Germination responses of a non-dormant seed population of Amaranthus patulus Bertol. to constant temperatures in the sub-optimal range

Abstract. The germination responses of a nondormant seed population of Amaranthus patulus Bertol, at constant sub-optimal temperatures in the range of 10–34°C were analysed through a detailed time-course study. Although a final germination percentage of nearly 100% was attained at temperatures above 18°C, it fell abruptly to zero with decreasing temperature from 17 to 10°C. The final germination percentage, v. temperature plotted on a normal probability scale yielded a straight line, indicating normality of the lower limit temperature within seed population with an estimated mean of 13.75°C and a standard deviation of 1.50°C. Simple linear relationships were obtained between the temperature and the germination rates, i.e., the reciprocals of the time taken to germinate by the subpopulations with 20–80% germination. The linear relationships were characterized by similar base temperatures or theoretical limit temperatures of about 11°C but there was a variation in the required 'thermal times' (θ), the distribution of which could be approximated for the seed population by the following distribution function: where m is the median of the distribution and A is a parameter characterizing the pattern of the distribution. When the germination rates were calculated after subtracting 10–14 h from the time actually consumed in germination, linear Arrhenius relationships were obtained. The apparent activation energy estimated from the linear regression of Arrhenius plot was approximately 100 kJ mol⁻¹ with all 20–80% subpopulations.     

Germination Responses of Taraxacum platycarpum Seeds to Temperature

Abstract Germination responses of Taraxacum platycarpum seeds to temperature were examined under laboratory conditions to investigate the emergence-season choice mechanism of the seeds. Almost all the newly collected seeds were non-dormant. Under constant temperature conditions, maximum percentage germination (approximately 90%) was attained at temperatures 6–16°C, where simple linear relationships were observed between the temperature and the rates of germination, i.e. the reciprocals of the time taken to germinate by seed subpopulations with 10–80% germination. Thermal time required for germination of the subpopulations ranged from 600 Kh (degree Kelvin × hours) to 1500 Kh with a relatively constant base temperature of about 2.5°C. Lower limit temperature for germination was slightly below 6°C. Higher limit temperature for germination has the normal distribution with the mean ±SD of 19±2.5°C. Pre-exposure of imbibed seeds to temperatures higher than the higher limit temperature for germination, 25 and 30°C, had no effect on the germinability and the rate of germination at a circa-optimum temperature. Moist chilling treatment at 4°C caused an increase in the variation of germination rate within the seed population, but no evidence for dormancy-inducing or breaking effects was obtained.     

Urbanization alters plastic responses in the common dandelion Taraxacum officinale

Urban environments expose species to contrasting selection pressures relative to rural areas due to altered microclimatic conditions, habitat fragmentation, and changes in species interactions. To improve our understanding on how urbanization impacts selection through biotic interactions, we assessed differences in plant defense and tolerance, dispersal, and flowering phenology of a common plant species (Taraxacum officinale) along an urbanization gradient and their reaction norms in response to a biotic stressor (i.e., herbivory). We raised plants from 45 lines collected along an urbanization gradient under common garden conditions and assessed the impact of herbivory on plant growth (i.e., aboveground biomass), dispersal capacity (i.e., seed morphology), and plant phenology (i.e., early seed production) by exposing half of our plants to two events of herbivory (i.e., grazing by locusts). Independent from their genetic background, all plants consistently increased their resistance to herbivores by which the second exposure to locusts resulted in lower levels of damage suffered. Herbivory had consistent effects on seed pappus length, with seeds showing a longer pappus (and, hence, increased dispersal capacities) regardless of urbanization level. Aboveground plant biomass was neither affected by urbanization nor herbivore presence. In contrast to consistent responses in plant defenses and pappus length, plant fitness did vary between lines. Urban lines had a reduced early seed production following herbivory while rural and suburban lines did not show any plastic response. Our results show that herbivory affects plant phenotypes but more importantly that differences in herbivory reaction norms exist between urban and rural populations.     

Responses of Sexual and Apomictic Genotypes of Taraxacum officinale to Variation in Light

Abstract: The mode of reproduction, sexual or asexual, will influence the way populations respond to selective pressures. This can cause genetic and ecological divergence between sexual and asexual forms of the same species. Here we examine differences in morphology and phenology between sexual and apomictic types of dandelion, Taraxacum officinale. Sexual and apomictic dandelions were collected from a mixed population on the banks of the river Rhine, The Netherlands. Clonal copies of both sexual and apomictic genotypes were planted in an experimental garden under two light levels. Sexual plants flowered four days later on average than apomicts, but the number of capitula was the same. Apomicts had longer leaves and were heavier than sexual plants, especially under shaded conditions. In apomicts plasticity for leaf length and height was larger than in sexuals, but for most other measured traits no differences in plasticity were observed. Trait values of apomicts were within the same range as those of sexual plants.     

Ecophysiological basis of the Jack-and-Master strategy: Taraxacum officinale (dandelion) as an example of a successful invader

Aims Successful invasive plants are often assumed to display significant levels of phenotypic plasticity. Three possible strategies by which phenotypic plasticity may allow invasive plant species to thrive in changing environments have been suggested: (i) via plasticity in morphological or physiological traits, invasive plants are able to maintain a higher fitness than native plants in a range of environments, including stressful or low-resource habitats: a 'Jack-of-all-trades' strategy; (ii) phenotypic plasticity allows the invader to better exploit resources available in low stress or favorable habitats, showing higher fitness than native ones: a 'Master-of-some' strategy and (iii) a combination of these abilities, the 'Jack-and-Master' strategy.Methods We evaluated these strategies in the successful invader Taraxacum officinale in a controlled experiment mimicking natural environmental gradients. We set up three environmental gradients consisting of factorial arrays of two levels of temperature/light, temperature/water and light/water, respectively. We compared several ecophysiological traits, as well as the reaction norm in fitness-related traits, in both T. officinale and the closely related native Hypochaeris thrincioides subjected to these environmental scenarios.Important findings Overall, T. officinale showed significantly greater accumulation of biomass and higher survival than the native H. thrincioides, with this difference being more pronounced toward both ends of each gradient. T. officinale also showed significantly higher plasticity than its native counterpart in several ecophysiological traits. Therefore, T. officinale exhibits a Jack-and-Master strategy as it is able to maintain higher biomass and survival in unfavorable conditions, as well as to increase fitness when conditions are favorable. We suggest that this strategy is partly based on ecophysiological responses to the environment, and that it may contribute to explaining the successful invasion of T. officinale across different habitats.     

A comparison of phenotypic plasticity in the native dandelion Taraxacum ceratophorum and its invasive congener T. officinale

We compared plastic responses to variation in the light environment for sympatric populations of native and exotic dandelion species, Taraxacum ceratophorum and Taraxacum officinale. Plasticity in leaf size, inflorescence height, reproductive phenology and dispersal-related traits were measured under experimentally altered light quality (red : far-red light ratio, R : FR) and light intensity (photosynthetically active radiation, PAR). To test whether differences in means and reaction norms of dispersal-related traits between species affected colonization potential, we created seed-dispersal models based on seed-fall rate and release height. Differences in plasticity between species were not systematic, but varied in direction and magnitude among traits. Taraxacum officinale produced larger leaves that exhibited greater plasticity in size under variable light intensity than T. ceratophorum. Plasticity in scape length at flowering occurred in relation to R : FR ratio in both species, but tended to be greater in T. ceratophorum. Seed-bearing scapes of T. officinale were taller and more canalized in height across light regimes than scapes of T. ceratophorum. Seeds of T. officinale were smaller than seeds of T. ceratophorum. Models predict greater dispersal in T. officinale within open and vegetated habitats. In contrast to the idea that plasticity promotes invasiveness, results suggest that the lack of plasticity in dispersal-related traits enhances the colonization potential of T. officinale.     

Thermal range predicts bird population resilience to extreme high temperatures

The identification of the characteristics of species that make them susceptible or resilient to climate change has been elusive because non-climatic influences may dominate short- and medium-term changes in population and distribution sizes. Here we studied the 2003 French heat wave, during which other confounding variables remained essentially unchanged, with a correlational approach. We tested the relationship between population resilience and thermal range by analysing the responses of 71 bird species to a 6-month heat wave. Species with small thermal ranges showed the sharpest decreases in population growth rate between 2003 and 2004 in locations with the highest temperature anomalies. Thermal range explained the resilience of birds to the heat wave independently of other potential predictors, although it correlated with nest location and broad habitat type used by species. The geographically deduced thermal range appears to be a reliable predictor of the resilience of these endothermic species to extreme temperatures.     

Germination responses of the Rubia fruticosa Ait. seed dispersal system in different experimental seasons

In this paper, we present results on germination patterns of the seed dispersal system of an endemic Macaronesian plant (Rubia fruticosa). Seeds from this plant are mainly dispersed by endemic lizards and native warblers; therefore, we included three different treatments: control seeds, seeds extracted from lizards and seeds found in warbler droppings. Seeds from the same pool of every treatment were germinated in two different seasons, one in autumn, coinciding with the arrival of the first rains, and another in spring, coinciding with the arrival of the dry season. A clear differential pattern of germination success was observed between the two seasons. Seeds planted in autumn achieved a higher percentage of germination than those sown in spring in all treatments. The great robustness of these results seems to indicate that germination timing is strongly selected in R. fruticosa and this evolutionary trend probably extends to other vascular plants growing in xeric coastal environments of the Macaronesian islands.     

The effect of parent plant nutrition on seed size, viability and vigour and on germination of wheat and triticale at different temperatures

Seed was obtained from two adjacent trials of wheat (cv. Avalon) and triticale (cv. Lasko) harvested in 1990. The treatments consisted of factorial combinations of five levels of spring nitrogen (50, 100, 150, 200 and 250 kg N ha^-1) with fungicide treatments designed to control foliar diseases. Seed was graded by size into different fractions. Application of foliar fungicides to the parent plant increased the proportion of large seed in wheat but not in triticale. Fungicide application had no effect on seed vigour (quantified as the value of K; after controlled deterioration), final germination, median germination time (t₅₀) or rate of germination. Parent plant nutrition altered the proportions of seed in different size categories in triticale and wheat. In laboratory germination tests, final germination of wheat increased with seed size. Large (> 3.0 mm diameter) wheat seed from plots receiving 50 kg N ha“¹ had a lower final germination than large seed from plots which had received greater amounts of nitrogen, but t₅₀ and K; were similar for all nitrogen levels. In triticale, the higher the seed weight, the higher the final germination, the faster the rate and the higher the Ki. Final germination, t₅₀ and K;, were all positively correlated with the amount of nitrogen received by the parent plants. Parent plant nutrition had no effect on the response to temperature of final germination, t₅₀, or rate of germination for large or small seeds of both wheat and triticale.     

Mathematical description of the seed germination dependency on time and temperature

Abstract. A mathematical model which describes the germination percentage dependency on time and temperature of a seed population was derived from the experimental results with a seed population of Amaranthus patulus Bertol. under sub-optimal temperature conditions (Washitani & Takenaka, 1984). The equation of the model which is a modified thermal time model is where G is germination percentage at a certain time after the start of imbibition ( t ) at a certain temperature ( T ), μ _{T 1} and σ _{T 1} are the mean and standard deviation of lower limit temperature among the seeds belonging to the seed population, and Tb, m , and A are the parameters characterizing the linear relationship between the rate and temperature, namely, Tb is the base temperature, m , the median of the required thermal time and A , a parameter determining the pattern of the variation of the required thermal time within seed population, respectively. The equation yields time courses for germination which are very similar to those observed by experiment.     

野豌豆属4种植物种子萌发的积温模型分析

以青藏高原野豌豆属窄叶野豌豆(Vicia angustifolia)、山野豌豆(V. amoena)、歪头菜(V. unijuga) 3种野生植物与一种当地栽培植物救荒野豌豆(箭筈豌豆) (V. sativa) ‘兰箭3号’种子为材料, 在5、10、15、20、25及30 ℃下进行萌发实验, 应用种子萌发的积温模型对上述4种植物萌发对温度的响应特征进行了比较分析。结果表明: 1)基于萌发速率(1/T_g)对种子萌发温度最低温T_b值的估计受萌发率(g)的影响较小; 与此不同, 除‘兰箭3号’种子外, 对萌发最高温T_c值的估计, 受到g的显著影响。 这表明种群内所有种子个体萌发的T_b值相对恒定, 但T_c值在有些物种中变异较大; 2)基于重复概率单位回归分析估计的种子萌发T_b值与基于萌发速率估计的值较为接近; 而由此方法估计的T_c值则与萌发率为50%时的估计值较为接近; 3)相比多年生豆科植物歪头菜和山野豌豆, 一年生豆科植物箭筈豌豆‘兰箭3号’与窄叶野豌豆具有相对较低的T_b与T_c值; 4)积温模型可准确地预测休眠破除后豆科植物种子在不同温度条件下的萌发进程。     

Seed germination temperatures of eight Mexican Agave species with economic importance

The genetic diversity of Agave plants is threatened by clonal commercial reproduction and climatic change. Sexual reproduction is uncommon and research on seed germination is scarce. The present study evaluated the seed germination of Agave lechuguilla, Agave striata, Agave americana var. marginata, Agave asperrima, Agave cupreata, Agave duranguesis, Agave angustifolia ssp. tequilana and Agave salmiana at constant temperatures (10, 15, 20, 25, 30, 35 and 40°C). Initial imbibition (after the first 12 h) was significantly variable among species, positively correlated with seed weight (r = 0.6560, P < 0.001) and increased with temperature (from 35% at 10°C to 66% at 40°C). Temperature affected maximum imbibition (83–150%) for A. asperrima, A. lechuguilla, A. salmiana and A. striata; other species averaged 110%. Most germination kinetics best fitted a logistic model, whereas only a few treatments fit a Weibull model. The time to germination onset diminished (P < 0.05) from 125–173 h at 15°C to 68–84 h at 25°C, and then ascended to 84–196 h at 35°C. The mean germination rate and seed germination percentage after 312 h peaked at 25°C (0.50–0.95% seeds/h and 85–99%, respectively) and fell (P < 0.05) to near zero at 10 and 40°C. Temperatures of 10, 35 and 40°C were partially lethal to A. asperrima, A. duranguensis and A. salmiana seeds. The time to germination onset, seed germination percentage after 312 h and mean germination rate are best described by a Gaussian distribution, with its optimum at approximately 25°C. Thus, optimum temperatures are related to the ecological characteristics of each species area.     

Germination behaviour of Conyza bonariensis to constant and alternating temperatures across different populations

Conyza bonariensis is one of the most problematic weed species throughout the world. It is considered highly noxious due to its interference with human activities, and especially the competition it poses with economically important crops. This research investigated the temperature requirements for seed germination of four populations of C. bonariensis with distinct origin and the influence of daily alternating temperatures. For this, a set of germination tests were performed in growth chambers to explore the effect of constant and alternating temperatures. Seeds of the four populations (from Lleida, Badajoz and Seville, Spain and Bahía Blanca, Argentina) were maintained at constant temperatures ranging from 5 to 35°C. The final germination and cardinal temperatures (base, optimum and maximum) of each population were obtained. We also tested the influence of daily alternating temperatures on final germination. To do so, seeds were exposed to two temperature regimes: 5/15, 10/20, 15/25, 20/30 and 25/35°C night/day temperature (intervals increasing 5°C, with constant oscillation of 10°C) and to 18/22, 16/24, 14/26, 12/28 and 10/30°C night/day temperature (intervals with average of 20°C, but increasing the oscillation in 4°C between intervals). In general, all populations behaved similarly, with the highest germination percentages occurring in the optimum temperature range (between 21.7°C and 22.3°C) for both constant and alternating temperatures. In general, climatic origin affected germination response, where seeds obtained from the coldest origin exhibited the highest germination percentage at the lowest temperature assayed. In addition, we observed that the alternating temperatures can positively affect total germination, especially in oscillations that were further from the average optimum temperature (20°C), with high germination percentage for the oscillations of 15/25, 20/30, 18/22, 16/24, 14/26, 12/28 and 10/30°C in all populations. The cardinal temperatures obtained were significantly different across the populations. These results provide information that will facilitate a better understanding of the behaviour of Conyza and improve current field emergence models.     

Germination responses to temperature and water potential in Jatropha curcas seeds: a hydrotime model explains the difference between dormancy expression and dormancy induction at different incubation temperatures

Background and Aims

Jatropha curcas is a drought-resistant tree whose seeds are a good source of oil that can be used for producing biodiesel. A successful crop establishment depends on a rapid and uniform germination of the seed. In this work we aimed to characterize the responses of J. curcas seeds to temperature and water availability, using thermal time and hydrotime analysis,

Methods

Thermal and hydrotime analysis was performed on germination data obtained from the incubation of seeds at different temperatures and at different water potentials.

Key Results

Base and optimum temperatures were 14·4 and 30 °C, respectively. Approximately 20 % of the seed population displayed absolute dormancy and part of it displayed relative dormancy which was progressively expressed in further fractions when incubation temperatures departed from 25 °C. The thermal time model, but not the hydrotime model, failed to describe adequately final germination percentages at temperatures other than 25 °C. The hydrotime constant, θ_H, was reduced when the incubation temperature was increased up to 30 °C, the base water potential for 50 % germination,Ψ_b(50), was less negative at 20 and 30 °C than at 25 °C, indicating either expression or induction of dormancy. At 20 °C this less negative Ψ_b(50) explained satisfactorily the germination curves obtained at all water potentials, while at 30 °C it had to be corrected towards even less negative values to match observed curves at water potentials below 0. Hence, Ψ_b(50) appeared to have been further displaced to less negative values as exposure to 30 °C was prolonged by osmoticum. These results suggest expression of dormancy at 20 °C and induction of secondary dormancy above 25 °C. This was confirmed by an experiment showing that inhibition of germination imposed by temperatures higher than 30 °C, but not that imposed at 20 °C, is a permanent effect.

Conclusions

This study revealed (a) the extremely narrow thermal range within which dormancy problems (either through expression or induction of dormancy) may not be encountered; and (b) the high sensitivity displayed by these seeds to water shortage. In addition, this work is the first one in which temperature effects on dormancy expression could be discriminated from those on dormancy induction using a hydrotime analysis.     

Germination of Prosopis juliflora (Sw.) D.C. seeds at different osmotic potentials and temperatures

The effects of osmolytes, osmotic potential (Ψ_s), temperature, and their interactions on the germinability, germination rate, and other germination parameters of the invasive shrub Prosopis juliflora, which grows in the semiarid environmental conditions of the Caatinga in northeast Brazil, were evaluated. To study the effects of polyethylene glycol (PEG) and NaCl stress and temperature on germination, two separate experiments were carried out at the Plant Ecophysiology Laboratory of the Federal University of Pernambuco in 2011. The overall germinability decreased significantly with increases in both PEG (one‐way anova , F_4,75 = 111.21, P ≤ 0.001) and NaCl (one‐way anova , F_4,75 = 12.82, P ≤ 0.001); however, the effects were more accentuated with PEG than NaCl. The PEG‐treated seeds maintained their germinability, even when they were subjected to a Ψ_s = ?1 MPa after being rinsed and allowed to germinate on deionized water. In contrast, NaCl‐treated seeds usually lost their ability to germinate; this fact was possibly linked to the accumulation of Na⁺ and Cl^? in the cells, which may have contributed to a loss of membrane function that led to the death of the embryos. Although numerous studies describing seed germination in the presence of osmolytes have been conducted, studies that show the interactions between osmolytes, osmotic potentials, and temperature are scarce. The present study is the first to describe these interactions for P. juliflora seeds.     

Effect of short‐duration high temperatures on weed seed germination

Thermal soil disinfestation techniques are effective reducers of weed seedbank and weed emergence. Two experiments (Expt 1 and Expt 2) were conducted to test the effect of brief exposure to varying temperatures on the seed germination of Amaranthus retroflexus, Echinochloa crus‐galli, Galinsoga quadriradiata, Portulaca oleracea, Setaria viridis and Solanum nigrum. To this end, species seeds were moistened with loamy‐sand soil and placed into test tubes. The tubes were heated rapidly and then cooled by dipping them into a hot water bath until target temperatures were achieved. Expt 1 temperatures ranged between 55°C and 85°C at 5°C intervals and Expt 2 ranged between 48°C and 86°C at 2°C intervals. Thereafter, the tubes were dipped into a cooling (1°C) water bath. Exposure to target temperatures ranged between 2 s and 5 s. Soil temperatures were monitored using embedded thermocouples. A log‐logistic dose–response model described the effect of heating on seed germinability; temperatures required for 99% reductions were calculated. On the basis of the predictive model equation used, weed species' germination sensitivity to high temperature exposure can be ranked as follows: E. crus‐galli (79.6°C), S. viridis (75.8°C), S. nigrum (74.6°C), P. oleracea (72.2°C), A. retroflexus (70.9°C) and G. quadriradiata (68.1°C). The interval between no effects to complete seed devitalisation occurred at temperatures varying from 6.5°C to 15.7°C. Seed size and weight varied directly with heat tolerance. Study results not only inform the timing and optimal adjustment for effective thermal soil treatment, but also demonstrate a relatively simple and generalizable methodology for use in other studies.