Edward W. Chester

Seed germination ecophysiology of four summer annual mudflat species of Cyperaceae

Abstract Cyperus erythrorhizos Muhl., C. flavicomus Michx., Fimbristylis autumnalis (L.) R.S exhumed seeds of C. erythrorhizos, C. flavicomus, F. autumnalis and F. vahlii germinated to a maximum of 7%, 4%, 73% and 0%, respectively, in darkness. Seeds flooded from October to April or May came out of dormancy during winter, and those flooded from February to August did not re-enter dormancy. Thus, the major effect of flooding is prevention of germination while mudflats are under water. Since seeds do not enter dormancy in spring or summer, they can germinate any time during the growing season if the mudflats become exposed.

Effect of flooding on annual dormancy cycles in buried seeds of two wetlandCarex species

Buried seeds ofCarex comosa andC. sticta were exposed to nonflooded and flooded conditions and natural seasonal temperature changes for 30.5 and 33 mo, respectively. At 1-, 2- or 6-mo intervals, exhumed seeds were tested for germination in light and darkness over a range of daily thermoperiods. Freshly-matured seeds of both species were conditionally dormant; maximum germination was at 35/20°C, in light. Dormancy decreased in nonflooded and flooded seeds ofC. comosa during late autumn and winter, but the decrease was greater in flooded than in nonflooded seeds. Nonflooded and flooded seeds ofC. stricta gained the ability to germinate in light during the first summer of burial and in darkness during the following winter. Seeds of neither species germinated while they were buried in pots of soil under either nonflooded or flooded conditions in the nonheated greenhouse. Nonflooded and flooded seeds of both species incubated in light and flooded seeds ofC. comosa incubated in darkness had an annual conditional dormancy/nondormancy cycle, being conditionally dormant in summer and autumn and nondormant in spring. However, nonflooded seeds ofC. comosa incubated in darkness remained dormant, germinating to only 1%. Most nonflooded and flooded seeds ofC. stricta incubated in darkness had an annual dormancy/nondormancy cycle, being dormant in summer and nondormant in spring. Thus, flooding influenced the annual changes in dormancy states of buried seeds ofC. comosa, but it had no effect on seeds ofC. stricta.

Deep Complex Morphophysiological Dormancy in Seeds of Thaspium pinnatifidum (Apiaceae)

Seeds of Thaspium pinnatifidum (Buckl.) Gray (Apiaceae) were dormant at maturity in early autumn. Embryos in fresh seeds were 0.7 mm long, but by the time germination began in early February they had grown to 3.6 mm. Seventy-six percent of the seeds sown in autumn in a nonheated greenhouse germinated the following late winter and early spring, with the peak occurring between February 4 and 11, when mean daily maximum and minimum air temperatures were 13.3 and 8.7 C, respectively. An additional 15% germinated the second season. Seeds cold stratified at 5 C for 12 wk germinated to 69% and 64% in light and darkness, respectively, at 5 C. After 4 wk at 5 or 15/6 C, embryos in seeds were 1.5 mm and 2.3 mm in length, respectively. Seeds transferred from 5 to 15/6 C germinated to 61% after 4 wk, whereas those kept at 15/6 C for 8 wk germinated to only 2%. Thus, cold stratification was required to break physiological dormancy and stimulate growth of the underdeveloped embryos. Since gibberellic acid did not substitute for cold stratification and dry, room-temperature laboratory storage did not reduce the length of the cold stratification period required to break dormancy, seeds have deep complex morphophysiological dormancy.

Effect of flooding on the annual dormancy cycle and on germination of seeds of the summer annual Schoenoplectus purshianus (Cyperaceae)

Abstract Seeds of Schoenoplectus purshianus were dormant at maturity in early autumn and thus did not germinate in light or darkness at 15/6, 20/10, 25/15, 30/15 or 35/20°C. Seeds buried in either flooded or nonflooded soil and exposed to natural seasonal temperature changes for 32 months exhibited an annual dormancy/nondormancy cycle each year when tested in light under either flooded or nonflooded conditions. That is, seeds came out of dormancy during autumn and winter and could germinate to high percentages in spring. Burial prevented seeds from germinating, thus they re-entered dormancy by summer and could not germinate at any temperature. Seeds buried in flooded soil and tested in darkness on wet sand (=nonflooded) also exhibited an annual dormancy/nondormancy cycle, but those buried in nonflooded soil and tested in darkness on wet sand never germinated to more than 2%. Optimum habitat conditions for high year-to-year germination are a regime of nonflooding in winter and flooding in spring.

Seed germination ecology of the aquatic winter annual Hottonia inflata

Abstract Freshly collected seeds of Hottonia inflata were buried in soil under flooded conditions in May 1991 and exposed to seasonal temperature cycles for 39 months. Seeds were exhumed at 1-, 2- or 4-month intervals and tested for germination on moist sand in light (14 h daily photoperiod) and in continuous darkness under 12 12 h thermoperiods of 15 6 , 20 10 , 25 15 , 30 15 and 35/20°C. On three dates, seed germination was tested under flooded conditions. In May 1991, 95% of the seeds were dormant, and they came out of dormancy during the summer of 1991, first gaining the ability to germinate to 90% or more in light at 30/15 and 35/20°C. By October 1991, seeds were non-dormant and germinated to 90–100% in light at 20 10 to 35 20 °C. Seeds remained non-dormant during the rest of the study and did not exhibit cyclic changes in germination requirements. Only 1–13% of the seeds germinated in light at 15/6°C or in darkness at any thermoperiod. Seeds incubated under flooded conditions germinated to 0–2% at 15 6 °C and to 58–95% at 20 10 , 25 15 and 30 15 °C. Although seeds have the potential to germinate in the field from April–October, the species functions as a winter annual. Apparently, its life cycle is regulated by favourable (e.g. low water level) and unfavourable (e.g. high water level) conditions for germination in its habitat in autumn and spring–summer, respectively.

Seed dormancy in the wetland winter annualPtilimnium nuttallii (Apiaceae)

Seeds (mericarps) ofPtilimnium nuttallii have linear, underdeveloped embryos that must grow from about 0.2 to 1.3 mm in length before the radicle emerges from the seed (fruit); thus, seeds have morphological dormancy (MD). In addition to MD, about 70% of the seeds also have physiological dormancy (PD) at maturity in mid-July; thus, seeds have morphophysiological dormancy (MPD). Whereas fresh seeds with only MD (26–29%) germinated in light during 2 weeks of incubation at 15/6, 20/10, and 25/15 °C, those with MPD required exposure to high temperatures (25/15, 30/15 °C), which broke PD, before they would germinate. The high temperature requirement for loss of PD means that seeds have nondeep simple MPD. Seeds buried in soil in July 1991 and exposed to natural seasonal temperature changes came out of PD during the summers of 1991–1996, and those failing to germinate in autumn of each year re-entered PD during winter. Thus, seeds have an annual dormancy/nondormancy cycle with respect to PD, and MD can be broken only while seeds are not physiologically dormant. Light was required for embryo growth in 55–98% of the seeds, but the light requirement varied with year. Consequently. embryo growth and germination are restricted to autumn, and seeds on the soil surface are more likely to germinate than those buried in soil. Results of this study indicate thatP. nuttallii is a strict winter annual with a long-term soil seed bank. The annual dormancy/nondormancy cycle would prevent germination in spring when high water levels in the habitat might be inhibitory for successful seedling establishment.

Annual dormancy cycle and influence of flooding in buried seeds of mudflat populations of the summer annual Leucospora multifida

AbstractDormant seeds of Leucospora multifida (Michx) Nutt. were used to determine: (1) temperature requirements for loss of dormancy; (2) if buried seeds undergo seasonal changes in their dormancy states; and (3) effects of flooding on dormancy loss and induction. Seeds came out of dormancy when buried at 5°C and 15/6°C for 12 weeks, but at 20/10°C, 25/15°C and 30/15°C, they gained the ability to germinate only at 30/15°C and 35/20°C. Seeds buried in soil and exposed to natural seasonal temperature changes under non-flooded conditions exhibited an annual conditional dormancy/non-dormancy cycle, being non-dormant in spring and early summer, and conditionally dormant in late summer and autumn. Non-dormant seeds required light for germination at all thermoperiods, and they did not germinate in light or darkness at 15/6°C. Flooding prevented non-dormant seeds from entering conditional dormancy in summer, and it prevented dormant seeds from coming out of dormancy in winter. However, if dormancy loss had start...

Seed germination ecology of two mesic woodland winter annuals, Nemophila aphylla and Phacelia ranunculacea (Hydrophyllaceae)

BASKIN, C. C., J. M. BASKIN (School of Biological Sciences, University of Kentucky, Lexington, KY 40506 and Center for Field Biology, Austin Peay State University, Clarksville, TN 37044) AND E. W. CHESTER (Center for Field Biology, Austin Peay State University, Clarksville, TN 37044). Seed germination ecology of two mesic woodland winter annuals, Nemophila aphylla and Phacelia ranunculacea (Hydrophyllaceae). Bull. Torrey Bot. Club 120: 29-37. 1993.Seeds of Nemophila aphylla and Phacelia ranunculacea (Hydrophyllaceae) were dormant at maturity in May and required high summer temperatures to afterripen enough to germinate at autumn temperatures in autumn. Seeds buried in soil in a nontemperature-controlled greenhouse in May 1989 germinated to 14-64% in light and to 7-51% in darkness at simulated autumn (15/6, 20/10?C) temperatures in October 1989. During autumn 1989, about 25 and 50% of the P. ranunculacea and N. aphylla seeds, respectively, germinated while buried in soil in the greenhouse. Seeds of P. ranunculacea exhibited an annual dormancy/nondormancy cycle during the next 24 months of burial, with only a few buried seeds germinating in the autumn of 1990, while those of N. aphylla that afterripened during the summer of 1990 germinated in the soil that autumn and did not re-enter dormancy. Thus, seed banks in the latter species are attributed to lack of afterripening and not to the ability of nondormant seeds to re-enter dormancy. Seeds sown on soil in the greenhouse in May 1989 germinated in the autumns of 1989-1991. Since seeds only germinated in autumn, the species are obligate winter annuals. Both species form persistent seed banks. Some seeds that germinated in soil samples from N. aphylla population sites were at least 2 years old, while some in two sets of samples from a P. ranunculacea site were 3 years old.

Morphophysiological dormancy in seeds of Chamaelirium luteum, a long-lived dioecious lily

for 12 wk broke physiological dormancy, but embryos in only about 20% of the seeds grew to - 1.3 mm during this treatment. After seeds that had been stratified in darkness at 5°C for 12 wk were transferred to light at a simulated spring temperature (20/10°C = April), embryos in all seeds grew to -1.3 mm within 12 days. Furthermore, seeds receiving 12 wk of stratification in light at 5°C had germinated to 90-100% after 14 days in light at 15/6, 20/10, 25/15, and 30/15°C. Seeds receiving light only during the stratification period at 5°C or only during the 2-wk incubation period at the four temperature regimes germinated to 33-100%, whereas those in continuous darkness during both stratification at 5°C and incubation germinated to only 1-7%. Seeds of C. luteum have nondeep simple MPD. In the field, mature seeds are dispersed in mid- to late autumn, and cold stratification during winter breaks physiological, but not morphological, dormancy of the embryo. However, embryo growth and germination occur rapidly as temperatures begin to increase in early spring.

Effects of flooding and temperature on dormancy break in seeds of the summer annual mudflat species Ammannia coccinea and Rotala ramosior (Lythraceae)

Studies were undertaken on seeds of the summer annual mudflat species Ammannia coccinea and Rotala ramosior to determine the (1) effects of flooding during late autumn to late spring on dormancy break and (2) optimum temperature for dormancy break. At maturity in autumn, about 65–100% of the seeds of these species were dormant. Seeds of both species buried under flooded and under nonflooded conditions in a nonheated greenhouse germinated to 70–98% at 30(day)/15(night)°C and at 35/20°C the following June or July; seeds required light for germination. As dormancy break occurred, seeds of R. ramosior showed a decrease in the minimum temperature for germination, but those of A. coccinea did not. In another experiment, seeds buried under nonflooded conditions in the nonheated greenhouse were flooded in November, December, February, March, April, or May, and all flooded seeds and nonflooded controls were exhumned and tested in July. With few exceptions, seeds of both species flooded for short (May–July) or long (November-July) periods germinated to significantly higher percentages over a range of temperatures when exhumed in July than did seeds that had not been flooded. In a third experiment, seeds of both species were incubated on moist sand in darkness at 5, 15/6, 20/10, and 30/15°C for 0, 3, 6, 9, and 12 wk and then tested in light at 15/6, 20/10, 25/15, 30/15, and 35/20°C. The optimal temperature regime for dormancy break in seeds of R. ramosior and A. coccinea was 20/10 and 30/15°C, respectively. In the nonheated greenhouse, some dormancy break began in buried seeds of both species during late autumn and winter, and it continued as temperatures increased in spring and/or early summer. The ability of seeds of both species to come out of dormancy during flooding at field temperatures from late autumn to early summer means that seeds are nondormant when mudflats become dewatered in summer.