ALBERT

Notes: Buried seeds of Capsella bursa-pastoris exhibit an annual conditional dormancy/non-dormancy cycle. Seeds after-ripen during summer and remain non-dormant during autumn and winter. Seeds enter conditional dormancy in early spring, first showing marked decreases in ability to germinate at high (35/20°C) and then at lower (30/15, 25/15°C) temperatures. Seeds do not lose the ability to germinate to high percentages at March (15/6°C) and April (20/10°C) temperatures in March and April. Thus, C. bursa-pastoris is a facultative winter annual, germinating in both autumn and spring if seeds are exposed to light. However, because some seeds retain the ability to germinate at 30/15 and 25/15°C, they could do so throughout the growing season in regions with cool, moist summers. Conditional dormancy developed in all seeds given 12 weeks at 5°C and subsequently kept for 4 weeks each at March (15/6°C), April (20/10°C) and May (25/15°C) temperatures. Thus, seeds of C. bursa-pastoris enter conditional dormancy as temperatures increase in spring.

Notes: Seedling emergence of Lamium amplexicaule L. and L. purpureum L. was monitored in field plots tilled at various times during the growing season, and the number of viable seeds in the soil was determined. In plots tilled in early spring, only seeds of L. amplexicaule, germinated, but seeds of both species germinated in the same plots in autumn without further disturbance. Additional seeds of L. amplexicaule, but not of L. purpureum, germinated the following spring. In plots tilled in late spring and summer, seeds of L. amplexicaule germinated in autumn and the following spring, whereas seeds of L. purpureum germinated only in autumn. The number of viable seeds in the top 13 mm layer of soil ranged from 189 to 1216 m−2 for L. amplexicaule and from 131 to 854 for L. purpureum. These field results support those obtained in previous glasshouse-laboratory physiological studies on the annual dormancy cycle in the two Lamium species. Levée au champ de Lamium amplexicaule L. et L. purpureum L. par rapport au cycle annuel de la dormance des grainesLa levée de jeunes plants de Lamium amplexicaule L. et L. purpureum L. a été observée sur des parcelles cultivées à différentes époques de la saison de végétation et le nombre de graines viables au sol a été déterminé. Sur les parcelles cultivées en début de printemps, seules les graines de L. amplexicaule ont germé, mais des graines des deux espèces ont germé en automne sur les mêmes parcelles, sans cultivation ultérieure. Au printemps suivant, une nouvelle germination a été constatée chez L. amplexicaule mais pas chez L. purpureum. Sur les parcelles cultivées en fin de printemps et en été, des graines de L. amplexicaule ont germé en automne et aussi au printemps suivant, tandis que les graines de L. purpureum n'ont germé qu'en automne. Le nombre de graines viables dans les premiers 13 mm de la surface du sol allait de 189 à 1216 m−2 pour L. amplexicaule et de 131 à 854 pour L. purpureum. Ces résultats sur le terrain confirment ceux obtenus dans des études physiologiques sur le cycle annuel de dormance chez les deux espèces de Lamium, menées préalablement en serre et au laboratoire. Ueber dus Auflaufen von Lamium amplexicaule L. und L. purpureum L. in Bezug auf den Jährlichen Zyklus der Samenruhe unter FeldbedingungenIn Parzellen, die zu verschiedenen Zeitpunkten während der Vegetationsperiode einer Boden-bearbeitung unterworfen worden waren, wurde sowohl das Auflaufen der Keimlinge von Lamium amplexicaule L. und L. purpureum L. registriert, als auch die Anzahl lebenfähiger Samen festgestellt. Nach Bodenbearbeitung im frühen Frühling keimten nur Samen von L. amplexicaule; im Herbst keimten in denselben Parzellen jedoch Samen beider Arten, sofern der Boden nicht mehr bewegt worden war. Weitere Samen von L. amplexicaule, nicht aber von L. purpureum, keimten im folgenden Frühling. In Parzellen, die im späten Frühling und im Sommer bearbeitet worden waren, keimte L. amplexicaule im Herbst und im folgenden Frühling. Während L. purpureum nur im Herbst keimte. Die Anzahl lebensfähiger Samen m−2 in den obersten 13 mm des Bodens bewegte sich zwischen 189 und 1216 für L. amplexicaule und zwischen 131 und 854 für L. purpureum. Diese Feldresultate bestätigen Ergebnisse von Gewüchshausstudien über den jährlichen Zyklus der Samenruhe der beiden Lamium-Arten.

Notes: Spring-produced seeds of Lamium amplexicaule L. were buried in pots of soil in an unheated glasshouse in June 1978, and at 1–2-month intervals, for 27 months, they were exhumed and tested for germination in light and darkness at temperatures simulating those in the habitat from early spring to late autumn. Freshly-matured seeds were dormant, but by autumn 85% or more germinated in light at 15/6, 20/10, 25/15 and 30/15°C but only 7% or less in darkness. During late autumn and winter germination in light decreased at 25/15 and 30/15 °C but not at 15/6 and 20/10 °C, and germination in darkness increased at 15/6 and 20/10 °C. During late winter and early spring germination in light at 15/6 and 20/10 °C decreased, and seeds lost the ability to germinate in darkness. By the second autumn of burial, seeds germinated to near 100% in light at 15/6 to 30/15 °C and to 10–25% in darkness at 15/6 and 20/10 °C. The cycle of germination responses was repeated during the second winter and spring and the third summer of burial. Autumn-produced seeds were dormant when buried in November 1979, but by spring they germinated to 81 and 36% at 15/6 and 20/10 °C, respectively, in light. These seeds afterripened further during summer. The consequence of seasonal changes in germination responses is that (1) seeds can germinate in the habitat in late summer, autumn and spring but not in early- to mid-summer or in late autumn and winter and (2) during both germination seasons, seeds produced during the previous spring(s) and/or autumn(s) can germinate.

Notes: Fresh seeds of Lamium purpureum L. were dormant at maturity, and when buried and exposed to natural seasonal temperature changes they exhibited an annual dormancy/non-dormancy cycle. During burial in summer, fresh seeds and those that had been buried for 1 year afterripened and thus were non-dormant by September and October; light was required for germination. During autumn and winter seeds re-entered dormancy, and during the following summer they became non-dormant again. Dormant seeds afterripened when buried and stored over a range of temperatures, becoming conditionally dormant at low (5, 15/6°C) and non-dormant at high (20/10, 25/15, 30/15 and 35/20°C) temperatures. Conditionally dormant seeds germinated to high percentages at 5, 15/6 and 20/10°C, while non-dormant seeds germinated to high percentages additionally at 25/15, 30/15 and 35/20°C. Low temperatures caused non-dormant seeds to re-enter dormancy, while high temperatures caused a sharp decline in germination only at 30/15 and 5°C. The temperature responses of L. purpureum seeds are compared to those of L. amplexicaule L.

Notes: Thlaspi perfoliatum L. is an introduced winter annual that grows in waste places, but it is not a troublesome agricultural weed. Seeds are dormant at maturity in spring, afterripen during summer and germinate in autumn. Non-dormant seeds that fail to germinate in autumn are induced into secondary dormancy by low winter temperatures and cannot germinate the following spring. These seeds afterripen during summer and germinate in autumn if conditions are suitable. Vernalization is not an absolute requirement for flowering. However, it shortens the time to flowering, and vernalized plants produce many more flowers and seeds than non-vernalized plants. Results from studies on seed germination and flowering of T. perfoliatum are compared to published studies on the life cycle of Thlaspi arvense L., an introduced species that is a serious agricultural weed which behaves both as a winter and a summer annual.

Notes: The annual dormancy cycle was investigated in buried seeds of Polygonum aviculare L. exposed to natural temperature changes in Lexington, Kentucky, U.S.A. Seeds were exhumed monthly from December 1984 to February 1987 and tested in light (14-h daily photoperiod) and continuous darkness at 12/12-h daily alternating temperature regimes of 15/6, 20/10, 25/15, 30/15 and 35/20°C. During autumn and winter, seeds became non-dormant, and in March 1985 they germinated to 95-100% at all thermoperiods in light and to 7-61% in darkness. Seeds remained non-dormant during spring but became more specific in their germination requirements in early summer. During July and August 1985, seeds germinated to 17-53% in light at 30/15 and 35/20°C but to 0-10% at all other test conditions. By September, about 65% of the seeds were dormant, but the others were able to germinate under the higher alternating temperatures in light. A similar seasonal cycle was recorded in the following year through to the spring of 1987. The results confirm the seasonal pattern of dormancy in this species (Courtney, 1968) but indicate that alternating temperatures combined with light are important in determining germination potential in P. aviculare.

Notes: Summary. Most freshly-matured seeds of Thlaspi arvense L. (Brassicaceae) were dormant at maturity in May. Seeds sown on soil germinated in autumn and spring, but mostly in autumn. Buried seeds exhumed at monthly intervals and tested in light and darkness over a range of thermoperiods exhibited annual dormancy/non-dormancy cycles. However, the dormant period was short, usually only in April, but sometimes May, and in some years 1–6% of the seeds remained conditionally dormant. After-ripening occurred during summer, and seeds were non-dormant during autumn. Seeds entered conditional dormancy in winter and dormancy in late winter or early spring. When buried dormant seeds were kept at 25/15, 30/15 or 35/20°C for 12 weeks, they gained the ability to germinate to 95–100% at 15/6, 20/10, 25/15, 30/15 and 35/20°C. After burial for 12 weeks at 15/6 and 20/10°C, seeds germinated to 80–100% at 15/6, 20/10 and 25/15°C. but to only 11–64% at 30/15 and 35/20°C. After 4 weeks at 5°C, initially-dormant seeds germinated to 100% at all thermoperiods except 35/20°C, where only 15% of them germinated. However, after 18 weeks at 5°C, only 0–1% of the seeds germinated at all thermoperiods. Most non-dormant seeds exposed to 1, 5 and 15/6°C for 16 weeks were induced into dormancy; 1–15% entered conditional dormancy and thus germinated only at 15/6, 20/10 and 25/15°C. This study indicates that seeds of winter annual plants of T. arvense are non-dormant in autumn and enter dormancy in winter, while those from summer annuals are dormant in autumn and become non-dormant during winter.

Notes: Spring-produced seeds of Lamium amplexicaule L. were dormant at maturity in May and after-ripened when buried and stored over a range of temperatures, becoming conditionally dormant at low (5, 15/6 and 20/10°C) and non-dormant at high (25/15, 30/15 and 35/20°C) temperatures. Conditionally dormant seeds germinated to high percentages at 5 and 15/6°C, and non-dormant seeds germinated to high percentages at 5, 15/6, 20/10, 25/15 and 30/15°C. Seeds that became conditionally dormant at 5°C afterripened completely (i.e. became non-dormant) after transfer to 30/15°C. Buried seeds that became non-dormant in a non-temperature-controlled glasshouse during summer were still non-dormant after 12 weeks of storage at 30/15°C, while those stored at 5°C for 12 weeks had entered conditional dormancy. Thus, low temperatures cause reversal of the afterripening that takes place at high temperatures, but not that which takes place both at low and at high temperatures. Low winter temperatures cause dormant autumn-produced seeds and non-dormant seeds in the soil seed pool to become conditionally dormant. The ecological consequences of these responses to temperature are discussed in relation to the timing of seed germination in nature.

Notes: Temperature requirements for after-ripening were investigated in seeds of the weedy winter annuals Arabidopsis thaliana, Arenaria serpyllifolia, Capsella bursar-pastoris, Cardamine hirsuta. Cerastium viscosum, Draba verna, Holosteum umbellatum, Stellaria media and Thlaspi per-foliata. Fresh seeds of seven species were innately dormant, and those of A. serpyllifolia and C. viscosum were conditionally dormant. (Dormancy terminology follows Vegis, 1964.) Seeds of each species were buried in moist soil at 5, 15/6, 20/10, 25/15, 30/15 and 35/20°C from time of maturation in spring until the third week of September. Buried seeds at each temperature were then exhumed and tested in light at all six temperatures. Seeds of all species became non-dormant at 25/15, 30/15 and 35/20°C, except for those of D. verna, H. umbellatum, A. serpyllifolia and C. viscosum, which rotted during burial at 35/20°C At 20/10°C. seeds of T perfoliata and D. verna became conditionally dormant, and those of the other seven species became non-dormant. Thalaspi perfoliata and D. verna seeds did not after-ripen at 5 or 15/6°C, while those of H. umbellatum and C. hirsuta became conditionally dormant at 15/6°C but remained innately dormant at 5°C. The other five species became conditionally dormant at both 5 and 15/6°C; they germinated at low, bill not at high, temperatures. Thus, after-ripening in seeds of winter annuals is fully promoted by high summer temperatures and wholly or partially inhibited by low winter temperatures.Exigences en temperature pour la post-maturation des semences de neuf espèces annuelles d'hiverLes exigences en température pour la post-maturation ont étéétudiées chez les semences d'adventices annuelles d'hiver, Arabidopsis thaliana, Arenaria serpyllifolia, Cupsella bursa-pastoris, Cardamine hirsuta, Cerastium viscosum, Draba verna, Holosteum umbellatum, Stellaria media et thlaspi perfaliata. Les semences fraîches de sept espèces manifestaient une dormance absolue, et celles de A. serpyllifolia et C. viscosum une dormance relative (selon la terminologie de Vegis, 1964). Des semences de chaque espèce ont été enfouies dans du sol humide à 5, 15/6, 20/10, 25/15, 30/15 et 35/20°C, à partir de la période de maturation au printemps et jusqu'à la troisième semaine de Septembre. Les semences enfouies aux différentes températures ont été ensuite exhumées et soumise, sous éclairement, aux six conditions de température. Les semences de toutes les espèces ont perdu leur dormance à 25/15, 30/15 et 3/20°C, excepté celles de D. verna. H. umbellatum, A. serpillyfolia et C. viscosum, qui ont pourri après a voir été enfouies à 35/20°C. A 20/10°C, les semences de. T. perfoliata et D. verna ont acquis une dormance relative et celles des sept autres espèces sont devenues non dormantes Les semences de Thlaspi perfoliata et D. verna n'ont pas subi de post-maturation à 5 ou 15/6°C, alors que celles de H. umbellatum et de C. hirsuta ont acquis une dormance relative à 15/6°C mais ont conserveé une dormance absolue à 5°C. Les cinq autres espèces ont acquis une dormance conditionnelle à la fois à 5 et 15/6°C; elles ont germéà basse, mais non à haute température. Par consèquent, la post-maturation des semences d'annuelles d'hiver est totalement induite par les fortes tempéeratures d'été, et totalement ou partiellement inhibée par les basses températures d'hiver.Temperaturhedürftnisse zur Nachreifung von Samen bei neun WinterannuellenIn dieser Studie wurden die Temperaturanor- derungen zur Nachreifung der Samen der winter-annuellen Unkräuter Arabidopsis thaliana, Arenaria serpyllifolia, Capsella bursa-pastoris, Cardamine hirsuta, Cerasthim viscosum, Draba verna, Holosteum umbellatum, Stellaria media und Thlaspi perfoliata untersucht. Junge Samen von sieben Arten sind von Natur aus obligat und diejenigen von A. serpyllifolia und C. viscosum je nach Umständen dormant (Terminologie der Samenruhe nach Vegis, 1964). Samen jeder Species wurden zur Zeit der Samenreife im Frühling in feuchtem Boden eingegraben und darin bei Temperaturen von 5, 15/6, 20/10, 25/15, 30/15 und 35/20°C bis zur dritten Woche im September belassen. Danach wurden die Samen ausgegraben und unter Lichleninfluss bei allen sechs Temperaturstufen auf ihre Keimfähigkeit getestet. Mit Ausnahme der Samen von D. Verna, H. umbellatum, A. serpyllifolia und C. viscosum, welche bei 35/20°C im Boden verfault waren, wurden alle anderen Arten bei 25/15, 30/15 und 35/20°C keimfähig. Bei 20/10 °C kamen die Samen von T. perfoliata und D. verna in einen bedingten Ruhezustand, während diejenigen der anderen sieben Arten voll keimfähig wurden. T. perfoliata und D. verna reiften bei 5 oder 15/6°C nicht nach: H. umbellatum und C. hirsuta wurden bei 15/6 °C bedingt keimfähig, blieben aber bei 5°C völlig dormant. Die anderen fünf Arten erreichten bei 5 und 15/6°C eine bedingte Dormanz; sie keimten bei niedrigen, nicht aber bei hohen Temperaturen. Aus diesen Ergenissen lässt sich schlicssen, dass die Nachreifung von Samen winterannueller Arien durch hohe Sommertemperaturen voll entwickelt, durch niedrige Wintertemperaturen aber ganz oder teilweise gehemmt wird.