Germination responses of buried seeds of Capsella bursa-pastoris exposed to seasonal temperature changes

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.     

Temperature requirements for after-ripening in seeds of nine winter annuals

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'hiver Les 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 Winterannuellen In 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.     

Role of temperature in regulating timing of germination in soil seed reserves of Lamium purpureum L.

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.     

The role of light and alternating temperatures on germination of Polygonum aviculare seeds exhumed on various dates

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.     

Germination ecology of seeds of the annual weeds Capsella bursa-pastoris and Descurainia sophia originating from high northern latitudes

Low temperatures may inhibit dormancy break in seeds of winter annuals, therefore it was hypothesized that seeds of Capsella bursa‐pastoris and Descurainia sophia that mature at high latitudes in late summer–early autumn would not germinate until they had been exposed to high summer temperatures. Consequently, germination would be delayed until the second autumn. Most freshly matured seeds of both species collected in August and September in southern Sweden were dormant. After 3 weeks of burial at simulated August (20/10°C) and September (15/6°C) temperatures, 28 and 27%, respectively, of the C. bursa‐pastoris and 56 and 59%, respectively, of the D. sophia seeds germinated in light at 15/6°C. In contrast, in germination phenology studies conducted in Sweden, only a few seeds of either species germinated during the first autumn following dispersal. However, there was a peak of germination of both species the following spring, demonstrating that dormancy was lost during exposure to the low habitat temperatures between late summer and early autumn and spring. Nearly 100% of the seeds of both species subjected to simulated annual seasonal temperature changes were viable after 30.5 months of burial. In the burial study, exhumed seeds of C. bursa‐pastoris were capable of germinating to 98–100% in light at the simulated spring–autumn temperature regime (15/6°C) in both spring and autumn, while those of D. sophia did so only in autumn. In early spring, however, seeds of D. sophia germinated to 17–50% at 15/6°C. Thus, most seeds of these two annual weeds that mature in late summer do not germinate in the first autumn, but they may do so the following spring or in some subsequent autumn or spring.     

Effect of temperature during burial on dormant and non-dormant seeds of Lamium amplexicaule L. and ecological implications

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.     

Seasonal changes in the germination responses of buried Lamium amplexicaule seeds

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.     

Dormancy and viability of Phalaris minor seed in a rice–wheat cropping system

Seed placement, soil temperature and soil moisture content influenced the process of after-ripening in Phalaris minor seeds. Seeds of P. minor collected from the soil just after wheat harvesting exhibited higher germination than seeds from P. minor threshed directly. There was a pronounced impact of periodic inhabitation of seed into the soil on germination after its dispersal. Germination was strongly inhibited when the seed was kept in soil at more than field capacity (FC) or in water. Maximum germination of seed incubated in soil at FC occurred at 30°C while a temperature of 40°C favoured after-ripening of seed when mixed with dry soil or kept dry without any medium. Release from conditional dormancy was quicker in the seed retrieved from the soil kept at 20°C than at 10°C. Seed release from conditional dormancy and germination increased with a rise in temperature from 30 to 40°C when the seed was retrieved from incubation in soil at FC for 70 days. The seed kept immersed in water was least responsive to a rise in temperature. Seed recovered from dry soil, or kept without any medium, responded quickly at both temperatures. Light enhanced the germination of Phalaris minor seed. The seedbank subjected to rice (Oryza sativa) field management conditions lost vigour in comparison with the seed stored in laboratory. There was significant variability in seed viability when exposed to differential water management conditions in rice.     

A high temperature requirement for after ripening of imbibed dormant Poa annua L. seeds

Freshly harvested seeds of Poa annua L. collected in south Louisiana were stored in moist soil at seven temperatures between 5°C and 35°C. At monthly intervals, seed lots were removed and germinated at each of the seven temperatures. Seed were dormant for at least 1 month at all test temperatures. Seeds stored for 2 months at 30 and 35°C showed conditional dormancy; there was 100% germination at 10 or 15°C, and poorer germination at 5 or 20°C. Seeds started to lose viability after 2 months at 35°C and were dead after 7 months. In seeds stored at 10–30°C, there were increased percentages and a wider range of germination temperatures as storage time or storage temperatures increased. Seeds stored at 10°C remained dormant for 9 months, but by 12 months of storage the seeds germinated only at 5 or 10°C. Nearly all seeds stored at the same temperatures in air dry soil remained dormant for 6 months, regardless of storage temperature. These results differ from other reports of low temperatures breaking seed dormancy in Poa annua L. and suggest an adaptation to subtropical climates.     

Untersuchungen zur Keimungsbiologie von sechs tropischen Segetalarten

Investigation of the germination of six tropical arable weeds With the six tropical arable weeds studied, Ageratum conyzoides, Blechum brownei, Crassocephalum crepidioides, Mikania micrantha, Paspalum conjugatum and P. paniculatum, the lowest temperatures at which germination occurred were in the range 10–15(20)°C, the highest up to 40°C, with the optimum between 20°C and 35°C. Ageratum conyzoides and C. crepidioides had lower limits of germination temperature than the other species, which explains their comparatively greater incidence at high altitudes. The germination temperatures of both representative temperate species, Alopecurus myosuroides and Chenopodium album were 5 (minimum), 15–25 (optimum) and 40°C (maximum). The germination of A. conyzoides, C. crepidioides and M. micrantha was reduced by an osmotic potential of - 1 bar. At -4·7 bar only P. paniculatum of the tropical species germinated to a small extent whilst Al. myosuroides and Ch. album were not affected. After storage for a year in dry conditions all six tropical species only germinated when exposed to light. After 20 months, C. crepidioides and P. paniculatum would also do so in the dark.     

Seed germination, seedling establishment and growth patterns of wrinklegrass (Ischaemum rugosum Salisb.)

Several laboratory and glasshouse experiments were conducted to assess seed germination, seedling establishment and growth patterns of wrinklegrass (Ischaemum rugosum Salisb.) influenced by temperature and light regimes, and chemical media. Wrinklegrass was a positively photoblastic species, and seed germination was temperature‐dependent and light‐mediated. Seeds soaked in distilled water for 24 h, or oven‐dried at the respective temperature regimes of 15, 20, 25, 30, 35, or 40°C prior to treatment in distilled water and incubated in darkness, failed to germinate. Likewise, no germination prevailed when the seeds were exposed to similar temperature regimes and treated with 0.2 m KNO₃, 5% H₂O₂ or 0.01 m HNO₃, and incubated under continuous darkness. Seeds treated with 5% H₂O₂ at 30°C, or oven‐dried and treated with 0.01% M HNO₃ at 35°C registered 10 and 20% germination. Approximately 75 and 90% of the light‐exposed seeds for all treatments germinated in the first three and six days at 25°C. No germination occurred at 15°C in the first three days after treatment. Seeds subjected to 40°C for six days after treatment recorded 36% germination. The optimum temperatures for seed germination were 25–30°C. Seed drying and soaking treatments widened the windows of the optimal temperatures for wrinklegrass germination. The acidic media of KNO₃, H₂O₂ or HNO₃ favored seed germination. Less than 5% of seed germination occurred with burial or water inundation at depths exceeding 2 cm. Seed burial or inundation at ≥2 cm depths inhibited seed germination. Seeds sown onto moist paddy soils registered ca. 50% germination. Free‐floating seeds on the water surface registered ca. 98% germination within the first six days after seeding. The mean number of seedlings that survived was inversely proportional to water depths, with close to 100% mortality at the 14 cm depths of inundation. Both plant height and seedling survival were linearly proportional to the amount of root mass of seedlings which penetrated the soil. The weed was a prolific seed producer (ca. 6000 seeds/genet or 18 000 seeds/genet per year). The vegetative and reproductive efforts of each wrinklegrass plant registered values of 0.68 and 0.32, respectively.     

Effects of temperature and leaf age on conidial germination and disease development of powdery mildew on rubber tree

Powdery mildew is an important disease of rubber trees worldwide. To assess the effects of temperature and leaf age on conidial germination and disease development, conidia were inoculated onto rubber tree seedlings with leaves at three phenological stages (copper bronze, colour-changing, and light green) and then incubated at six constant temperatures (10, 15, 20, 25, 30, and 35°C). Leaf age did not affect conidial germination (p = .296) whilst temperature did (p < .0001), although conidia were able to germinate at all tested temperatures. The estimated optimal temperature for conidial germination was 23.2°C. Leaf age, temperature, and their interactions had significant effects on conidial infection and hypha number (p < .0001). At 10 and 35°C, more than 2 and 4 days were needed for infection to complete, respectively, compared to <2, 1, 0.5, and 0.5 days for 15, 20, 25, and 35°C, respectively. Sporulation and mildew symptoms were only observed on those inoculated leaves of all stages at 20 and 25°C, and at the copper bronze stage only at 15°C. The latent period on the copper bronze leaves at 15°C was longer (9 days) than at 20 and 25°C (4 days). The latent period at 20 and 25°C increased from 4 to 7 days as the leaf development stage increased from copper bronze to light green. Therefore, temperature affected germination and postgermination growth of rubber tree powdery mildew, whereas leaf age primarily affected postgermination growth of the pathogen.     

Factors affecting seed germination and emergence of button grass (Dactyloctenium radulans) (R.Br.) P.Beauv.

Button grass (Dactyloctenium radulans) is a native, widely spread summer grass weed species in Australia. However, limited information is available on the seed germination biology of this species. Experiments were conducted to evaluate the effect of environmental factors on the germination and emergence of two populations of D. radulans. The seeds of these populations were collected separately from Dalby, Queensland and Coleambally, NSW. Seeds were germinated at a range of constant and alternating temperatures (25/15, 30/20, 35/25 and 40/30°C day/night). The greatest seed germination was at a constant temperature of 30°C. Seed germination was reduced at the lowest alternating temperature (25/15°C). Germination of both populations was strongly stimulated by light, suggesting a great amount of emergence of D. radulans on bare ground, such as crop seedbeds. Germination of the D. radulans population collected from a northern cotton farming system (Dalby) was more tolerant to a greater range of salt stress than the population sourced from the south (Coleambally). Seeds of both populations germinated over a wide range of pH, between 4 and 10. However, germination was the greatest in a high pH buffer solution, indicating that the species prefers to germinate in alkaline soil. These results showed that D. radulans seeds possess a wide range of tolerance mechanisms to different environmental stresses. Information obtained in this study will help in developing more sustainable and effective integrated weed management strategies for the control of this weed and weeds with similar responses in summer cropping systems, such as cotton.     

Exploring Chenopodium album adaptive traits in response to light and temperature stresses

Weeds within canopies are depleted in photosynthetic photon flux density (PPFD) and red to far‐red light ratio (R/FR) throughout their lifespan. Growth cabinet studies isolated PPFD and R/FR effects and explored the adaptive traits of a model invasive weed, Chenopodium album, to light and temperature. Reproductive development progressed rapidly at high temperature, yet the rate of leaf appearance was insensitive to temperatures of 25:15 and 10:5°C (day:night). Low R/FR effects were detected early in the life cycle, but by seed set, growth was influenced by low PPFD. C. album adapted to the simulated canopy environments by delaying seed set, growing taller and producing more leaf area per mol of accumulated incident PPFD. Low PPFD reduced seed number per plant and the carbon/nitrogen ratio of those seeds, but not seed weight. PPFD was a primary cue for many shade avoidance traits and only under low PPFD did R/FR modify the expression of these traits. This study elucidates the adaptive strategies that make C. album a persistent weed.     

Effects of after-harvest period and environmental factors on seed dormancy of Amaranthus species

The effects of seven constant temperatures (10–40°C at 5°C intervals) and seven after‐harvest periods (30–540 days after harvest) were evaluated on seed germination of nine Amaranthus species (A. albus, A. blitoides, A. cruentus, A. deflexus, A. graecizans, A. hybridus, A. lividus, A. retroflexus and A. viridis). Seeds of A. blitoides and A. viridis were also tested at alternating temperatures of 10/30°C (12/12 h thermoperiod) in continuous darkness and in an alternating 12/12 h dark/light photoperiod. With the exception of A. blitoides and A. viridis, germination increased as temperature increased from 20 to 35°C; the latter representing the optimum temperature (70–100% germination). At 10 and 15°C constant temperatures, no significant seed germination occurred in A. albus, A. deflexus, A. graecizans and A. lividus, while in A. cruentus, A. hybridus and A. retroflexus there was no germination at 10°C, but at 15°C more than 60% germination occurred. Germination was influenced strongly by after‐ripening period in A. cruentus, A. hybridus and A. retroflexus, partially in A. deflexus, and barely in A. graecizans and A. lividus. Seeds of A. blitoides and A. viridis required alternating temperatures and light to achieve high germination percentage (>90%). Primary dormancy in Amaranthus plays a fundamental role in extending germination over a longer period, so that the probability of seedling survival is maximised. The present study adds to the understanding of the environmental control and germination ecology of Amaranthus species and provides data that can contribute to predicting weed emergence dynamics.     

Germination and seedling emergence of glyphosate‐resistant and susceptible biotypes of goosegrass (Eleusine indica[L.] Gaertn.)

Effects of environmental factors on the germination and seedling emergence of glyphosate‐resistant (R) and ‐susceptible (S) biotypes of Eleusine indica (L.) Gaertn. were examined under laboratory and greenhouse conditions. The R biotype exhibited a higher germination percentage compared with the S biotype at constant temperatures of 20 and 35°C under dark conditions, and alternating temperatures of 30/25°C, and 35/25°C during a 12 h photo period. For both biotypes, germination was optimal at alternating temperatures of 30/20°C and 35/20°C. However, there was no significant difference (P > 0.05) in the germination between the R and S biotypes at these temperature regimes. The germination of both biotypes was inhibited by osmotic stress imposed by a water potential of ?0.80 MPa. When the moisture stress was released and the seeds were subsequently transferred to distilled water, the germination was enhanced to approximately 90% and 16% for the R and S biotype seeds, respectively. Higher emergence rates were obtained in shallow seed depths (0 or 2 cm) compared to deep depths. Emergence percentage of the R biotype was higher than that of the S biotype at 0 cm and 2 cm depths. The maximum emergence percentage of the R biotype was higher than that of S biotype when seeds were sown on the surface of either loamy or clay loam soil taken from three different sites.     

Dormancy,germination and emergence of Urochloa panicoides regulated by temperature

Urochloa panicoides is an annual weed of summer crops. In Argentina, in subhumid areas with monsoon rainfall, it germinates and establishes in a single flush. To (i) identify the environmental factors that modify its seed dormancy level and germination and (ii) quantify the parameters describing the thermal behaviour of the germination and emergence dynamics of this weed under non‐limiting water conditions, we established a set of germination experiments performed (i) under controlled conditions using seeds after ripened for 3 or 6 months in different thermal and hydric conditions and (ii) under field conditions, where the soil temperature was modified by applying different shading levels. Seed dormancy level remained high with 3 months after ripening in all treatments. After 6 months, seeds stored at 4°C in dry conditions did not germinate at any temperature, while seeds stored at 25°C in dry conditions and in situ germinated c. 20% and 60% respectively. Germination percentage was higher in seeds harvested before their natural dispersal. The base, optimum and maximum temperatures for seed germination were 6, 35 and 45°C respectively. Shading reduced the number of emerged seedlings, possibly by reducing the soil thermal amplitude. The results explained the dormancy‐breaking mechanism of U. panicoides that allows a high germination rate in the field when rainfall occurs.     

Field emergence and temperature requirements for germination in Solanum sarrachoides Sendt.

Emergence of Solanum sarrachoides began in late April, reached a peak in May or June and ceased in September. This pattern closely resembled that for S. nigrum L., whereas almost all seedlings of S. dulcamara L. emerged in April. Fresh seeds of S. sarrachoides were dormant but developed a capacity for germination at 25 and 30°C and at alternating (16 h low/8 h high) temperatures of 4/25, 10/25, 10/30 and 20/30°C when stored dry. kept moist at 4°C or buried in the field. Buried seeds also became capable of germinating at 10. 15 and 20°C and the temperature range for germination was widest during April-June. Induced dormancy developed during August and the range narrowed. The consistent seasonal emergence pattern appears to be associated with cyclic changes in the dormancy status of buried seeds.     

Asian spiderflower (Cleome viscosa) germination ecology in southern Iran

Cleome viscosa is one of the most important weeds of warm‐season crops in southern Iran. Laboratory experiments were conducted to assess the impact of environmental factors on seed germination of C. viscosa . Freshly harvested seeds exhibited dormancy that was relieved (>90%) after immersion for 20 min in concentrated sulfuric acid. Regardless of the temperature regime, the final percentage of germination in light/dark (69.3%) was significantly higher than in complete darkness (58.3%). The optimum temperature for germination was 35/25°C in both light and dark. No germination was observed at constant temperatures of either 15 or 45°C. The thermal thresholds for seed germination, the base (T _b) and the mean ceiling germination temperatures (T _c(50)) were estimated to be 18.8 and 39.9°C, respectively. A base water potential ( Ψ _b(50) ) of ?0.96 MPa was identified for C. viscosa seeds. The response threshold of C. viscosa to reduce 50% of maximum germination for salinity was estimated to be 255 mM. Seeds that were placed on the soil surface had the highest percentage of seedling emergence (77.3%), and no seedlings emerged from seeds placed at a depth of 6 cm. The findings of this study could help to improve the integrated weed management strategies for this species.     

Comparative biology and life table of Habrobracon hebetor (Hymenoptera: Braconidae) on Anagasta kuehniella (Lepidoptera: Pyralidae) at five constant temperatures

Habrobracon hebetor Say is an ectoparasitoid that has been used as a control agent of various lepidopteran pests. Temperature-dependent life table and thermal characteristics of H. hebetor are important in understanding the dynamics of host–parasitoid relationships and for optimizing biocontrol programmes. The influence of five constant temperatures (15, 20, 25, 30 and 35 °C) on the biology of H. hebetor when parasitizing Anagasta kuehniella Zeller was studied. The survival rate of immature stages increased from 16.67% to 83.81% as temperature increased from 15 to 30 °C and then decreased at 35 °C. Total development time ranged from 45.70 days at 15 °C to 7.10 days at 35 °C. The lower temperature threshold for immature stages varied slightly around a value of 11–12 °C. The net reproductive rate (R₀) values were significantly different among temperatures and the highest value was found at 30 °C (85.10). The high survival rate and net reproductive rate combined with a relatively short generation time at 30 °C resulted in the intrinsic rate of increase (r_m) being highest (0.312 d^?1) at this temperature. Considering the acquired results, the temperature range between 25 and 30 °C was optimal for H. hebetor.