Thermoinhibition uncovers a role for strigolactones in Arabidopsis seed germination

Strigolactones are host factors that stimulate seed germination of parasitic plant species such as Striga and Orobanche. This hormone is also important in shoot branching architecture and photomorphogenic development. Strigolactone biosynthetic and signaling mutants in model systems, unlike parasitic plants, only show seed germination phenotypes under limited growth condition. To understand the roles of strigolactones in seed germination, it is necessary to develop a tractable experimental system using model plants such as Arabidopsis. Here, we report that thermoinhibition, which involves exposing seeds to high temperatures, uncovers a clear role for strigolactones in promoting Arabidopsis seed germination. Both strigolactone biosynthetic and signaling mutants showed increased sensitivity to seed thermoinhibition. The synthetic strigolactone GR24 rescued germination of thermoinbibited biosynthetic mutant seeds but not a signaling mutant. Hormone analysis revealed that strigolactones alleviate thermoinhibition by modulating levels of the two plant hormones, GA and ABA. We also showed that GR24 was able to counteract secondary dormancy in Arabidopsis ecotype Columbia (Col) and Cape Verde island (Cvi). Systematic hormone analysis of germinating Striga helmonthica seeds suggested a common mechanism between the parasitic and non-parasitic seeds with respect to how hormones regulate germination. Thus, our simple assay system using Arabidopsis thermoinhibition allows comparisons to determine similarities and differences between parasitic plants and model experimental systems for the use of strigolactones.     

Genetic variation for lettuce seed thermoinhibition is associated with temperature-sensitive expression of abscisic Acid, gibberellin, and ethylene biosynthesis, metabolism, and response genes

Lettuce (Lactuca sativa ‘Salinas’) seeds fail to germinate when imbibed at temperatures above 25°C to 30°C (termed thermoinhibition). However, seeds of an accession of Lactuca serriola (UC96US23) do not exhibit thermoinhibition up to 37°C in the light. Comparative genetics, physiology, and gene expression were analyzed in these genotypes to determine the mechanisms governing the regulation of seed germination by temperature. Germination of the two genotypes was differentially sensitive to abscisic acid (ABA) and gibberellin (GA) at elevated temperatures. Quantitative trait loci associated with these phenotypes colocated with a major quantitative trait locus (Htg6.1) from UC96US23 conferring germination thermotolerance. ABA contents were elevated in Salinas seeds that exhibited thermoinhibition, consistent with the ability of fluridone (an ABA biosynthesis inhibitor) to improve germination at high temperatures. Expression of many genes involved in ABA, GA, and ethylene biosynthesis, metabolism, and response was differentially affected by high temperature and light in the two genotypes. In general, ABA-related genes were more highly expressed when germination was inhibited, and GA- and ethylene-related genes were more highly expressed when germination was permitted. In particular, LsNCED4, a gene encoding an enzyme in the ABA biosynthetic pathway, was up-regulated by high temperature only in Salinas seeds and also colocated with Htg6.1. The temperature sensitivity of expression of LsNCED4 may determine the upper temperature limit for lettuce seed germination and may indirectly influence other regulatory pathways via interconnected effects of increased ABA biosynthesis.     

Nitrate,abscisic acid and gibberellin interactions on the thermoinhibition of lettuce seed germination

Germination of lettuce seeds has obvious thermoinhibition, but the mechanism for thermoinhibition of seed germination is poorly understood. Here, we investigated the interactions of nitrate, abscisic acid (ABA) and gibberellin on seed germination at high temperatures to understand further the mechanism for thermoinhibition of seed germination. Our results showed that lettuce (Lactuca sativa L. ‘Jianye Xianfeng No. 1’) seeds exhibited notable thermoinhibiton of germination at ≥17°C in darkness, and at ≥23°C in light, but the thermoinhibited seeds did not exhibit secondary dormancy. Thermoinhibition of seed germination at 23 or 25°C in light was notably decreased by 5 and 10 mM nitrate, and the stimulatory effects were markedly prevented by nitric oxide (NO) scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. The sensitivity of seed germination to exogenous ABA increased with increasing temperature. Thermoinhibition of seed germination was markedly decreased by fluridone (an inhibitor of ABA biosynthesis) and GA₃, and was increased by diniconazole (an inhibitor of the ABA-catabolizing enzyme ABA 8′-hydroxylase) and paclobutrazol (an inhibitor of GA biosynthetic pathway). The effect of fluridone in decreasing thermoinhibition of seed germination was obviously antagonized by paclobutrazol, and that of GA₃ was notably added to by fluridone, and that of nitrate was antagonized by paclobutrazol, diniconazole and ABA and was added to by GA₃ and fluridone. Our data show that thermoinhibition of lettuce seed germination is decreased by nitrate in a NO-dependent manner, which is antagonized by ABA, diniconazole and paclobutrazol and added by fluridone.     

Hormonal regulation of tomato seed germination at a supraoptimal temperature

Germination of tomato cv. New Yorker seed is inhibited at 35°C. This thermoinhibition was partially counteracted by application of GA₄₊₇ alone, the compound applied in combination with ACC or ethephon markedly enhancing the process. The latter compound alone was not able to induce germination at 35 °C. Thermoinhibition of seeds at 35 °C was also counteracted by fluridone, an inhibitor of ABA biosynthesis. At 25 °C, an optimal temperature, ABA inhibited germination of New Yorker seeds. Although another known growth inhibitor MeJA, when applied at an optimal temperature (25 °C), had also a slightly inhibitory effect on germination of those seeds and clearly delayed the process, inhibitors of its biosynthetic pathway (ibuprofen, indoprofen, antypiryne and salicylic acid) did not remove thermoinhibition at 35 °C. An increase in endo-β-mannanase activity after 24 hours of incubation at 35 °C was observed in the seeds incubated in the presence of gibberellins, ACC, ethephon, fluridone used alone and in combinations, but it was not clearly correlated with the effects of these compounds on alleviation of seed germination. However, fluridone present in the same incubation medium at 35 °C with ABA was able to counteract the inhibitory effect of ABA on endo-β-mannanase activity. The results of our study suggest that gibberellins, ethylene (produced from ACC or ethephon) and ABA, but not jasmonates, regulate tomato seed germination at supraoptimal temperatures. Alleviation of thermoinhibition of New Yorker seed germination by plant growth regulators and fluridone is partially associated with their controlling endo-β-mannanase activity.     

Abscisic acid in the thermoinhibition of lettuce seed germination and enhancement of its catabolism by gibberellin

Germination of lettuce (Lactuca sativa L. cv. 'Grand Rapids') seeds was inhibited at high temperatures (thermoinhibition). Thermoinhibition at 28 degrees C was prevented by the application of fluridone, an inhibitor of abscisic acid (ABA) biosynthesis. At 33 degrees C, the sensitivity of the seeds to ABA increased, and fluridone on its own was no longer effective. However, a combined application of fluridone and gibberellic acid (GA3) was able to restore the germination. Exogenous GA3 lowered endogenous ABA content in the seeds, enhancing catabolism of ABA and export of the catabolites from the intact seeds. The fluridone application also decreased the ABA content. Consequently, the combined application of fluridone and GA3 decreased the ABA content to a sufficiently low level to allow germination at 33 degrees C. There was no significant temperature-dependent change in endogenous GA1 contents. It is concluded that ABA is an important factor in the regulation of thermoinhibition of lettuce seed germination, and that GA affects the temperature responsiveness of the seeds through ABA metabolism.     

Use of the Weibull Function to Calculate Cardinal Temperatures in Faba Bean

The onset of germination of faba bean seeds at constant temperaturewas progressively delayed as that temperature diverged froman optimum of 25.5 ?C. At temperatures below 10 ?C, or above28 ?C, the maximum germination percentage fell to below 90%.There was no germination at 39 ?C. Positive and negative linearrelationships were established between the constant temperaturesand the rates of progress of germination to different percentiles,at sub-optimal and supra-optimal temperatures, respectively.Like germination rates, base temperature (T_b) declined from3.71 to –0.83 ?C as the percentile value increased from10% to 80%. Caution was urged in extrapolating beyond the experimentaldata set. Differences in the ceiling temperature (T_c) with percentilecould not be discerned. Cumulative germination progress curves at each temperature weremodelled by the Weibull, logistic, and cumulative normal distributionfunctions. Cardinal temperatures (T_b and T_c) calculated fromthese data reasonably approximated the actual data. The Weibullfunction demonstrated a good approximation at all percentilelevels, while the logistic and cumulative normal distributionfunctions, as a result of their inherent symmetry, deviatedat the extreme percentiles. It was concluded that the Weibullfunction not only accurately modelled cumulative germinationbut could also be used in the calculation of cardinal temperatures. Key words: Seed germination rate, cardinal temperatures, faba bean, Weibull function, probit and logic scales     

Isolation and characterization of high temperature-resistant germination mutants of Arabidopsis thaliana

Temperature is a primary environmental cue for seed germination of many weeds and vegetables. To investigate the mechanism of germination regulation by temperature, we selected five high temperature (thermoinhibition)-resistant germination mutants (TRW lines) from 20,000 T-DNA insertion lines of Arabidopsis. Segregation analyses indicated that each of the five lines had single locus recessive mutations. The seeds of TRW134-15 and TRW187 showed reduced sensitivity to ABA and also to the gibberrellin biosynthesis inhibitor, paclobutrazol. Genetic and nucleotide sequencing analyses indicated that TRW187 is a new allele of abi3 (abi3-14). TRW71-1 exhibited a maternal effect for both thermoinhibition-resistant and transparent testa phenotypes, and genetic analysis revealed that the mutation was allelic to tt7 (tt7-4 sib). Interestingly, the seeds of reduced dormancy mutants rdo1, rdo2, rdo3 and rdo4 were also thermoinhibition tolerant, and all the TRW seeds showed reduced dormancy. Like rdo3, TRW13-1 had shorter siliques and slightly shorter stems than the wild type. The mutation of TRW13-1 was mapped to the bottom arm of chromosome 1 where rdo3 has also been mapped, but the two mutants are not allelic. We designated TRW13-1 as thermoinhibition-resistant germination 1 (trg1). We also mapped the ABA-insensitive mutation of TRW134-15 to the bottom arm of chromosome 5 and named it trg2. These results show that both embryo/endosperm and maternal factors contribute to germination inhibition at supraoptimal temperatures in Arabidopsis. In addition, we confirm the role of ABA in thermoinhibition of seed germination and a link between seed physiological dormancy and response to high temperature.     

Germination studies ofMolucella laevis

Seed of Molucella laevis (L.) was gathered in the years 1963-66 and germinated soon after harvest and at various intervals subsequently. All seeds showed dormancy on gathering. There was a noticeable fluctuation in the percentage of seeds germinating during storage. Dormancy persisted throughout the years of the experiment. It appears that a number of factors are operative in the inhibition of germination of M. laevis seeds. One factor is an inhibitor which may be adsorbed by active charcoal or heavy loam, involving some process which requires fluctuating temperature. Rupture of the seed-coat also improved the germination of dormant seed, and a subsequent water rinse for 24 hrs. further increased the percent of germinating seed. Immersion of the seed for 48 hrs. in gibberellic acid (G.A.), 400 ppm, greatly improved germination, but it did not completely overcome dormancy. The maximum effect (93% germinating seed) was obtained when seed pre-treated with G.A. was germinated on top of active charcoal. The optimal germination temperature was found to be a daily alternating one of 16 hrs. at 15°C and 8 hrs. at 30°C with light supplied at the latter temperature. Treated seed was sown in the screen house and found to develop normal plants and seed. The dormancy-breaking effect of G.A. lasts for at least 180 days. The fluctuations in germination of seed pre-treated with gibberellin were similar to those of the untreated ones. The effect of dormancy-breaking factors differed with year of gathering and date of application. Thus, M. laevis seeds display much heterogeneity in their germination potential.     

Combined effect of temperature and water stress on seed germination of four Leptocereus spp. (Cactaceae) from Cuban dry forests

Cactus seeds have developed adaptations to survive with high temperatures and low soil moisture in their habitats. We studied the effect of the combination of four water potentials (0, −0.2, −0.4 and −0.6 MPa) and two temperatures (25°C and 35°C) on germination and seedling mass of four endemic Leptocereus species from Cuba. There were two semi-arid coast species (L. arboreus and L. santamarinae) and two species inhabiting dry inland karstic hills (L. ekmanii and L. scopulophilus). We hypothesized that: (a) a decrease in water potential and an increase in temperature would result in low and slow germination, as well as in low seedling mass, and (b) ungerminated cactus seeds after exposure to combined water and temperature stress would have a high recovery capacity. The minimum time required for the seeds to start germination (Tmin), mean germination time (MGT) and germinability were evaluated. In addition, seed mass, the recovery after treatments of non-germinated seeds, and the seedling fresh mass obtained under different treatments were compared between species. In general, germination was only obtained at 25°C and germinability and seedling mass were drastically affected by the reduction from 0 MPa to −0.2 MPa. Seeds showed thermoinhibition at 35°C at all water potentials. There was a tendency to increase the MGT with decreasing water potential in three species. Low seed recovery occurred at all combined treatments for three species. If the predictions of increased temperature and decreased rainfall for the Caribbean region occur, a reduction in the germination of the Leptocereus species studied is expected.     

Development of a Threshold Model to Predict Germination of Populus tomentosa Seeds after Harvest and Storage under Ambient Condition

Effects of temperature, storage time and their combination on germination of aspen (Populus tomentosa) seeds were investigated. Aspen seeds were germinated at 5 to 30°C at 5°C intervals after storage for a period of time under 28°C and 75% relative humidity. The effect of temperature on aspen seed germination could not be effectively described by the thermal time (TT) model, which underestimated the germination rate at 5°C and poorly predicted the time courses of germination at 10, 20, 25 and 30°C. A modified TT model (MTT) which assumed a two-phased linear relationship between germination rate and temperature was more accurate in predicting the germination rate and percentage and had a higher likelihood of being correct than the TT model. The maximum lifetime threshold (MLT) model accurately described the effect of storage time on seed germination across all the germination temperatures. An aging thermal time (ATT) model combining both the TT and MLT models was developed to describe the effect of both temperature and storage time on seed germination. When the ATT model was applied to germination data across all the temperatures and storage times, it produced a relatively poor fit. Adjusting the ATT model to separately fit germination data at low and high temperatures in the suboptimal range increased the models accuracy for predicting seed germination. Both the MLT and ATT models indicate that germination of aspen seeds have distinct physiological responses to temperature within a suboptimal range.     

Effect of age on germination of dormant seeds

Dormant seeds in permanent soil seed banks change their germination probability with age, but little is known about the causes of this behavior. Since current adaptive models do not consider the age structure of the seed bank, the sole explanation is that changes in germination probability with age are due to passive accidental factors or to decay of seeds. Here we present a theory, which shows that such changes might be adaptive. In particular, in density-regulated populations, the evolutionarily stable fraction of germinating seeds in each age class increases with age whenever age-dependent mortality of dormant seeds remains constant or increases with age.     

Development of combined imbibition and hydrothermal threshold models to simulate maize (Zea mays) and chickpea (Cicer arietinum) seed germination in variable environments

* The ability of hydrothermal time (HTT) and virtual osmotic potential (VOP) models to describe the kinetics of maize (Zea mays) and chickpea (Cicer arietinum) seed germination under variable conditions of water potential was investigated with a view to gaining an improved understanding of the impact of on-farm seed priming on seedling establishment through simulation. * Germination and/or imbibition time courses were recorded over a wide range of constant temperatures and water potentials and simple stepwise changes in water potential. * Both models adequately described germination under constant environmental conditions, but not conditions of water potential that varied. To test the hypothesis that this inaccuracy resulted from the use of ambient water potential, a parsimonious model of seed imbibition was developed to calibrate the HTT and VOP models (IHTT and IVOP) and drive them with estimates of seed water potential. * The IHTT and IVOP models described germination during stepwise changes in water potential more accurately than the conventional models, and should contribute to improved predictions of germination time in the field.     

The Effects of Priming and 'Natural' Differences in Quality amongst Onion Seed Lots on the Response of the Rate of Germination to Temperature and the Identification of the Characteristics under Genotypic Control

Ellis, R. H. and Butcher, P. D. 1988. The effects of primingand ‘natural’ differences in quality amongst onionseed lots on the response of the rate of germination to temperatureand the identification of the characteristics under genotypiccontrol —J. exp. Bot. 39: 935–950. A screening procedure was applied to define the response ofthe rate of seed germination to sub-and supra-optimal temperaturesfor different lots or sub-lots of two onion (Allium cepa L.)cultivars.Three sub-lots of the cultivar White Lisbon were derived froma control lot by osmotic priming (–1.4 MPa, 20 °C.7 d) alone, by priming and drying and by priming, drying andsubsequently storing the seeds for 7 weeks at 2–5 °C.The major effect of priming was to reduce the thermal time forgermination at both sub- and supra-optimal temperatures. Primingalone also altered the distribution of thermal times at sub-optimaltemperatures. A new equation is presented to describe this variation.In contrast, priming had no consistent effect on base temperature(T_b and little effect on the distribution of ceiling temperatures[T_e(G)]. For the control lot of White Lisbon T_b was 4°C,whilst the best common estimate of T_b for all four sub-lotswas 3.5°C. The mean estimate of T_c(50) for the control,primed and primed and dried sub-lots was 35.5°C.Comparisonof three lots of the cultivar Senshyu Semi Globe Yellow of widely-differingviability showed substantial differences in the thermal timefor germination at sub-optimal temperatures, but no significantdifferences in T_b (P>0.10), the common estimate being 4°C.There was a significant negative correlation between probitpercentage viability and the logarithm of the thermal time for50% germination at sub-optimal temperatures amongst the threelots (P<0.05). The work suggests that base temperature forgermination is a genotypic characteristic which is unaffectedby differences in seed quality. It also shows that the effectof priming, quantified as a reduction in thermal time requirementsfor germination, varies amongst the seeds within a lot. Key words: -Onion, seed germination rate, temperature, priming     

Protein mobilization in germinating mung bean seeds involves vacuolar sorting receptors and multivesicular bodies

Plants accumulate and store proteins in protein storage vacuoles (PSVs) during seed development and maturation. Upon seed germination, these storage proteins are mobilized to provide nutrients for seedling growth. However, little is known about the molecular mechanisms of protein degradation during seed germination. Here we test the hypothesis that vacuolar sorting receptor (VSR) proteins play a role in mediating protein degradation in germinating seeds. We demonstrate that both VSR proteins and hydrolytic enzymes are synthesized de novo during mung bean (Vigna radiata) seed germination. Immunogold electron microscopy with VSR antibodies demonstrate that VSRs mainly locate to the peripheral membrane of multivesicular bodies (MVBs), presumably as recycling receptors in day 1 germinating seeds, but become internalized to the MVB lumen, presumably for degradation at day 3 germination. Chemical cross-linking and immunoprecipitation with VSR antibodies have identified the cysteine protease aleurain as a specific VSR-interacting protein in germinating seeds. Further confocal immunofluorescence and immunogold electron microscopy studies demonstrate that VSR and aleurain colocalize to MVBs as well as PSVs in germinating seeds. Thus, MVBs in germinating seeds exercise dual functions: as a storage compartment for proteases that are physically separated from PSVs in the mature seed and as an intermediate compartment for VSR-mediated delivery of proteases from the Golgi apparatus to the PSV for protein degradation during seed germination.     

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。     

Control processes in the induction and relief of thermoinhibition of lettuce seed germination : actions of phytochrome and endogenous ethylene

Germination of lettuce seeds (Lactuca sativa L. cv Grand Rapids) in the dark was nearly 100% at 20°C but was inhibited at 27°C and higher temperatures (thermoinhibition). A single 5-minute exposure to red light completely overcame the inhibition at temperatures up to 28°C, above which the effectiveness of single light exposures gradually declined to reach a negligible level at 32°C. However, the promotive effect of light could be extended to 34°C by repeated irradiations. At any one temperature, increased frequency of irradiations increased germination percentage, and with each degree increase in temperature, increasingly frequent irradiations were necessary to elicit maximal germination. Loss of the effectiveness of single irradiations with increase in temperature may result either from acceleration of the thermal reversion of the far red-absorbing form of phytochrome or decrease in seed sensitivity toward a given percentage of the far red-absorbing form of phytochrome. Using continuous red light to induce germination, the role of endogenous C₂H₄ in germination at 32°C was studied. Ethylene evolution from irradiated seeds began to increase 2 hours prior to radicle protrusion, whereas the dark-incubated (nongerminating) seeds produced a low, constant amount of C₂H₄ throughout the 24 hour incubation period. Inhibition of C₂H₄ synthesis with 2-aminoethoxyvinyl glycine and/or inhibition of C₂H₄ action with 2,5-norbornadiene blocked the promotive effect of light. Exogenous C₂H₄ overcame these blockages. The results showed that participation by endogenous C₂H₄ was essential for the light-induced relief of thermoinhibition of lettuce seed germination. However, light did not act exclusively via C₂H₄ since exogenous C₂H₄ alone in darkness did not promote germination.     

A molecular framework for the embryo growth in germinating seeds of Solanum lycocarpum A. St.‐Hil., a nurse plant species

We are clarifying how the functional embryo growth occurs in germinating seeds of Solanum lycocarpum A. St.‐Hil., a nurse plant with a central role in forest dynamics in the Cerrado (a biodiversity hotspot). For that, we used classical seed germination measurements (germinability, mean germination time, mean germination rate, coefficient of variation of the germination time, synchronisation index and germination time range) and gene expression of mRNA codifying key proteins/enzymes for the success in the seed–seedling transition (Cyclin, Actin, Small Heat Shock Protein, Glutathione S‐transferase, Malate Dehydrogenase, Alcohol Dehydrogenase). Our findings demonstrate: (a) Although germination kinetics in S. lycocarpum seeds is slower than that in tomato seeds, the fold change of genes codifying key enzymes for the embryo development is similar in germinating seeds of both species. (b) The genes used here are useful, from a technical point of view, for classifying commercial seed samples of the species in relation to physiological quality. More notably, cyclin and malate dehydrogenase genes have a greater expression, both in germination sensu stricto and in immediate post‐germination. (c) A molecular framework for the embryo growth in germinating seeds of S. lycocarpum can be a functional explication for the species to be a nurse plant. Thus, the overlapping of classical and contemporary measurements is especially interesting to those species playing a central role in the environment, such as nurse plants, and may represent a new conservationist paradigm.     

Hydrothermal time analysis of watermelon (<Emphasis Type="Italic">Citrullus vulgaris</Emphasis> cv. ‘Crimson sweet’) seed germination

This study evaluated the ability of a hydrothermal time model (HTT) to describe the kinetics of watermelon (Citrullus vulgaris cv. ‘Crimson sweet’) seed germination under different temperatures (T) and water potentials (ψ) and also to determine the cardinal temperatures of watermelon. Results indicated that ψ influenced germination rate and germination percentage. For this seed lot, cardinal temperatures were 10 °C for T _b, 28.34 °C for T _o and 40.8 °C for T _c in the control (0 MPa) treatment. There was a decrease in hydrotime constant (θ _H) when T was increased to T _o and then remained constant at supra-optimal temperatures (30 MPah^?1). Also, at temperatures above T _o, ψ _b(50) values increased linearly with T. The k _T value (the slope of the relationship between ψ _b(50) and T exceeds T _o) of this seed lot was calculated as 0.076 MPa°Ch^?1. Results this study show that when the HTT model is applied, it can accurately describe ψ _b(g) and the course of germination around Ts (R ² = 0.82). Moreover, the ψ _b(50) was estimated to be ?0.96 MPa based on this model. Consequently, the germination response of watermelon for all Ts and ψs can be adequately described by the HTT model and enabling it to be used as a predictive tool in watermelon seed germination simulation models.     

Selection for low dormancy in annual ryegrass (Lolium rigidum) seeds results in high constitutive expression of a glucose-responsive α-amylase isoform

Background and Aims

α-Amylase in grass caryopses (seeds) is usually expressed upon commencement of germination and is rarely seen in dry, mature seeds. A heat-stable α-amylase activity was unexpectedly selected for expression in dry annual ryegrass (Lolium rigidum) seeds during targeted selection for low primary dormancy. The aim of this study was to characterize this constitutive activity biochemically and determine if its presence conferred insensitivity to the germination inhibitors abscisic acid and benzoxazolinone.

Methods

α-Amylase activity in developing, mature and germinating seeds from the selected (low-dormancy) and a field-collected (dormant) population was characterized by native activity PAGE. The response of seed germination and α-amylase activity to abscisic acid and benzoxazolinone was assessed. Using an alginate affinity matrix, α-amylase was purified from dry and germinating seeds for analysis of its enzymatic properties.

Key Results

The constitutive α-amylase activity appeared late during seed development and was mainly localized in the aleurone; in germinating seeds, this activity was responsive to both glucose and gibberellin. It migrated differently on native PAGE compared with the major activities in germinating seeds of the dormant population, but the enzymatic properties of α-amylase purified from the low-dormancy and dormant seeds were largely indistinguishable. Seed imbibition on benzoxazolinone had little effect on the low-dormancy seeds but greatly inhibited germination and α-amylase activity in the dormant population.

Conclusions

The constitutive α-amylase activity in annual ryegrass seeds selected for low dormancy is electrophoretically different from that in germinating seeds and its presence confers insensitivity to benzoxazolinone. The concurrent selection of low dormancy and constitutive α-amylase activity may help to enhance seedling establishment under competitive conditions.     

Seed Dispersal Phenology and Germination Characteristics of a Drought-Prone Vegetation in Southeastern Brazil

Seed germination is determined by the environmental conditions typical of a habitat and also by the geographical origin of the source species pool. During the Quaternary, Brazilian Atlantic Rain Forest species expanded their distribution into the sandy coastal plains (restingas). Periods of water shortage, however, are frequent in the sandy substrate of the restinga. We investigated whether the germination characteristics of restinga species are more related to their biogeographical origin in the humid forest or to water shortage on sandy substrates. We characterized the seed dispersal phenology of a restinga community and conducted experiments to determine the water requirements for seed germination and the short-term seed dehydration sensitivity of different species. Species shed seeds throughout the year in the restinga. When subjected to Ψ=−0.37 MPa, seed germination percentage decreased and germination time increased in six of ten species when compared with Ψ=0 MPa. Most species showed high seed moisture content (MC>40 %) at seed dispersal. Seeds took 3–17 d to dehydrate when subjected to relative humidity≤76 percent and only two of eight species had seeds sensitive to short-term dehydration. Thus, rather than a specific set of germination characteristics related to humid or dry habitats, we gathered evidence to show that the germination characteristics of restinga species represent a multiplicity of responses that may be found in both kinds of habitat.