Roberto L. Benech-Arnold

ENVIRONMENTAL CONTROL OF DORMANCY IN WEED SEED BANKS IN SOIL

Dormancy is a common attribute of many weed seed populations and this usually hampers the task of predicting timing and extent of emergence of weeds. Both the number of established plants and the timing of emergence of a weed are strongly related to the dynamics of dormancy release of the seed population. In this paper, we discuss the different factors that affect dormancy in weed seed banks in soil, aiming to set a conceptual basis that will facilitate the construction of predictive models. From the long list of factors that are known to control dormancy under field conditions, we distinguish those that modify the dormancy level of the population (i.e. soil temperature and soil hydric conditions) from those that terminate dormancy or in other words, remove the ultimate constraints for seed germination once the degree of dormancy is sufficiently low (i.e. light, fluctuating temperatures, nitrate concentration). We also discuss the effect of agricultural practices on dormancy of weed seed populations, making reference to studies that have evinced clearly the factor(s) involved in determining a particular pattern of response. Overall, we stress the importance of clarifying, both qualitatively and quantitatively, the interaction between soil thermal and hydric conditions in the modification of the degree of dormancy of seed populations. Similarly, it is essential that we understand the extent to which such changes in dormancy comprise changes in sensitivity to factors that terminate dormancy. # 2000 Elsevier Science B.V. All rights reserved.

A quantitative analysis of dormancy loss dynamics in Polygonum aviculare L. seeds: Development of a thermal time model based on changes in seed population thermal parameters

A model for simulating Polygonum aviculare L. seed dormancy loss in relation to stratification temperature was developed. The model employs the lower limit temperature for germination (T l ) as an index of seed population dormancy status. While population mean for T l (T l(50) ) and T l distribution within the population (s Tl ) 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 (T h ) are held constant. In order to relate changes in T l(50) and s Tl to variable time and temperature, a stratification thermal time index (S tt ) was developed, which consists of the accumulation of thermal time units under a threshold temperature for dormancy loss to occur. Therefore, T l(50) and s Tl varied in relation to the accumulation of S tt according 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.

Termination of hull-imposed dormancy in developing barley grains is correlated with changes in embryonic ABA levels and sensitivity

We studied changes in dormancy (as imposed by the different structures surrounding the embryo, namely, endosperm, pericarp and glumellae) and its relationship with changes in embryonic ABA levels and sensitivity, in developing grains of two commercial barley cultivars: B 1215 and Quilmes Palomar, which have, respectively, a low and high dormancy level at harvest and, consequently, a contrasting sprouting behaviour in rainy years. Dormancy imposed by endosperm plus pericarp was gradually and similarly alleviated throughout development in both cultivars. The presence of the hull (glumellae), in contrast, completely inhibited germination of grains from both cultivars until physiological maturity (PM). From there on, hull-imposed dormancy was removed abruptly in B 1215 grains, while in Q. Palomar ones, it was removed at a much lower rate. This difference determined the contrasting sprouting behaviour of these two cultivars within the ‘time window’ going from PM to crop harvest. Embryonic ABA content and sensitivity were similar in the two cultivars throughout development until PM. From there on, ABA content and sensitivity in B 1215 embryos declined dramatically coinciding with the abrupt termination of hull-imposed dormancy observed in this cultivar. In contrast, ABA levels in Q. Palomar embryos remained high for longer and sensitivity to ABA declined at a much slower rate. This correlation suggests that hull-imposed dormancy in barley might be regulated by embryonic ABA levels and / or sensitivity. Inhibition of GA synthesis with paclobutrazol applied after anthesis lowered the germination capacity of grains from both cultivars without altering that of the naked caryopses, thus further suggesting that hull-imposed dormancy is under hormonal control.

A quantitative analysis of seed responses to cycle-doses of fluctuating temperatures in relation to dormancy: Development of a thermal time model for Polygonum aviculare L. seeds

The sensitivity of Polygonum aviculare L. 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 (S tt ) scale by accumulating thermal time units under a threshold temperature for dormancy loss to occur. Results showed that those seeds that had accumulated similar S tt units 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 similar S tt , obtaining a family of germination response curves in relation to S tt accumulation during storage. Based on these results, a simulation model was constructed relating dynamic changes in the parameters that determine germination response curves in relation to S tt accumulation. 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.

The role of fluctuations in soil water content on the regulation of dormancy changes in buried seeds of Polygonum aviculare L.

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 of Polygonum aviculare L. stored at dormancy-releasing temperatures. Effects of fluctuations in soil water content on the dormancy status of P. aviculare seeds 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 germinate in 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.

Advances in weed management strategies

Abstract Weed problems are currently solved by using inputs such as herbicides and soil tilling and by improving our knowledge of the ecophysiological process governing weed infestations and their competitive outcome on crop yield. In this paper, we review three different approaches for studying weed population dynamics (demographic, mechanistic and long-term studies) and discuss some of the advances in weed management strategies deriving from them. We present encouraging examples of successful strategies designed from population studies in annual and perennial weeds, as well as some ideas of how we should expand our knowledge base to achieve sustainable weed management strategies.

Predicting changes in dormancy level in natural seed soil banks

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.

A predictive model for dormancy loss in Polygonum aviculare L. seeds based on changes in population hydrotime parameters

Changes in population hydrotime parameters were determined during stratification in Polygonum aviculare L. 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 (Ψ b (50)) decreased concomitantly with seed dormancy, while the hydrotime constant (θ H ) and the standard deviation of base water potential (θ Ψb ) displayed only minor changes. Based on these results, a model for simulating P. aviculare seed dormancy loss in relation to low temperature was developed. The model employs Ψ b (50) as an index of mean seed population dormancy status. While Ψ b (50) was allowed to vary as seeds were released from dormancy, θ H and θ Ψb were held constant. Changes in Ψ b (50) were related to the time and temperature, using a previously developed thermal stratification time index ( S tt ), which quantifies the accumulation of thermal time units below a threshold temperature required for dormancy loss to occur. Therefore, Ψ b (50) varied in relation to the accumulation of S tt according 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.

Cloning and expression of a sorghum gene with homology to maize vp1. Its potential involvement in pre-harvest sprouting resistance.

Pre-harvest sprouting (PHS) in sorghum is related to the lack of a normal dormancy level during seed development and maturation. Based on previous evidence that seed dormancy in maize is controlled by the vp1 gene, we used a PCR-based approach to isolate two Sorghum bicolor genomic and cDNA clones from two genotypes exhibiting different PHS behaviour and sensitivity to abscisic acid (ABA). The two 699 amino acid predicted protein sequences differ in two residues at positions 341 (Gly or Cys within the repression domain) and 448 (Pro or Ser) and show over 80, 70 and 60% homology to maize, rice and oat VP1 proteins respectively.Expression analysis of the sorghum vp1 gene in the two lines shows a slightly higher level of vp1 mRNA in the embryos susceptible to PHS than in those resistant to PHS during embryogenesis. However, timing of expression was different between these genotypes during this developmental process. Whereas for the former the main peak of expression was observed at 20 days after pollination (DAP), the peak in the latter was found at later developmental stages when seed maturation was almost complete.Under favourable germination conditions and in the presence of fluridone (an inhibitor of ABA biosynthesis), sorghum vp1 mRNA showed to be consistently correlated with sensitivity to ABA but not with ABA content and dormancy.

Dormancy in cereals (not too much, not so little): about the mechanisms behind this trait

As in other cultivated species, dormancy can be seen as a problem in cereal production, either due to its short duration or to its long persistence. Indeed, cereal crops lacking enough dormancy at harvest can be exposed to pre-harvest sprouting damage, while a long-lasting dormancy can interfere with processes that rely on rapid germination, such as malting or the emergence of a uniform crop. Because the ancestors of cereal species evolved under very diverse environments worldwide, different mechanisms have arisen as a way of sensing an appropriate germination environment (a crucial factor for winter or summer annuals such as cereals). In addition, different species (and even different varieties within the same species) display diverse grain morphology, allowing some structures to impose dormancy in some cereals but not in others. As in seeds from many other species, the antagonism between the plant hormones abscisic acid and gibberellins is instrumental in cereal grains for the inception, expression, release and re-induction of dormancy. However, the way in which this antagonism operates is different for the various species and involves different molecular steps as regulatory sites. Environmental signals (i.e. temperature, light quality and quantity, oxygen levels) can modulate this hormonal control of dormancy differently, depending on the species. The practical implications of knowledge accumulated in this field are discussed.