Base Temperature For Germination Research Articles

Dormancy-break and germination of buried milk thistle (Silybum marianum (L.) Gaertn.) seeds

Milk thistle (Silybum marianum (L.) Gaertn.) is a medicinal/oil crop plant that produces dormant seeds at maturity. Seeds are released before harvest into the soil seed bank and hence can cause weed control problems in agricultural ecosystems. So far, information on dynamic changes of dormancy state, viability and persistence of milk thistle seeds in the soil seed bank is lacking. Accordingly, in this study the effects of ex-situ (dry in laboratory) and in-situ (buried in soil) storage conditions on dormancy-break and germination were investigated. Freshly harvested (FH) seeds were tested for germination at constant temperature ranging from 5 to 35 °C under both light/dark and dark conditions with or without GA3. Also, FH seeds were buried in June 2019 at depths of 5, 10, and 20 cm in a field and exhumed after 2–12 months of burial and assessed for germination. FH seeds did not germinate at any test conditions, but GA3 and after-ripening in dry storage increased germination, indicating that seeds have non-deep physiological dormancy (PD). With increased dry after-ripening, the ceiling temperature for germination (30 °C) did not change but the base temperature for germination decreased to 5 °C, i.e, seeds exhibited type 2 non-deep PD. Increased seed burial depth increased the length of the in situ after ripening period necessary for complete dormancy alleviation. Furthermore, persistence of seeds in the soil was affected by burial depth. Seeds buried at 5 cm had a transient seed bank and those at 10 and 20 cm a persistent seed bank.

Limiting temperatures of embryo growth and seed germination of Fraxinus excelsior L. (Oleaceae): a threshold model approach

Key messageWe have examined thermal control for embryo growth and seed germination in European ash. Using historical data (span of 36 years) of embryo index in matured seeds, we showed that maturation is not influenced by the annual average temperature. Secondary dormancy in ash’s seeds is easily induced, and the thermal window for both embryo growth and seed germination was between 10 and 20 °C.ContextSeed germination plays a crucial role in establishing plant communities. Ash seeds are characterized by deep morphophysiological dormancy, usually overcome by 32 weeks of stratification. Observed climate changes might disrupt ash’s thermal window for embryo growth and seed germination. Thus, proper examination of seed adaptability can be important for forest management.AimsWe investigated the effect of temperature on the embryo index (ratio between embryo and seed length) as well as embryo growth and germination of European ash (Fraxinus excelsior L.) seeds under different thermal conditions.MethodsWe used seeds collected at the same site in Poland between 1985 and 2021 to first analyze the influence of the growing season on the initial embryo index in mature seeds. Subsequently, we examined embryo growth and germination of seeds at constant temperatures of 3°, 10°, 15°, 20°, 25°, and 30 °C from seeds collected in 2021. Using the modeled rate of embryo growth and germination, we predicted the cardinal temperatures for those processes in ash seeds.ResultsThe embryo index of ash seeds varied; however, there was no relationship between the index and year of seed collection. The base temperature for embryo growth was 1.4 ± 1.1 °C, with a ceiling temperature of 29.9 ± 2.0 °C and an optimal temperature of 15.7 ± 0.3 °C. The base temperature for germination was at − 1.8 ± 0.3 °C. Due to the induction of secondary dormancy, the ceiling temperature of germination was impossible to establish.ConclusionsThe initial embryo index of mature ash seeds seems to be an individual trait and was not affected by local average annual temperatures. The most effective temperature ranges for embryo growth and germination were similar, however narrow (between 10 and 15 °C). Temperatures above 20 °C inducted secondary dormancy, hampering embryo growth as well as germination. Such a narrow thermal window for embryo growth can potentially act as a limiting factor for the natural regeneration of ash seeds.

Seed dormancy and germination characteristics of Osteomeles schwerinae, a riparian plant endemic to the Hengduan Mountains and adjacent regions

Osteomeles schwerinae, an endemic plant with ecological restoration functions from the hot/warm-dry river valleys of the Hengduan Mountains and adjacent regions, has low seed germination in cultivation. This study attempted to improve seed germination by testing the effect of several treatments including dry after-ripening (DAR), GA3, light and temperature treatments for seeds from three populations. DAR increased germination at 25/15°C but not at 15/5°C, as compared with fresh seeds. DAR was sufficient to break dormancy; GA3 could not significantly increase germination percentage, indicating that seeds of O. schwerinae appear to have non-deep physiological dormancy. After the dormancy is broken, temperature significantly affects germination. Seeds germinated significantly better at higher temperatures (20-25°C) than at lower (< 15°C) and extreme high temperatures (> 30°C). A base temperature for germination (Tb) of 4.9-9.3°C and a thermal time requirement for 50% germination (50) ranging from 158.7 to 217.4°Cd were identified for non-dormant DAR seeds, depending on the population. This study provides a theoretical basis for dormancy-break and germination of O. schwerinae seeds.

Evaluating process-based sugarcane models for simulating genotypic and environmental effects observed in an international dataset

Crop modelling has the potential to assist plant breeding by identifying favourable genotypic (G) traits for specific environments (Es). Sugarcane crop models have not been rigorously evaluated against a factorial GxE dataset. It is imperative that models are evaluated in this way before they are applied to plant breeding problems. Our objectives were to (1) calibrate, (2) assess, and (3) identify weaknesses and recommend improvements to, three sugarcane models, DSSAT-Canegro, Mosicas and APSIM-Sugar, in relation to their predictions of observed E, G and GxE interaction effects in response to abiotic factors (temperature and solar radiation). Data from an international GxE growth analysis trial were used; these consisted of five irrigated experiments at four sites (Belle Glade, Florida, USA; Chiredzi, Zimbabwe; La Mare, Reunion Island; and Pongola, South Africa), with cultivars N41, R570 and CP88-1762. Observed G and E effects on final above-ground dry mass (ADM) yields were explained in terms of seasonal radiation interception (FIPARa) and seasonal average radiation use efficiency (RUEa). Calibration was undertaken where possible by translating phenotypic parameters derived from observations into model input trait parameter values representing genetic traits. E and G effects on FIPARa were generally simulated satisfactorily, while GxE interaction effects were poorly predicted due to inadequate responses to temperature. E, G and GxE effects on RUEa were poorly predicted by all models, although data shortcomings (arising from uncertainty regarding date of primary shoot emergence and impacts of lodging) prevented us from making strong conclusions in this regard. Models accurately predicted G differences in RUEa during mid-season biomass sampling periods where data confidence was greater. Although the models were able to predict final ADM yield per G and per E reasonably well, none of the models predicted GxE interaction effects well. All models also under-estimated the variation in RUEa and ADM. Recommendations for experimental protocols for exploring RUEa are made. Our key recommendations for future work to improve models for sugarcane breeding applications are to explore G-specific thermal time base temperatures for germination and canopy development processes, and to improve linkages between carbon availability and canopy development.

Germination characteristics of Cakile edentula (Brassicaceae) seeds from two different climate zones

Different species have different germination strategies to cope with various climates they encounter when spreading, in which temperature and water are two of the most important factors that affect seed germination. Such differences in their coping strategies, therefore, could determine the fate of a species after its invasion into different environments. Cakile edentula is an invasive species originating from North America that invaded Australian coastlines. However, it has gradually disappeared in most of those habitats and can only be found in a few places nowadays. To explore whether germination strategy has had any contribution to such a phenomenon, we collected seeds from two populations with different climate patterns, i.e., subtropical and temperate. Germination experiments were performed using temperature and water potential hierarchies to detect differences of germination characteristics between the two populations. Based on the experimental data, a hydrothermal time model was utilized to further analyze and predict the germination characteristics of seeds from two populations. Results showed that germination percentage of C. edentula was affected by both temperature and water. The temperature and water requirements for germination of C. edentula varied significantly between populations. The population from the sub-tropical area displayed higher base water potential and base temperature for germination. However, there were a large fraction of seeds remaining dormancy, leading to a relatively low germination performance. Overall, our study has shown that C. edentula populations from subtropical and temperate climate zones differed in seed germination behavior, indicating its adaptation after invasion. Such results suggested that there was a wider environmental range than expected for seed germination in C. edentula, potentially promoting its invasion.

Modeling germination of smallflower umbrella sedge (Cyperus difformisL.) seeds from rice fields in California across suboptimal temperatures

AbstractSmallflower umbrella sedge is a prolific C3weed commonly found in rice fields in 47 countries. The increasing infestation of herbicide-resistant smallflower umbrella sedge populations threatens rice production. Our objectives for this study were to characterize thermal requirements for germination of smallflower umbrella sedge seeds from rice fields in California and to parameterize a population thermal-time model for smallflower umbrella sedge germination. Because the use of modeling techniques is hampered by the lack of thermal-time model parameters for smallflower umbrella sedge seed germination, trials were carried out by placing field-collected seeds in a thermogradient table set at constant temperatures of 11.7 to 41.7 C. Germination was assessed daily for 30 d, and the whole experiment was repeated a month later. Using probit regression analysis, thermal time to median germination [θT(50)], base temperature for germination (Tb), and SD of thermal times for germination [σθT(50)] were estimated from germination data, and model parameters were derived using the Solver tool in Microsoft Excel®. Germination rates increased linearly below the estimated optimum temperatures of 33.5 to 36 C. EstimatedTbaveraged 16.7 C, whereasθT(50)equaled 17.1 degree-days and σθT(50)was only 0.1 degree-day. The estimatedTbfor smallflower umbrella sedge is remarkably higher than that ofjaponicaandindicatypes of rice, as well asTbof important weeds in theEchinochloacomplex. Relative to the latter, smallflower umbrella sedge has lower thermal-time requirements to germination and greater germination synchronicity. However, it would also initiate germination much later because of its higherTb, given low soil temperatures early in the rice growing season in California. When integrated into weed growth models, these results might help optimize the timing and efficacy of smallflower umbrella sedge control measures.

Germination and emergence of Neslia paniculata (L.) Desv.

Seed characteristics, either in size or in structure, can affect germination, hence the emergence of a plant species. In the present work germination and emergence of Neslia paniculata, a Brassicaceae species that produces fruits containing one seed covered by a hard fruit coat, was studied. Germination experiments of seed with and without the fruit coat were performed at constant temperatures from 5 to 23 °C under controlled conditions and germination was measured daily. Additionally, seeds were sown in the field and emergence was periodically followed in three sites: Almenar (Spain), Morris (USA) and Riga (Latvia). Both germination and emergence were modelled with Boltzmann and Weibull mathematical functions. Neslia paniculata is a winter species adapted to low temperatures, with low base temperatures for germination (0.79 °C) and emergence (-3.6 °C). Seeds present both physiological and physical dormancy, the latter provoked by a thick fruit coat that prevents the seed from germinating unless it is degraded. In contrast to other Brassicaceae species studied (Camelina microcarpa; Thlaspi arvense), N. paniculata emergence lacked a response to light. This might have been caused by its fruit coat rendering the embryo non-sensitive to light. For this same reason the best emergence models were developed using a hydrothermal time basis. The relatively larger seed size of N. paniculata species explains the need for considering moisture in the soil at deeper layers (8–10 cm) compared to other Brassicaceae (4–6 cm in C. microcarpa and 4–8 cm in T. arvense), as their radicles can elongate deeper before emergence occurs. The factors to be considered for germination and emergence models are seed dormancy characteristics, either physical, physiological or both, seed size, which allow seedlings to reach deeper moist soil layers prior to emergence, and their seed position in the soil, which determines the temperature for germination. This research deepens in the knowledge of the germination and emergence biology of N. paniculata, with new models for parametrizing these stages, which can help in either germination laboratory studies and field emergence predictions, either for control or conservation purposes.

Morphological and functional seed traits of the wild medicinal plant Dioscorea strydomiana, the most threatened yam in the world.

Morphological and functional seed traits have important roles in characterising the species regeneration niche and help to understand the reproductive biology of rare and threatened plants, which can thus support appropriate plant conservation measures. Seed morphometric and dispersal kinetics of the critically endangered Dioscorea strydomiana were measured and compared with those of four other Dioscorea species, and seed germination response under constant temperatures (5-35°C) was compared with that of the congeneric and widespread D.sylvatica. Seed mass of D.strydomiana (ca. 14mg) was twice that of D.sylvatica, but similar to or smaller than the other species examined. Seeds of D.strydomiana have the lowest speed of descent and lowest variability in most of the morphological traits considered, suggesting lower phenotypic plasticity but higher variance in the wing-loading value. Seeds of D.strydomiana reached maximum germination at 15°C (ca. 47%), which decreased slightly to ca. 37% at 25°C and was completely inhibited at 35°C. D.sylvatica seeds started to germinate at 10°C (ca. 3%), reached 75-80% germination at 15-20°C and maximum (ca. 90%) at 25-30°C. Base temperatures for germination (Tb ) were 9.3 and 5.7°C, for D.strydomiana and D.sylvatica, respectively. Due to the higher germination percentages of D.sylvatica, ceiling and optimum temperatures could also be modelled for this species, suggesting higher sensitivity to high temperature for seeds of D.strydomiana. The detected poor seed lot quality of D.strydomiana suggests difficulties in reproduction from seed, highlighting the need for further investigation and conservation actions for this threatened yam species.

Yellow bristle grass (Setaria pumila) germination biology

The presence of Setaria pumila in dairy pastures lowers feed quality and may cause grazing avoidance. More information is required about the germination biology of this weed to improve the effectiveness of control strategies. Base temperature and germination profile were determined on a thermo-gradient table. For emergence depth, seeds were buried at 0—50 mm and time to emerge recorded. Seed persistence was tracked at seven field sites over 5 years and in a laboratory-based controlled-ageing test. Base temperature for germination was 10.2˚C, germination was most rapid at 30—35˚C and most numerous from 20—30˚C. Emergence occurred from 2—50 mm depth but mostly from 2—15 mm. The controlled-ageing test indicated the seed was viable for <3 years. However, some Setaria pumila seed continued to germinate after 5 years of burial in all but the clay soil. Farmers should try to prevent this species from seeding for at least 5 years to reduce the seed bank. Soil inversion could promote dormancy and seed persistence. If followed by effective control, cultivation could stimulate germination and further reduce the seed bank.

Seed germination niche of the halophyte Suaeda maritima to combined salinity and temperature is characterised by a halothermal time model

Sensitivity to salinity during seed germination potentially serves as a niche specifier for halophytes. Quantification of germination niche with respect to salinity, and any interaction with temperature, is therefore important to understand, particularly regarding the impacts of sea level rise in response to climate change. Population-based germination models quantify the response to temperature and water potential. However, there is limited applicability to salinity. Here, we develop a new approach considering thermal time (for temperature), halo time (for salinity) and halothermal time (for combined temperature and salinity), to quantify the germination of Suaeda maritima, a model halophyte with a worldwide distribution. We find salt concentration has a profound effect on the estimated base temperature for germination, Tb, being c. −4 °C in 600 mM NaCl compared with c. 1 °C in water. The predicted maximum concentration of NaCl for 50% germination to occur (NaClmax50) is 1280 mM at 5 °C and decreases (lower concentration) as the temperature increases. Overall, germination becomes slower as the concentration of salt increases (i.e., longer halo time) or the temperature decreases (i.e., longer thermal time). Combining all parameters into a halothermal time model is most successful at 5 and 10 °C. However, Na+ accumulation in the seed which coincides with maintained seed moisture content, regardless of salt concentration, influences the model fit. The models reveal two different germination niches: at cold temperatures, germination is slow but temperature and salinity are not necessarily limiting; and at warmer temperatures, germination is fastest but constrained by NaClmax50.

Assessing seed priming, sowing date, and mulch film to improve the germination and survival of direct-sown Miscanthus sinensis in the United Kingdom.

Direct sowing of Miscanthus seed could lower crop establishment costs, and increase the rate of grower uptake and biomass supply for the emerging bio‐economy. A replicated field trial was conducted at two contrasting UK sites: Aberystwyth (ABR) in mid‐Wales and Blankney (BLK) in Lincolnshire. These sites encompass the west–east meteorological gradient in the United Kingdom where the growing season at ABR is cooler and wetter while BLK is warmer and drier. Primed and unprimed Miscanthus sinensis seeds were sown directly onto the soil surface with and without a clear biodegradable mulch film, at nine dates interspersed from May to October. Average daily mean soil surface temperatures measured over the first 2 months after sowing under the mulch film were higher than control plots (2.7°C ABR and 4.2°C BLK). At both sites, the film covering also affected soil volumetric moisture relative to uncovered control plots (−3% ABR and 8% BLK), demonstrating the negative impact of mulch film when sowing on dry soil. Over nine sowings, seed germination at ABR under film varied between −28% and +18% of germination under control conditions. Seedlings from the first three sowings at both sites under film had sufficient physiological maturity to survive the first winter period. At BLK, mulch film significantly increased tiller count and height in both the first and second years after sowing. At ABR, where temperatures were lower, film covering significantly increased tiller height but not count. Water priming had no significant effect on seed viability or germination in the field tests. Base temperatures for germination of primed and unprimed seeds on a thermal gradient plate were 7.0°C and 5.7°C, respectively, with a ± 1.7°C confidence interval. Based on our results for M. sinensis in the United Kingdom, we recommend the sowing of unprimed seed in May under film and only when the soil is moist.

Seed Germination and Seedling Recruitment Behavior of Winged Sea Lavender (Limonium lobatum) in Southern Australia

AbstractWinged sea lavender [Limonium lobatum(L.f. Chaz)] is emerging as a significant weed of field crops in southern Australia. Several environmental factors affecting germination and seedling recruitment were examined to provide a better understanding of the behavior of its seedbank. At maturity, weed seeds were dormant for a period of around 2 mo, but dormancy was easily broken with scarification or by pretreatment with 564 mM NaOCL for 30 min, which confirms the role of the seed coat in regulating seed germination. Exposure to light significantly increased germination. Seeds were able to germinate over a broad range of temperatures (5 to 30 C), with maximum germination (~92%) at temperatures between 10 and 30 C. At 20 to 25 C, 50% germination was reached within 1.3 to 2 d, and the predicted base temperature for germination of the two populations ranged from 1.4 to 3.9 C. The NaCl concentration required to inhibit germination by 50% was 230 mM, with some seeds capable of germination at salinity levels as high as 480 mM. These results indicated greater tolerance to salinity inL. lobatumthan many other Australian agricultural weed species previously investigated. Seedling emergence was the highest (51% to 57%) for seeds present on the soil surface and was significantly reduced by burial at 1 cm (≤11%) and 2 cm (≤2%), with no emergence at 5 cm. Under field conditions, seedling recruitment varied considerably among the three experimental sites. The level of seedling recruitment was negatively associated with rainfall received at the site, organic carbon (OC) level, and microbial biomass of the soil. Rapid decay of weed seeds in high-OC soils appears to be an important determinant of seedling recruitment in this species and could explain greater occurrence ofL. lobatumon soils with low OC and low microbial activity in low-rainfall areas of southern Australia. Furthermore, many such soils in southern Australia are affected by salinity, which would enableL. lobatumto be more competitive with crops and other weeds present at a site.

Understanding biological and ecological factors affecting seed germination of the multipurpose tree Anogeissus leiocarpa.

Anogeissus leiocarpa (DC.) Guill. & Perr. (Combretaceae) has important economic and cultural value in West Africa as source of wood, dye and medicine. Although this tree is in high demand by local communities, its planting remains limited due to its very low propagation via seed. In this study, X-rays were used to select filled fruits in order to characterise their morphology and seed germination responses to treatment with sulphuric acid and different incubation temperatures. Morphological observations highlighted a straight orthotropous seed structure. The increase in mass detected for both intact and scarified fruits through imbibition tests, as well as morphological observations of fruits soaked in methylene blue solution, confirmed that they are water-permeable, although acid-scarified fruits reached significantly higher mass increment values than intact ones. Acid scarification (10min soaking in 98% H2 SO4 ) positively affected seed germination rate but not final germination proportions. When intact fruits where incubated at a range of temperatures, no seeds germinated at 10°C, while maximum seed germination (ca. 80%) was reached at 20°C. T50 values ranged from a minimum of ca. 12days at 25°C to a maximum of ca. 34days at 15 and 35°C. A theoretical base temperature for germination (Tb ) of ca. 10°C and a thermal requirement for 50% germination (S) of ca. 195°Cd were also identified for intact fruits. The results of this study revealed the seed germination characteristics driven by fruit and seed morphology of this species, which will help in its wider propagation in plantations.

Base temperatures for germination of selected weed species in Iran

Weed emergence models require the estimation of base temperature for germination (T<sub>b</sub>) that was estimated for Abutilon theophrasti, Echinochloa crus-galli, Amaranthus retroflexus, Sorghum halepense, Amaranthus albus, and Amaranthus hybridus in Iran, to calibrate an existing model called AlertInf. Two statistical procedures were adopted: Model 1 ‒ linear regression of germination rate and Model 2 – probit analysis. Model 1 provided lower T<sub>b</sub> values. Abutilon theophrasti and A. hybridus presented lower T<sub>b</sub> values (about 4 and 8°C), while the remaining species had values above 10°C. Since the estimated values of T<sub>b</sub> were in agreement with those adopted for the Alertinf model in Italy, the first step was achieved to adapt AlertInf to Iranian conditions.

Hydropriming accelerates seed germination of Medicago sativa under stressful conditions: A thermal and hydrotime model approach

This study determined the effects of priming on germination in response to temperature, water potential and NaCl. Thermal and hydrotime models were utilized to evaluate changes in parameters of the model after priming. Priming reduced the amount of thermal time in both cultivars, but slightly increased the base temperature for germination from 1.0 to 3.5°C in “Longdong”. Priming significantly increased germination rate at high water potential but had no effect at low water potential. Further, priming reduced the hydrotime constant but made the median base water potential value slightly more positive in both cultivars. Thus, priming increased germination rate in water but decrease it under severe water stress. Germination rate was significantly increased in both cultivars under salinity (NaCl) stress. Moreover, priming improved seedling growth in response to temperature, water and salinity stress in both cultivars.

Plant Development and Seed Biology of Windmillgrass (Chloris truncata) in Southern Australia

Windmillgrass is a major weed in agricultural systems in northern Australia, and it has now become more common in southern Australia. Because little information is available on the biology of this weed species in southern Australia, studies were conducted to investigate plant development and seed biology. Under irrigated field conditions in South Australia, windmillgrass required 748 to 786 growing degree days from emergence to mature seed production. Freshly harvested seed had low dormancy with 16% to 40% germination. Seeds required light exposure for germination and less than 2% germination was observed in complete darkness. Seed could germinate over a wide temperature range (10 to 40 C) with maximum germination at 20 to 25 C. At 25 to 30 C, 50% germination occurred within 27.3 to 45.5 h, and the predicted base temperature for germination of the two populations investigated ranged from 9.2 to 11.2 C. The sodium chloride concentration and osmotic potential required to inhibit germination by 50% were 51 to 73 mM and −0.27 MPa, respectively. Seedling emergence was completely inhibited by burial of seed, which is consistent with its absolute requirement for light exposure to begin germination. Under field conditions, there was no clear effect of burial depth on seed viability in the first 2 yr with average rainfall, and seeds were completely nonviable after 12 mo. However, in the third year, with low spring–summer rainfall, buried seeds (37% viability after 14 mo) persisted longer than those left on the soil surface (6% viability after 14 mo). This study provides important information on plant development and seed biology of windmillgrass that will contribute to the development of a management program for this weed species in southern Australia.

Temperature but not moisture response of germination shows phylogenetic constraints while both interact with seed mass and lifespan

AbstractUnderstanding how plant traits interact with climate to determine plant niches is decisive for predicting climate change impacts. While lifespan and seed size modify the importance of germination timing, germination traits such as base temperature and base water potential directly translate climatic conditions into germination timing, impacting performance in later life stages. Yet we do not know how base temperature, base water potential, seed mass, lifespan and climate are related. We tested the relationships between base temperature and base water potential for germination, seed size and lifespan while controlling for bioclimatic regions. We also quantified the phylogenetic signal in germination traits and seed size using Pagel's λ. We used a worldwide data set of germination responses to temperature and moisture, seed size and lifespan of 240 seed plants from 49 families. Both germination temperature and moisture are negatively related to seed size. Annual plants show a negative relation between seed size and base water potential, whereas perennials display a negative relation between base temperature and seed mass. Pagel's λ highlighted the slow evolution of base temperature for germination, comparable to seed mass while base water potential was revealed to be labile. In the future, base water potential and seed mass can be used when moisture niches of plants are to be predicted. Lifespan, seed size and base temperature should be taken into account when analysing thermal limits of species distributions.

Thermal requirements for seed germination of underutilized Lippia species

Seed germination requirements of three underutilized Lippia (Verbenaceae) species were investigated. Seeds of L. graveolens, L. javanica and L. multiflora were incubated at a range of constant temperatures (5–35°C) in the light (12h light/12h darkness). The effect of GA3 and KNO3 on seed germination was also tested in the same range of germination temperatures. Germination responses to accumulated temperature were then characterized by a thermal time approach. Two different trends of germination response to temperature were detected among species, regardless of the applied treatment. Seeds of L. graveolens and L. javanica started to germinate at 15°C, reached their maximum germination at 25°C and then decreased at warmer temperatures, while those of L. multiflora reached their maximum at the highest tested temperature (35°C). Base temperature for germination (Tb, °C) and the thermal constant for 50% germination (S, °Cd) of untreated seeds were ca. 2.5 and 7.5°C and 76.92 and 84.3°Cd, for L. graveolens and L. javanica, respectively. A Tb of ca. 20°C could only be roughly estimated for L. multiflora, due to the low germination achieved at temperatures <35°C. GA3 and KNO3 had an overall inefficacy on enhancing seed germination of these species. Thermal thresholds of treated seeds could also be estimated for L. javanica, with a Tb of ca. 11°C for both treatments and a S of 70.92 and 69.44°Cd for GA3 and KNO3, respectively. Practical applications of the findings of this study may support the propagation of these underutilized Lippia species.

Effect of temperature and cold stratification on seed germination of the Mediterranean wild aromatic Clinopodium sandalioticum (Lamiaceae)

A germination study was carried out on seeds of Clinopodium sandalioticum (Bacch. & Brullo) Bacch. & Brullo ex Peruzzi & Conti (Lamiaceae), a wild aromatic plant endemic to Sardinia. Seeds were incubated at a range of constant (5–25°C) and an alternating temperatures regime (25/10°C), with 12 hours of irradiance per day. The results achieved at 10°C were also compared with those obtained after a period of cold stratification at 5°C for three months. Final seed germination ranged from ca. 28% (5°C) to ca. 72% (25/10°C). A base temperature for germination (Tb) of ca. 5°C and a thermal constant for 50% germination (S) of 89.3°Cd were identified and an optimal temperature for germination (To) was estimated to be comprised between 20 and 25°C. Cold stratification negatively affected seed viability and germination at 10°C. Although a typical “Mediterranean germination syndrome”, could not be detected for C. sandalioticum seeds, these results were coherent with those previously reported for other Mediterranean Lamiaceae species.

Estimation and Comparison of Base Temperatures for Germination of European Populations of Velvetleaf (Abutilon theophrasti) and Jimsonweed (Datura stramonium)

Weed emergence models require biological parameters such as base temperature for germination, determination of which is costly and time-consuming. Transferability of these parameters across different populations may therefore represent one of the main constraints for the development and practical use of emergence models at a large scale. A collaborative project was undertaken to assess the interpopulation variability of base temperature for germination in different European populations of velvetleaf and jimsonweed and evaluate possible applicative consequences in terms of weed control. Seeds were collected in Italy, Portugal, and Spain, and each population was then sown in every country, obtaining nine seed batches named as experimental lots. Base temperature for germination was estimated for each experimental lot to calculate lot-specific accumulation of growing degree days (GDD) under three dissimilar climatic scenarios. Threshold date (TD50) was calculated as the date when GDD accumulation of a given experimental lot surpassed the values corresponding to 50% of cumulative field emergence of seedlings. GDD accumulation and TD50were then used as indicators to identify differences among experimental lots within each climatic scenario. No significant differences were detected among base temperatures estimated for velvetleaf experimental lots or among their patterns of accumulation of GDD and TD50values within climatic scenarios. Each value of base temperature determined for a single experimental lot could therefore be adopted to model germination for all the lots regardless of the population of origin or cultivation site. In contrast, the population of origin affected the base temperature for jimsonweed, with significantly higher values for experimental lots of the Portuguese population. From an applicative perspective, differences among patterns of accumulation of GDD and TD50of several experimental lots within each climatic scenario suggest the need to use population-specific values as base temperature for germination and emergence modeling of jimsonweed.