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Context Tall fescue (Schedonorus arundinaceus) is one of the most important cultivated forage grasses in temperate regions but its association with some Epichloë fungal endophytes usually makes it unsuitable for livestock feeding due to toxic alkaloids. However, re-inoculation of plants with non-toxic endophytes can result in positive effects for livestock feeding. Aims Assess the effects of the non-toxic AR584 endophyte on seed viability and germination in two tall fescue cultivars. Methods The effects of AR584 endophyte (presence/absence) and tall fescue cultivar (INIA Aurora/Taita) were evaluated on seed viability and germination responses across a range of temperature and water availability regimes. Response to constant temperature and water availability was characterised by the thermal-time and hydro-time models, respectively. Key results Under accelerated aging conditions, endophyte presence reduced seed viability in cultivar Taita. For both cultivars, endophyte-infected seeds showed a lower germination rate than endophyte-free seeds, except for INIA Aurora under low incubation temperatures. Also, endophyte-infected seeds in both cultivars displayed lower germination rates and final germination as the water potential was lowered. Conclusions A strong cultivar-by-endophyte interaction for seed germination indicates the need to understand how specific cultivar-endophyte combinations best fit a given environmental condition, as defined by temperature and water availability. Implications The information we present can be valuable not only for farmers who have to decide what to cultivate but also for seed companies that have to preserve seed quality.

Fil: Longas, Maria de Las Mercedes. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Centro de Recursos Naturales Renovables de la Zona Semiarida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiarida; Argentina. Universidad Nacional del Sur. Departamento de Agronomia; Argentina

Temperature may regulate seed dormancy and germination and determine the geographical distribution of species. The present study investigated the thermal limits for seed germination of Polygonum ferrugineum (Polygonaceae), an aquatic emergent herb distributed throughout tropical and subtropical America. Seed germination responses to light and temperature were evaluated both before (control) and after stratification at 10, 15 and 20°C for 7, 14 and 28 d. Germination of control seeds was ~50% at 10 and 15°C, and they did not germinate from 20 to 30°C. The best stratification treatment was 7 d at 10°C, where seed germination was >76% in the dark for all temperatures, except at 30°C, and < 60% in light conditions. A thermal time approach was applied to the seed germination results. Base temperature (Tb) was 6.3°C for non-dormant seeds and optimal temperature (To) was 20.6°C, ceiling temperature (Tc () was 32.8°C, and thermal time requirement for 50% germination was 44.4°Cd. We concluded that a fraction of P. ferrugineum seeds is dormant, has a narrow thermal niche to germinate (10 and 15°C) and that cold stratification (10°C) alleviated dormancy and amplified the thermal range permissive for germination of the species. Consequently, P. ferrugineum is expected to occur in colder environments, for example, at high altitudes. Higher temperatures decrease the probabilities of alleviate dormancy and the ability of their seeds to germinate.

The timing of emergence of weed species has critical ecological and agronomical implications. In several species, emergence patterns largely depend on the level of dormancy of the seedbank, which is modulated by specific environmental factors. In addition, environmental conditions during seed maturation on the mother plant can have marked effects on the dormancy level at the time of seed dispersal. Hence, the maternal environment has been suggested to affect seedbank dormancy dynamics and subsequent emergence; however, this modulation has not been adequately examined under field conditions, and the mechanisms involved are only partly understood. Combining laboratory and field experiments with population-based models, we investigated how dormancy level and emergence in the field are affected by the sowing date and photoperiod experienced by the mother plant in Amaranthus hybridus, a troublesome weed worldwide. The results showed that an earlier sowing date and a longer photoperiod enhanced the level of dormancy by increasing the dormancy imposed by both the embryo and the seed coat. However, this did not affect the timing and extent of emergence in the field; on the contrary, the variations in dormancy level contributed to synchronizing the emergence of the next generation of plants with the time period that maximized population fitness. Our results largely correspond with effects previously observed in other species such as Polygonum aviculare and Arabidopsis, suggesting a common effect exists within different species.

Crop–weed interactions are affected by environmental alterations resulting from a crop’s presence, such as modifications in temperature, light quality and quantity, and moisture conditions that could modify weed performance. The objectives of this work were to study (1) how soybean [Glycine max (L.) Merr.] crop structure modifies the environment under the canopy and large crabgrass [Digitaria sanguinalis (L.) Scop.] plant structure, biomass, and seed production and dormancy; and (2) the relative importance of these environmental changes on the weed’s characteristics. A field experiment in a completely randomized block design with five replicates was performed to evaluate narrow and wide interrow spacing and soybean maturity groups 3 and 4. Measured variables were intercepted solar radiation (RAD); red–far red (R-FR) ratio; humidity; minimum, maximum, and alternating temperatures; and weed biomass, tillers per plant, height, and seed dormancy. Crop canopy reduced solar radiation, R-FR ratio, and daily average maximum and alternating temperatures. Soybean presence reduced the weed biomass, tillers and seeds per plant, and seed dormancy. High solar radiation intercepted by the crop during the reproductive phase was the main environmental variable related to reductions in weed biomass, tillers per plant, and fecundity. The combination of low temperature and solar radiation received by developing seeds was more related to seed dormancy than the rest of the variables. Crop management decisions focused on the fact that keeping the crop canopy alive for a longer time at the end of the season would not only reduce the weed growth but also seed dormancy.

Seeds of weed species require light to terminate dormancy and give way to germination. It is documented that sensitivity to light in Polygonum aviculare seeds increases during dormancy release. However, it is not known whether this sensitivity is lost during dormancy induction. The aim of this study was to investigate and quantify the changes in dormancy level of P. aviculare seeds during secondary dormancy induction as measured by changes in sensitivity to light in relation to soil temperature. Polygonum aviculare seeds were stratified at 5oC until obtaining a minimum dormancy level. The seeds were then induced into secondary dormancy by burying them in pots containing soil which were stored at 10, 15, 20 and 25oC for different time‐periods. Polygonum aviculare seeds were exhumed periodically, exposed to different light treatments (Pfr/Pₜₒₜₐₗ‐ₚyₜₕₒcₕᵣₒₘₑ = 0.76, 0.03 and 7.6 × 10⁻⁴) or darkness, and incubated at 15oC to test germination. Our results showed that the high sensitivity to light acquired during dormancy release and decreased during dormancy induction at a rate that was temperature‐dependent. These changes in sensitivity to light were quantified as a function of the accumulation of thermal‐time over a base‐temperature of 7.9oC during dormancy induction and were coupled with previous thermal‐time index established for the dormancy release process. Both thermal‐time indices allowed us to develop a model for the prediction of cyclic changes in sensitivity to light in relation to the thermal environment experienced by the seeds during burial. This model constitutes a valuable tool for predicting weed emergence in the field and to design management practices accordingly.

Polygonum aviculare seeds show high levels of primary dormancy (PD). Low winter temperatures alleviate dormancy and high spring temperatures induce seeds into secondary dormancy (SD), naturally establishing stable seedbanks cycling through years. The objective of this work was to elucidate the mechanism(s) involved in PD expression and release, and in SD induction in these seeds, and the extent to which abscisic acid (ABA) and gibberellins (GAs) are part of these mechanisms. Quantification of endogenous ABA both prior to and during incubation, and sensitivity to ABA and GAs, were assessed in seeds with contrasting dormancy. Expression analysis was performed for candidate genes involved in hormone metabolism and signaling. It was found that endogenous ABA content does not explain either dormancy release or dormancy induction; moreover, it does not seem to play a role in dormancy maintenance. However, dormancy modifications were commonly accompanied by changes in ABA sensitivity. Concomitantly, induction into SD, but not PD, was characterized by a increased PaABI-5 and PaPYL transcription, and a rise in GA sensitivity as a possible counterbalance effect. These results suggest that dormancy cycling in this species is related to changes in embryo sensitivity to ABA; however, this sensitivity appears to be controlled by different molecular mechanisms in primary and secondary dormant seeds.

Agricultural practices exert selective forces on weed populations. As these practices change over time, weed adaptive traits also evolve, allowing weeds to persist in the new environment. However, only weeds having individuals showing the trait with adaptive significance will be able to cope with these changes, thus allowing a sub-population to be selected for persistence. In addition, changes in agricultural practices can select new weed species showing functional traits with characteristics adaptive to the modified system. Seed dormancy has long been recognized as a trait with enormous adaptive value to adjust weed biology to cropping systems. In this paper, we illustrate with examples of success and failure, the value of seed dormancy as a functional trait to cope with long-term changes in crop production systems. We show that successful outcomes are mostly related to the existence of sufficient variability for the functioning of physiological mechanisms that control dormancy characteristics as influenced by the agricultural environment. Presented examples illustrate how knowledge about the relationship that exists between agricultural practices and their selective pressure on seed dormancy can be instrumental in predicting changes in weed biotype dormancy characteristics or foreseeing the appearance of new weed species in future agricultural scenarios. © 2019 Society of Chemical Industry.

Digitaria sanguinalis is a troublesome annual weed that causes important yield losses in different crops. Despite this, there is scarce information about different aspects of its biology under field conditions. New knowledge about the establishment process of this species will be of paramount importance in order to maximise the effectiveness of weed management. The aims of this paper were to evaluate the effect of stubble found on the surface on seed dormancy levels through the season, the effects of stubble and soyabean crop canopy on seedling emergence and to determine the field emergence pattern as a consequence of seed dormancy level at dispersal time. Seeds on the soil surface, which showed a high dormancy level at the beginning of autumn, were released from dormancy by low winter temperatures and germinated during spring as temperatures rose, showing a transient surface seedbank. Seeds covered by stubbles had delayed the emergence in the field due to lower alternating temperatures perceived by the surface seedbank. On the other hand, the presence of a soyabean crop and stubble together reduced the number of seedlings. Seeds with a high dormancy level at dispersal time showed a delayed emergence in the next season when compared with seeds with a lower dormancy level. However, the final number of seedlings was similar. Both stubble on surface and crop canopy are useful factors to lessen and delay the seedling emergence allowing the design of weed management strategies in order to diminish the population levels of this species.

Emergence at an appropriate time and place is critical for maximizing plant fitness and hence sophisticated mechanisms such as seed dormancy have evolved. Although maternal influence on different aspects of dormancy behavior has been identified, its impact under field conditions and its relation to plant fitness has not been fully determined. This study examined maternal effects in Polygonum aviculare on release of seed primary dormancy, responses to alternating temperatures, induction into secondary dormancy, and field emergence patterns as influenced by changes in the sowing date and photoperiod experienced by the mother plant. Maternal effects were quantified using population threshold models that allowed us to simulate and interpret the experimental results. We found that regulation of dormancy in P. aviculare seeds by the maternal environment is instrumental for maximizing plant fitness in the field. This regulation operates by changing the dormancy level of seeds dispersed at different times (as a consequence of differences in the sowing dates of mother plants) in order to synchronize most emergence to the seasonal period that ultimately guarantees the highest reproductive output of the new generation. Our results also showed that maternal photoperiod, which represents a clear seasonal cue, is involved in this regulation.

Seed germination is one of the earliest phenotypes expressed by plants, and the accuracy to germinate in the correct time and place is essential for plant population fitness. Cues that regulate germination have been exhaustively characterized in laboratory experiments. However, the way in which seed populations respond to these cues and the ecological meanings of this phenomenon are scarcely known. To help filling this gap, we studied the thermic regulation of seed germination and its link with local climatic characteristics in Festuca pallescens populations, an iconic perennial grass species of Patagonia. By coupling thermal time models with field and environmental data, we evaluated seed responsiveness to temperature during germination of nine Festuca pallescens populations distributed across their complete longitudinal range in North Patagonia. The idea behind this experimental design was to search for inter-population differences in early life traits relevant for recruitment of the species, and associate seed traits with local environmental characteristics. F. pallescens populations showed strong differences in seed sensitivity to temperature for the thermal regulation of the germination rate, described by the population mean thermal time for germination (θ(50)) and its standard deviation (σθ). The greatest values of θ(50) and σθ corresponded to populations inhabiting harsh environments. Moreover, θ(50) of the different populations showed strong correlations with their local thermal environmental parameters, indicating a relation between seed physiological traits and climate across the longitudinal gradient. We did not observe inter-population differences in the minimum temperature that allows germination (Tb = -0.47 ± 0.19 °C). Higher θ(50) and σθ in populations from harsh climates may constitute a strategy of the species to increase fitness in hostile habitats, preventing anticipated germination and promoting a slower seed bank depletion in regions where establishment is highly limited.

López, Aldana Soledad. Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Patagonia Norte. Estación Experimental Agropecuaria Bariloche (EEA Bariloche). Instituto de Investigaciones Forestales y Agropecuarias Bariloche (IFAB). Bariloche, Río Negro, Argentina. López, Aldana Soledad. CONICET - Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Patagonia Norte. Estación Experimental Agropecuaria Bariloche (EEA Bariloche). Instituto de Investigaciones Forestales y Agropecuarias Bariloche (IFAB). Bariloche, Río Negro, Argentina. López, Dardo Rubén. Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Córdoba. Estación Experimental Agropecuaria Manfredi (EEA Manfredi). Agencia De Extensión Rural Villa Dolores. Córdoba, Argentina. Arana, María Verónica. Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Patagonia Norte. Estación Experimental Agropecuaria Bariloche (EEA Bariloche). Instituto de Investigaciones Forestales y Agropecuarias Bariloche (IFAB). Bariloche, Río Negro, Argentina. Arana, María Verónica. CONICET - Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Patagonia Norte. Estación Experimental Agropecuaria Bariloche (EEA Bariloche). Instituto de Investigaciones Forestales y Agropecuarias Bariloche (IFAB). Bariloche, Río Negro, Argentina. Batlla, Diego. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura. Buenos Aires, Argentina. Marchelli, Paula. Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Patagonia Norte. Estación Experimental Agropecuaria Bariloche (EEA Bariloche). Instituto de Investigaciones Forestales y Agropecuarias Bariloche (IFAB). Bariloche, Río Negro, Argentina. Marchelli, Paula. CONICET - Instituto Nacional de Tecnología Agropecuaria (INTA). Centro Regional Patagonia Norte. Estación Experimental Agropecuaria Bariloche (EEA Bariloche). Instituto de Investigaciones Forestales y Agropecuarias Bariloche (IFAB). Bariloche, Río Negro, Argentina. Sensitivity to water availability is a key physiological trait for grassland species located in arid and semiarid environments, where successful germination is closely related to rainfall dynamics. Festuca pallescens inhabits diverse environments along a steep precipitation gradient in North Patagonia, thus ofering a suitable model for the study of germination behavior in response to water availability. \nBy analyzing germination in nine populations using a hydrotime model approach, we aimed to fnd within-species variation. Seed population behavior was analyzed under diferent hydric conditions using hydrotime model parameters (hydrotime, mean base water potential and its standard deviation). We estimated the mean base water potential for F. pallescens (micro b(50) = − 2.79 ± 0.45 MPa), which did not difer signifcantly between populations. However, the hydrotime parameter (θH) varied markedly, suggesting physiological adaptation to local environments. Higher values of H were found in populations located at the extremes of the distribution gradient, indicating that germination may be prevented or delayed in conditions that are suboptimal for the species. Since the variation in hydrotime model parameters did not follow a cline, micro-environmental cues may have the greatest infuence on the physiological behavior of the species, rather than the macroscale rainfall gradient. grafs., tbls.

Seedling emergence in the field is strongly related to the dynamics of dormancy release and induction of the seed bank, which is mainly regulated by soil temperature. However, there is limited information on how temperature-driven effects on dormancy changes are modulated by the seed hydration-level. We investigated the effect of seed water content (SWC) on the dormancy release and dormancy induction in Polygonum aviculare L. seeds. We characterised quantitatively the interaction between seed water content (SWC) and temperature through the measurement of changes in the lower limit temperature for seed germination (Tl) during dormancy changes for seeds with different SWC. These relationships were inserted in existing population-based threshold models and were run against field obtained data. The model considering SWC was able to predict P. aviculare field emergence patterns. However, failure to consider SWC led to overestimations in the emergence size and timing. Our results show that in humid temperate habitats, the occurrence of eventual water shortages during late-winter or spring (i.e. short periods of water content below 31% SWC) can affect soil temperature effects on seed dormancy, and might lead reductions in the emergence size rather than to significant temporal displacements in the emergence window. In conclusion, SWC plays an important role for the perception of temperature signals that drive dormancy changes in buried seeds.

Weeds are usually more vulnerable to control practices at the seedling stage or at early stages of their growth. Therefore, developing models to predict the timing and extent of weed emergence is useful to assist farmers and agronomist to time pre- and post-emergence control practices to increase their efficacy. However, many important weeds forming persistence seedbanks in agricultural fields present dormancy. In those species, the number of established seedlings is strongly related to the dormancy level of the seedbank, and the timing of seedling emergence depends on the seasonal variation in seedbank dormancy level. Therefore, if we pretend to predict timing and extent of seedling emergence, we should include the regulation of the seedbank dormancy level in our predictive models. In this chapter, we present a conceptual framework to understand how dormancy and germination of weed seedbanks are regulated by the environment. This framework is based on the distinction between those factors that regulate seasonal changes in the seedbank dormancy level (i.e. temperature in interaction with seed moisture content) and those factors that terminate dormancy (i.e. light and alternating temperatures). Changes in the seedbank dormancy level are related to changes in the range of environmental conditions permissive for seed germination, as, for example, the thermal range permissive for germination which is defined by the lower and the higher limit temperatures. Seeds germinate when environmental conditions are within the permissive range, for example, seeds begging to accumulate thermal time towards germination once soil temperature overlaps the permissive thermal range. We present examples of how these concepts can be used to establish functional relationships between dormancy and germination regulating factors (i.e. temperature) and changes in seedbank population dormancy level and germination dynamics in order to develop mechanistic models to predict the timing and extent of weed seedling emergence in the field.

Published information regarding the effect of storage temperature on dormancy alleviation of sunflower achenes is contradictory and ambiguous. In the present study we explored the effect of temperature during dry storage on dormancy release in two sunflower genotypes, including a commercial hybrid and an inbred line. Dry storage at 25°C consistently accelerated dormancy release of achenes compared with 5°C. This response fits the general pattern reported for dry after-ripening in seeds of many other species. Depending on the genotype and the dormancy factor prevailing, higher temperature alleviated embryo dormancy and coat-imposed dormancy. Hormonal pathways involved in these changes were investigated at the physiological level. In both genotypes, sensitivity to abscisic acid (ABA) was reduced by storage at 25°C. Also, but only in one genotype, storage at 25°C reduced ABA levels upon imbibition and increased the response to a gibberellin (GA) synthesis inhibitor and to applied GA3, compared with storage at 5°C; these results support the idea that temperature affects both ABA and GA metabolism and signalling pathways during after-ripening. This information will be useful to define storage conditions for commercial sunflower achenes, and will also help focus future research on the underlying mechanisms of dormancy release during dry after-ripening in sunflower. Fil: Rodríguez, María Verónica. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Biología Aplicada y Alimentos. Cátedra de Fisiología Vegetal; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Bodrone, María Paula. Monsanto Argentina; Argentina Fil: Castellari, María Paula. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina

In seeds of many summer annuals low temperatures under moist conditions provoke dormancy release while high temperatures induce secondary dormancy. Seed dormancy level establishes the range of temperatures under which germination is possible. The range of temperatures permissive for seed germination is determined by two threshold limit temperatures: Lower limit temperature (Tl) and High limit temperature (Th). Numerous studies have been conducted to characterize the effect of temperature on dormancy release, but there is very few information on how temperature regulates secondary dormancy induction. Seeds of Polygonum aviculare were stratified at 1.6, 5 and 10 °C until achieving a minimum dormancy, and then were induced into secondary dormancy by further storage at 10, 15, 20, 25 and 30 °C. Based on obtained germination time course-curves we quantified changes in the thermal range permissive for seed germination through variations in the mean lower limit temperature for seed germination (Tl(50)) using a mathematical simulation germination model. Our data suggest that induction into secondary dormancy in P. aviculare seeds can be assessed quantitatively through changes in Tl(50).This changes could be described through a Dormancy Induction Thermal-Time Index (DItt), in which thermal time units are accumulated above a threshold temperature from which secondary dormancy is induced (7.9 °C). Additionally, the induction-rate into secondary dormancy was affected by the stratification temperature during dormancy release. We conclude that the effect of temperature on the rate of dormancy induction is not only dependent on prevailing temperature, but also on temperature experienced by seeds during previous dormancy release and the resulting dormancy status of the seed population. Fil: Malavert Pineda, Cristian Jonatan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cultivos Industriales; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina Fil: Benech-Arnold, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cultivos Industriales; Argentina

Cynara cardunculus is a troublesome weed in temperate grazing lands. Cynara cardunculus achenes are usually dormant at dispersal and require alternating temperatures to terminate dormancy and germinate. Laboratory and glasshouse experiments were conducted to determine (i) the treatments able to terminate dormancy and (ii) the effect of environmental factors and agronomic practices on germination and emergence of non-dormant (dry after-ripened) achenes. Scarification, hydrogen peroxide and sodium hypochlorite promoted germination of dormant achenes. Dry after-ripening and cold stratification were tested in two different populations. Dormancy of both populations was released from dormancy by dry after-ripening. In contrast, cold stratification allowed dormancy release in just one of the populations, while the other was induced into secondary dormancy. Germination of non-dormant (dry after-ripened) achenes was maximum in a range of temperatures from 10 to 20°C and was inhibited at higher temperatures. Reduction of osmotic potential below −0.6 MPa led to a decrease in final germination. These results explain synchronic emergence of C. cardunculus seedlings in autumn after dormancy release during summer. Maximum seedling emergence was close to 60% at soil depths of 1 cm and only decreased as depth increased over 6 cm. In contrast, seedling emergence was not reduced by the presence of cover residues, while a flooding duration of 21 days was required to suppress emergence significantly. These results suggest that the deep burial of achenes and agronomic practices that take advantage of synchronic emergence of achenes could be useful tools leading to better long-term management of C. cardunculus. Fil: Huarte, Hector Roberto. Universidad Nacional de Lomas de Zamora; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bolandelli, Federico. Universidad Nacional de Lomas de Zamora; Argentina Fil: Varisco, Daiana Ayelen. Universidad Nacional de Lomas de Zamora; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

Temperature is a key factor affecting both dormancy and germination. In non-dormant seeds, when temperature is within the thermal range permissive for germination, it just regulates germination velocity, while in seeds presenting dormancy it can also be affecting dormancy level, dormancy termination and the expression of dormancy itself. This dual effect of temperature on dormancy and germination often leads to misinterpretation of obtained germination results and confounds the analysis of temperature effects in seed populations presenting some degree of dormancy. In the present paper we discuss the effect of temperature in the regulation of dormancy level and its implications in dormancy expression, as an attempt to construct a conceptual framework that allows distinguishing between the effects of temperature on dormancy and germination. Finally, we present examples of how a better understanding of these effects could help us to interpret the mixed effects of temperature on both processes during incubation of seeds presenting dormancy. Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Benech Arnold, Roberto L.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

The success of large crabgrass (Digitaria sanguinalis) growing among summer crops in Argentina, may be partly explained by its escape from weed controls related to the emergence of different seedlings cohorts determined by seed dormancy and germination requirements. The objectives of this work were to evaluate the effect of temperature, red (R):far-red (FR) ratio and the possible role of the caryopses covering structures on the release of seed dormancy in D. sanguinalis. Therefore, the effects of moist pre-treatment duration, light and temperature, as well as the caryopsis covering structures, and imbibition with H2O2 and the extract of caryopses covers on seed germination, were investigated. Moist pre-treatment at 5 and 20 °C promoted dormancy release and fluctuating temperatures between 20/30 °C and light promoted germination. However, exposure to 30 min of light with a high R:FR ratio reduced germination. Removing or puncturing some of the caryopsis covering structures, as well as imbibition with 2.6 M H2O2 enhanced seed germination. Results suggest that the extended seedling emergence throughout the season could be due to the influence of the environmental factors studied here on dormancy release and germination, and that seed covering structures have an important role in seed dormancy imposition for this species. Fil: Oreja, Fernando Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal; Argentina Fil: de la Fuente, Elba Beatriz. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Producción Vegetal; Argentina

A rapid and uniform germination in the field is an important requirement for commercial hybrid sunflower seed. Persistence of dormancy after harvest can negatively affect this aspect of seed quality, and seed lots with some degree of dormancy cannot be commercialized. Seed dormancy intensity and duration can vary greatly among sunflower genotypes, but it is also subject to strong interactions with the maternal environment. In this paper we report results of investigations into the effect of temperature during sunflower fruit development on the level of dormancy after harvest. After conducting controlled (greenhouse) and field experiments (sowing dates and plastic tents), we found that higher temperatures during later stages of achene development significantly increased the level of dormancy at harvest and its persistence during storage. The impact of the maternal (thermal) environment on embryo and coat-imposed dormancy was also investigated. Results showed that although maturation under warmer environments reduced embryo dormancy, this effect was overcompensated for by the enhancement of inhibition imposed by the pericarp and the seed coat. In addition, the results obtained suggest that observed changes in dormancy in response to the maternal environment could be at least partially explained by changes in achene and/or embryo sensitivity to ABA. Results presented here should be useful when choosing a proper environment for the production of hybrid sunflower seed of high quality, avoiding high temperatures during later stages of fruit development. Fil: Bodrone, María Paula. Monsanto Argentina; Argentina Fil: Rodríguez, María Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Biología Aplicada y Alimentos. Cátedra de Fisiología Vegetal; Argentina Fil: Arisnabarreta Dupuy, Sebastián. Monsanto Argentina; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Biología Aplicada y Alimentos. Cátedra de Fisiología Vegetal; Argentina

Light regulates dormancy termination and the subsequent germination in many weed species. Under field conditions, the light environment of the seeds, which is perceived mainly by photoreceptors of the phytochrome family, provides essential information for cueing germination in the proper environmental situation. The light environment's spectral composition and irradiance allow weed seeds to sense their position in the soil profile, the presence of a leaf canopy capturing light and other resources and the occurrence of soil cultivation. From an agronomical point of view, the control of germination by light represents a potentially useful step in the life cycle of weeds for developing effective control practices. The goal of this article is to place current knowledge regarding photoreceptors, physiological and molecular bases of seed responses to light and their ecological implications within the context of weed management in agricultural systems. With that final objective, the authors intend to show how a better understanding of the way in which the light environment regulates dormancy termination and the subsequent germination of weed seeds could be used to develop more accurate control practices and to improve weed management strategies.

Sunflower (Helianthus annuus L.) achenes often display pericarp-imposed dormancy, which is long-lasting and causes serious problems to crop production and the seed industry. For this study we assessed an extensively used sunflower inbred line that has this type of dormancy. Our goals were (i) to determine the effect of pericarp on germination and to evaluate its impact on crop field emergence, (ii) to provide insight into the physiological basis of pericarp-imposed dormancy by determining the effects of abscisic acid (ABA) accumulation in the embryo and the embryo sensitivity to ABA during incubation at different temperatures, (iii) to assess the effect of oxidant agents and other compounds on dormancy termination, and (iv) to evaluate the feasibility of using oxidants to remove dormancy at an industrial scale. Incubation at high temperatures (i.e., 25 to 30°C) allowed the expression of dormancy, which was imposed by the pericarp and was accompanied by an increase in embryo sensitivity to ABA, but not in ABA concentration. Treated achenes with sodium hypochlorite, or their incubation in presence of an ethylene precursor or gibberellins overcame dormancy. ABA concentration decreased during incubation when treated with sodium hypochlorite. Application of sodium hypochlorite on a commercial seed lot (i.e., washing with 3 and 7%, after additional chemicals used by the industry were applied) resulted in higher germination compared with dormant non-treated controls. Field trials showed that pericarp-imposed dormancy reduced crop emergence in the inbred line tested herein. However, treating achenes with sodium hypochlorite using described industrial procedures improved field emergence. Fil: Dominguez, Constanza Pía. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. ; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. ; Argentina Fil: Rodríguez, María Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. ; Argentina Fil: Windauer, Liliana Beatriz. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Gerbaldo, Magdalena. Dow AgroSciences; Argentina Fil: Benech-Arnold, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. ; Argentina

Pre-harvest sprouting (PHS) is a serious risk when adapting quinoa (Chenopodium quinoa) seed production to different temperate environments. Two quinoa accessions, 2-Want and Chadmo were evaluated under field conditions in the Argentinean pampas over 2 years on five different sowing dates, to explore a range of climate conditions under which seed filling is manageable in this region. Both accessions exhibited dormancy during seed development and maturation under the conditions examined; however, dormancy expression was restricted to low temperatures in 2-Want, while seeds of Chadmo, originating from the humid island of Chiloe, southern Chile, expressed a high level of dormancy at all examined temperatures. Dormancy release was observed as a reduction in the lowest temperature permissible for seed germination, which broadened the optimal germination temperature window. Higher storage temperature increased the rate of dormancy release. The environment during seed development on the mother plant affected the levels and patterns of seed dormancy, with higher temperatures and longer photoperiods promoting dormancy. As dormancy was released before the next production period, the levels of dormancy observed in the accession would allow timely planting and uniform germination, while dormancy during seed maturation ensures the prevention of PHS. Chadmo showed deeper dormancy levels in all situations, compared with 2-Want, therefore greater PHS tolerance under various conditions in the pampas region can be expected for Chadmo, which makes this accession a better candidate to be included in adaptive breeding programmes for quinoa. Fil: Ceccato, Diana V.. Universidad Tecnológica Nacional. Facultad Regional Buenos Aires; Argentina Fil: Bertero, Hector Daniel. Universidad de Buenos Aires. Facultad de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina

The possibility of accurately predicting timing and extent of seedling emergence from natural seed soil banks has long been an objective of both ecologist and agriculturalist. However, as dormancy is a common attribute of many wild seed populations, we should first be able to predict dormancy changes if we intend to predict seedling emergence in the field. In this paper, we discuss the most relevant environmental factors affecting seed dormancy of natural seed soil banks, and present a conceptual framework as an attempt to understand how these factors affect seed-bank dormancy level. Based on this conceptual framework we show approaches that can be used to establish quantitative functional relationship between environmental factors regulating dormancy and changes in the seed-bank dormancy status. Finally, we briefly explain how we can utilize population-based threshold models as a framework to characterize and quantify changes in seed sensitivity to environmental factors as a consequence of dormancy loss and/or induction. © 2010 Springer Science+Business Media B.V. Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Benech-Arnold, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina

Mathematical models that predict emergence of weed seedbanks could be useful tools for determining the most suitable time for weed seedling control and, consequently, should result in a higher efficacy of applied control methods. To achieve this goal in dormant weed species, functional relationships should be established between environmental factors regulating dormancy and dormancy changes of seed populations. In the present work, we used a simple model and an optimisation procedure to quantify the effect of temperature on Polygonum aviculare seed dormancy release and induction, based on germination data. Dormancy release rate was inversely related to temperature, showing a decreasing logistic trend that results in no dormancy release for seeds exposed to 20 and 25 degrees C. In contrast, dormancy induction rates in absolute values were positively associated with temperature, showing a logistic trend in which dormancy induction was almost zero at low temperatures and maximal at 25 degrees C. Derived model parameters were used to simulate dormancy changes of P. aviculare seeds stored under controlled and field conditions. These results suggest that similar model structures could be used to quantify temperature effects on seed dormancy status of other weed species and to develop predictive models of weed emergence.

Models based on thermal-time approaches have been a useful tool for characterizing and predicting seed germination and dormancy release in relation to time and temperature. The aims of the present work were to evaluate the relative accuracy of different thermal-time approaches for the description of germination in Lithospermum arvense and to develop an after-ripening thermal-time model for predicting seed dormancy release. Fil: Chantre Balacca, Guillermo Ruben. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Sabbatini, Mario Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina Fil: Orioli, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina

Seeds integrate environmental cues that modulate their dormancy and germination. Although many mechanisms have been identified in laboratory experiments, their contribution to germination dynamics in existing communities and their involvement in defining species habitats remain elusive. By coupling mathematical models with ecological data we investigated the contribution of seed temperature responses to the dynamics of germination of three Nothofagus species that are sharply distributed across different altitudes in the Patagonian Andes. Seed responsiveness to temperature of the three Nothofagus species was linked to the thermal characteristics of their preferred ecological niche. In their natural distribution range, there was overlap in the timing of germination of the species, which was restricted to mid-spring. By contrast, outside their species distribution range, germination was temporally uncoupled with altitude. This phenomenon was described mathematically by the interplay between interspecific differences in seed population thermal parameters and the range in soil thermic environments across different altitudes. The observed interspecific variations in seed responsiveness to temperature and its environmental regulation, constitute a major determinant of the dynamics of Nothofagus germination across elevations. This phenomenon likely contributes to the maintenance of patterns of species abundance across altitude by placing germinated seeds in a favorable environment for plant growth. Fil: Arana, María Verónica. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Gonzalez Polo, Marina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación En Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; Argentina Fil: Martinez Meier, Alejandro. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche; Argentina Fil: Gallo, Leonardo Ariel. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche; Argentina Fil: Benech-arnold, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Sanchez, Rodolfo Augusto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas A la Agricultura; Argentina Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

It has been hypothesized that fluctuations in soil water content may affect the dormancy status of weed seed banks under field conditions. In this paper, we present results showing that fluctuations in soil water content affect the dormancy status of buried seeds ofPolygonum aviculareL. stored at dormancy-releasing temperatures. Effects of fluctuations in soil water content on the dormancy status ofP. aviculareseeds were evaluated by comparing changes in the range of temperatures and water potentials permissive for germination, and in the sensitivity to fluctuating temperatures, between seeds subjected to a moist soil regime (MS) or to a fluctuating soil water content regime (FS). In comparison to the dormancy release pattern observed for seeds subjected to MS, seeds subjected to FS generally showed an increase in their dormancy level after periods of storage under dry soil conditions, and a decrease in their dormancy level after periods of storage under moist soil conditions. These effects were more pronounced during early stages of the storage period, producing larger changes in the thermal and water potential range for seed germination than in the sensitivity of seeds to fluctuating temperatures. Seeds subjected to FS generally exhibited a lower mean low-limit temperature, lower mean thermal time and hydrotime requirements for germination, and a higher proportion of the seed population with the capacity to germinatein situ, than seeds subjected to MS. The results obtained suggested that fluctuations in soil water content could be an additional factor affecting dormancy and weed emergence patterns under field conditions.

Changes in population hydrotime parameters were determined during stratification inPolygonum aviculareL. seeds in order to model dormancy loss. Seeds buried in pots were stored at three temperatures (1.6, 7 and 12°C) for 110 d and were exhumed at regular intervals during the storage period. Exhumed seeds were incubated at different water potentials at 15°C and germination time courses were analysed to determine hydrotime parameters. The population mean base water potential (Ψb(50)) decreased concomitantly with seed dormancy, while the hydrotime constant (θH) and the standard deviation of base water potential (θΨb) displayed only minor changes. Based on these results, a model for simulatingP. aviculareseed dormancy loss in relation to low temperature was developed. The model employs Ψb(50) as an index of mean seed population dormancy status. While Ψb(50) was allowed to vary as seeds were released from dormancy, θHand θΨbwere held constant. Changes in Ψb(50) were related to the time and temperature, using a previously developed thermal stratification time index (Stt), which quantifies the accumulation of thermal time units below a threshold temperature required for dormancy loss to occur. Therefore, Ψb(50) varied in relation to the accumulation ofSttaccording to time and temperature. Model performance showed acceptable prediction of timing and percentage of germination of seeds buried in irrigated plots, but did not accurately predict germination of seeds exhumed from rain-fed plots. Thus, environmental factors other than temperature could also be involved in the regulation of dormancy status of buried seeds under field conditions.

The sensitivity ofPolygonum aviculareL. seeds to the dormancy-breaking effect of cycle-doses of fluctuating temperature changes as seeds lose dormancy due to storage under stratification temperatures. Sensitivity changes during seed stratification were characterized by a decrease in the number of cycles required to saturate the germination response, and by a progressive loss of the requirement for temperature fluctuations for dormancy breakage in increasing fractions of the seed population. The rate of these changes was dependent on the temperature at which seeds were stored for stratification; lower storage temperatures produced higher rates of change than higher storage temperatures. Germination curves, obtained in response to the effect of fluctuating temperature cycle-doses for seeds stratified at variable temperatures and times of storage, were brought to a common stratification thermal time (Stt) scale by accumulating thermal time units under a threshold temperature for dormancy loss to occur. Results showed that those seeds that had accumulated similarSttunits during stratification under different storage temperatures presented a similar germination response. Therefore, response-curve functions were adjusted to germination data of exhumed seeds that had accumulated similarStt, obtaining a family of germination response curves in relation toSttaccumulation during storage. Based on these results, a simulation model was constructed relating dynamic changes in the parameters that determine germination response curves in relation toSttaccumulation. The model was tested against independent data, showing a good description of the dynamics of changes in the fraction of the seed population requiring temperature fluctuation for dormancy breakage as dormancy release progressed.

A model for simulatingPolygonum aviculareL. seed dormancy loss in relation to stratification temperature was developed. The model employs the lower limit temperature for germination (Tl) as an index of seed population dormancy status. While population mean forTl(Tl(50)) andTldistribution within the population (σTl) are allowed to vary as seeds are released from dormancy, other thermal parameters characterizing the germination thermal responses (base, optimal and maximal temperatures, and thermal time required for germination) and the higher limit temperature for germination (Th) are held constant. In order to relate changes inTl(50)and σTlto variable time and temperature, a stratification thermal time index (Stt) was developed, which consists of the accumulation of thermal time units under a threshold temperature for dormancy loss to occur. Therefore,Tl(50)and σTlvaried in relation to the accumulation ofSttaccording to time and temperature. To derive model equations, changes in seed population thermal parameters were estimated for buried seeds stored at 1.6, 7 and 12°C for 110 d. Seeds were exhumed at regular intervals, and were incubated at 15°C and at a gradually increasing temperature regime in the range 6–25°C. The germination time-course curves obtained were reproduced using a mathematical model. Thermal parameters that best fit simulated and experimentally obtained germination time-course curves were determined. Model performance was evaluated against data of two unrelated experiments, showing acceptable prediction of timing and percentage of germination of seeds exhumed from field and controlled temperature conditions.

Summary 1. An early inhibition of germination in seeds of Silene gallica and Brassica campestris which were continuously exposed to the light environment under an establishing wheat canopy, was observed in two different experiments. Inhibition occurred c. 15 days after crop emergence, when the canopy leaf area index (LAI) was below one and the red (R):far-red (FR) ratio recorded under the canopy was well above 0·8. 2. This inhibitory effect was either overcome by filtering FR light through a solution of CuSO 4 or could be artificially imposed by simulating the canopy with filters yeilding a R:FR ratio of 0·95 and 0·8. These results show that light subtly enriched with FR was the environmental factor regulating germination below the developing canopy. 3. Exposure to canopy-filtered light pulses of 1 h (presumably sufficient to saturate a low fluence response, LFR) did not inhibit seed germination. Moreover, such treatment promotes germination up to an extent similar to that previously observed in the labor atory after a saturating pulse of R light. Instead, prolonged exposures were required to inhibit germination. These results, together with the relatively high R:FR ratios measured below the canopy in early stages of its establishment, suggest that a high irradiance response (HIR) would be involved in such a regulation. 4. This capacity to detect small environmental light-quality modifications when exposed to high irradiances, would allow the seeds from these species to detect the presence of a canopy in the very early stages of its establishment and to stay in ‘safe’ pre-germination phases when the probability of successful seedling establishment is low.

The effect of various combinations of temperature and relative humidity on dormancy alleviation of sunflower seeds during dry after-ripening was investigated. The rate of dormancy alleviation depended on both temperature and embryo moisture content (MC). Below an embryo MC of 0.1 g H2O g-1 dw, dormancy release was faster at 15 °C than at higher temperatures. This suggests that dormancy release at low MC was associated with negative activation energy, supported by Arrhenius plots, and low Q10 values. At higher MC, the rate of dormancy alleviation increased with temperature, correlating well with the temperature dependence of biochemical processes. These findings suggests the involvement of two distinct cellular mechanisms in dormancy release; non-enzymatic below 0.1 g H2O g-1 dw and associated with active metabolism above this value. The effects of temperature on seed dormancy release above the threshold MC were analysed using a population-based thermal time approach and a model predicting the rate of dormancy alleviation is provided. Sunflower embryo dormancy release was effective at temperatures above 8 °C (the base temperature for after-ripening, TbAR, was 8.17 °C), and the higher the after-ripening temperature above this threshold value, the higher was the rate of dormancy loss. Thermodynamic analyses of water sorption isotherms revealed that dormancy release was associated with less bound water and increased molecular mobility within the embryonic axes but not the cotyledons. It is proposed that the changes in water binding properties result from oxidative processes and can, in turn, allow metabolic activities. Fil: Bazin, J.. Universite de Paris VI; Francia Fil: Batlla, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Dussert, S.. No especifica; Fil: El-Maarouf-Bouteau, H.. Universite de Paris VI; Francia Fil: Bailly, C.. Universite de Paris VI; Francia

† Background and Aims It has been hypothesized that soil moisture conditions could affect the dormancy status of buried weed seeds, and, consequently, their sensitivity to light stimuli. In this study, an investigation is made of the effect of different soil moisture conditions during cold-induced dormancy loss on changes in the sensitivity of Polygonum aviculare seeds to light. † Methods Seeds buried in pots were stored under different constant and fluctuating soil moisture environments at dormancy-releasing temperatures. Seeds were exhumed at regular intervals during storage and were exposed to different light treatments. Changes in the germination response of seeds to light treatments during storage under the different moisture environments were compared in order to determine the effect of soil moisture on the sensitivity to light of P. aviculare seeds. † Key Results Seed acquisition of low-fluence responses during dormancy release was not affected by either soil moisture fluctuations or different constant soil moisture contents. On the contrary, different soil moisture environments affected seed acquisition of very low fluence responses and the capacity of seeds to germinate in the dark. † Conclusions The results indicate that under field conditions, the sensitivity to light of buried weed seeds could be affected by the soil moisture environment experienced during the dormancy release season, and this could affect their emergence pattern.

The effect of cold (stratification) temperature on changes in the sensitivity of Polygonum aviculare seeds to light was investigated. Seeds buried in pots were stored under stratification temperatures (1.6, 7 and 12 degrees C) for 137 d. Seeds exhumed at regular intervals during storage were exposed to different light treatments. Germination responses obtained for seeds exposed to different light treatments and stratification temperatures were used to develop a model to predict the sensitivity of buried seeds to light. Seed sensitivity to light increased as dormancy loss progressed, showing the successive acquisition of low-fluence responses (LFR), very low-fluence responses (VLFR), and the loss of the light requirement for germination for a fraction of the seed population. These changes were inversely correlated to stratification temperature, allowing the use of a thermal time index to relate observed changes in seed light sensitivity to stratification temperature. The rate of increase in sensitivity of P. aviculare seeds to light during stratification is inversely correlated to soil temperature, and these changes in light sensitivity could be predicted in relation to temperature using thermal-time models.