 20C. At constant 26C, however, germination was <3% regardless of whether the seeds were stratified. In the present study, seeds were not stratified and were stored at 20 to 25C following harvest. Thus, stratification clearly was not essential for obtaining 80% to 90%, germination, provided seeds were germinated under a 25 to 27C day/20C night cycle.
This research paper about growing 'Texas Gold' (Aquilegia hinckleyana) found that columbine seed should be ideally germinated under a 25 to 27C day/20C night cycle. Since this cannot be achieved in Texas until October or November, it is recommended that current season's seed be sent to Colorado for germination and plug production. The schedule is: Send the current season's seed in June or July; they grow plugs which are sent back to Texas in October or November; the plugs are planted directly into gallons which are finished in the field for sale in bloom in March and April. The grower who has been doing this for years is:
Welby Garden Company
TOLL FREE: 1-800-457-4725
Finnerty, T.L., J.M. Zajicek, and M.A. Hussey. 1992. Use of seed priming to bypass stratification requirements of three Aquilegia species. IlortScience 27:310313.
Hartmann, H.T., D.E. Kesler. and F.T. Davies, Jr. 1990. Plant propagation principles and practices. 5th ed. Prentice Hall, Englewood Cliffs, N.J.
Lott, E.J. 1985. New combinations in Chihuahuan Desert Aquilegia (Ranunculaceae). Phytologia 58:488.
Mayer. J.M. and A. Poljakoff-Mayber. 1989. The germination of seeds. 4th ed. Pergamon Press, Oxford. England.
Nau, J. 1989. Ball culture guide--The encyclopedia of seed germination. Ball Seed Co., West Chicago, Illinois.
Rao, V.S.. N. Sankhla, and A.A. Khan. 1975. Additiveergistic effects of kinetin and ethrel on germination, thermodormancy and polyribosome formation in lettuce seeds. Plant Physiol. 56:263-266.
Sankhla, N. and D. Sankhla. 1973. Lettuce seed germination: Prevention of thermodormancy by 2-chloroethanephosphonic acid. Experientia 29:731.
Sharples, G.C. 1973. Stimulation of lettuce seed germination at high temperatures by ethephon and kinetin. J. Amer. Soc. Hort. Sci. 98:209-212.
Thompson. K. and J.P. Grime. 1983. A comparative study of germination responses to diurnally fluctuating temperatures. J. Applied Ecol. 20:141-156.
Welch. W.C. 1989. Perennial garden color for Texas and the South. Taylor Publishing, Dallas.
This research paper about growing 'Texas Gold' (Aquilegia
hinckleyana)found that columbine seed should be ideally germinated under
a 25 to