Germination Phase Research Articles

Modeling high-severity fire, drought and climate change impacts on ponderosa pine regeneration

We describe a model to investigate the effects of high-severity fire and drought on ponderosa pine regeneration using a water balance approach. Based on literature reviews and an analysis of annual and monthly correlations against tree regeneration, climate envelopes are constructed to simulate conditions during the flowering, seed production, germination phase, seedling growth in the season following germination, and seedling growth in the 2 years following germination. The model was tested against observed regeneration at five sites in the Southwest that experienced high-severity fires during a drought from ca. 1945 to 1956. For validation purposes single fire events, as occurred at each field site, were simulated by altering maximum and minimum temperatures and runoff conditions in declining stages of severity for 7 years. To evaluate long term fire and climate impacts four sensitivity tests were conducted on climate records from 1914 through 2009: (1) a climate control with no modifications to temperature and precipitation inputs; (2) a permanently burned condition simulation; (3) a simulation where climate conditions were altered based on IPCC future climate change projections and; (4) a simulation with both climate change and fire modifications. Results show that annual data correlate poorly with ponderosa pine regeneration compared to monthly correlations. On average the model correctly predicts regeneration outcomes 62% of the time at the field sites. The sensitivity tests suggest that regeneration rates are reduced by 43% in post high-severity fire environments because of changed conditions during germination and post-germination establishment phases. Future climate projections result in mixed outcomes across the 5 sites and an average reduction of regeneration rates by 8%. Ponderosa pine forests on the dry end of the climate envelope are predicted to experience severe reduction in regeneration.

Heat and chilling induced disruption of redox homeostasis and its regulation by hydrogen peroxide in germinating rice seeds (Oryza sativa L., Cultivar Ratna).

Extremes of temperature (both heat and chilling) during early inbibitional phase of germination caused disruption of redox-homeostasis by increasing accumulation of reactive oxygen species (superoxide and hydrogen peroxide) and significant reduction of antioxidative defense (assessed in terms of total thiol content and activities of superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase) in germinating tissues of rice (Oryza sativa L., cultivar Ratna). Imbibitional heat and chilling stress also induced oxidative damage to newly assembled membrane system by aggravating membrane lipid peroxidation and protein oxidation [measured in terms of thiobarbituric acid reactive substances (TBARS), free carbonyl content (C = O groups) and membrane protein thiol level (MPTL)]. Treatment with standardized low titer hydrogen peroxide during early imbibitional phase of germination caused significant reversal in oxidative damages to the newly assembled membrane system imposed by heat and chilling stress [evident from the data of TBARS, C = O, MPTL, ROS accumulation, membrane permeability status, membrane injury index and oxidative stress index] in seedlings of experimental rice cultivar. Imbibitional H2O2 pretreatment also caused up-regulation of antioxidative defense (activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and total thiol content) in the heat and chilling stress-raised rice seedlings. When the parameters of early growth performances were assessed (in terms of relative growth index, biomass accumulation, relative germination performance, mean daily germination, T50 value), it clearly exhibited significant improvement of early growth performances of the experimental rice cultivar. The result proposes that an 'inductive pulse' of H2O2 is required to switch on some stress acclimatory metabolism through which plant restores redox homeostasis and prevents or repairs oxidative damages to newly assembled membrane system caused by unfavorable environmental cues during early germination to the rice cultivar Ratna. The importance of mitigating oxidative damages to membrane lipid and protein necessary for post-germinative growth under extremes of temperature is also suggested.

Development of a new wheat microarray from a durum wheat totipotent cDNA library used for a powdery mildew resistance study.

Totipotent cDNA libraries representative of all the potentially expressed sequences in a genome would be of great benefit to gene expression studies. Here, we report on an innovative method for creating such a library for durum wheat (Triticum turgidum L. var. durum) and its application for gene discovery. The use of suitable quantities of 5-azacytidine during the germination phase induced the demethylation of total DNA, and the resulting seedlings potentially express all of the genes present in the genome. A new wheat microarray consisting of 4925 unigenes was developed from the totipotent cDNA library and used to screen for genes that may contribute to differences in the disease resistance of two near-isogenic lines, the durum wheat cultivar Latino and the line 5BIL-42, which are respectively susceptible and resistant to powdery mildew. Fluorescently labeled cDNA was prepared from the RNA of seedlings of the two near-isogenic wheat lines after infection with a single powdery mildew isolate under controlled conditions in the greenhouse. Hybridization to the microarray identified six genes that were differently expressed in the two lines. Four of the sequences could be assigned putative functions based on their similarity to known genes in public databases. Physical mapping of the six genes localized them to two regions of the genome: the centromeric region of chromosome 5B, where the Pm36 resistance gene was previously localized, and chromosome 6B.

Enzymatic characterization of germination-specific cysteine protease-1 expressed transiently in cotyledons during the early phase of germination

Papain-like cysteine protease activity that shows a unique transient expression profile in cotyledons of daikon radish during germination was detected. The enzyme showed a distinct elution pattern on DEAE-cellulose compared with cathepsin B-like and Responsive to dessication-21 cysteine protease. Although this activity was not detected in seed prior to imbibition, the activity increased markedly and reached a maximum at 2 days after imbibition and then decreased rapidly and completely disappeared after 5 days. Using cystatin-Sepharose, the 26 kDa cysteine protease (DRCP26) was isolated from cotyledons at 2 days after imbibition. The deduced amino acid sequence from the cDNA nucleotide sequence indicated that DRCP26 is an orthologue of Arabidopsis unidentified protein, germination-specific cysteine protease-1, belonging to the C1 family of cysteine protease predicted from genetic information. In an effort to characterize the enzymatic properties of DRCP26, the enzyme was purified to homogeneity from cotyledons at 48 h after imbibition. The best synthetic substrate for the enzyme was carbobenzoxy-Phe-Arg-4-methylcoumaryl-7-amide. All model peptides were digested to small peptides by the enzyme, suggesting that DRCP26 possesses broad cleavage specificity. These results indicated that DRCP26 plays a role in the mobilization of storage proteins in the early phase of seed germination.

Germination and Seedling Responses of Subalpine Plants to Different Soil Substrates

In this study we explored the regeneration niche of 18 alpine plant species by testing the germination responses and seedling growth to soil mixtures made with gypsum, ophiolite, and quartzite, and a control soil. We analyzed different phases of germination: time of radicle emergence, germination duration, and germination percentage, and thereafter seedling performance measured using seedling biomasses. Species were grouped as acidophilous, basophilous and neutrophilous according to their Ellenberg indicator values. Seeds were germinated on each soil mixture in a greenhouse, and then seedlings were selected and transplanted outdoors to grow for one season at 2,050 m a.s.l. The time of radicle emergence, length of germination, final germination percentage, seedling biomass and specific leaf area (SLA) varied according to the soil mixtures depending on the pH group. There were some consistencies between the Ellenberg groups, which were built for adult plant species, and the way these seedlings responded to a particular soil mixture. For instance, the group of species predominantly occurring in basic soils during their adult life (basophilous) had the highest germination percentage and the shortest emergence time on the soil mixture composed of gypsum, but a low germination percentage and the smallest seedling biomass on the other two soil mixtures. In addition, the decrease in SLA for seedlings of the basophilous group when they were cultivated on acidic soil (especially quartzite) seems to indicate a weaker functioning of the plants. Our results highlight, first, the significance of the regeneration niche in the establishment of plants in a given soil environment, especially by emphasizing the links between germination and soil chemistry, and secondly, the consideration that different germination phases add more information about the plant community assemblage with respect to the soil environment.

RESISTANCE IN LYCOPERSICON ESCULENTUM VAR. CERASIFORME TO CLADOSPORIUM FULVUM

Leaf mould caused by Cladosporium fulvum is one of the most widespread tomato disease in greenhouse. The use of resistant cultivars is the most economic and decisive tool to control the disease. In a screening for leaf mould resistance numerous accessions of Lycopersicon pimpinellifolium, L. peruvianum and L. esculentum var. cerasiforme and the race 0 of C. fulvum were used. Among the tested accessions a general reaction of susceptibility was observed. Only the one accession of L. esculentum var. cerasiforme was resistant to C. fulvum. This accession was self-fertilized and signed as line R-28. The resistance reaction was evident on leaves with chlorotic spots that later became necrotic. Microscopic observations showed no differences between the resistant and the susceptible genotypes either in the phase of conidia germination and in elongation of germ tube. After one week on the susceptible plants conidiophores were evident whereas, on the leaves of line R-28 accession, fungi sporulation was never observed. In inheritance analysis, segregation ratios strongly fitted a single and dominant gene model conferring resistance in line R-28. Necrosis of leaves indicated that the resistance mechanism was a hypersensitive reaction. Resistance to C. fulvum found in the accession line R-28 is the first report about a monogenic and dominant gene in L. esculentum var. cerasiforme. It is important to emphasize that in previous studies in the accession line R-28 were also identified a resistance to Oidium neolycopersici conferred by a single recessive gene signed as ol-2.

Mechanistic studies of the spore photoproduct lyase via a single cysteine mutation.

5-Thyminyl-5,6-dihydrothymine (also called spore photoproduct or SP) is the exclusive DNA photodamage product in bacterial endospores. It is repaired by a radical SAM (S-adenosylmethionine) enzyme, the spore photoproduct lyase (SPL), at the bacterial early germination phase. Our previous studies proved that SPL utilizes the 5'-dA• generated by the SAM cleavage reaction to abstract the H(6proR) atom to initiate the SP repair process. The resulting thymine allylic radical was suggested to take an H atom from an unknown protein source, most likely cysteine 141. Here we show that C141 can be readily alkylated in the native SPL by an iodoacetamide treatment, suggesting that it is accessible to the TpT radical. SP repair by the SPL C141A mutant yields TpTSO(2)(-) and TpT simultaneously from the very beginning of the reaction; no lag phase is observed for TpTSO(2)(-) formation. Should any other protein residue serve as the H donor, its presence would result in TpT being the major product at least for the first enzyme turnover. These observations provide strong evidence to support C141 as the direct H atom donor. Moreover, because of the lack of this intrinsic H donor, the C141A mutant produces TpT via an unprecedented thymine cation radical reduction (proton-coupled electron transfer) process, contrasting to the H atom transfer mechanism in the wild-type (WT) SPL reaction. The C141A mutant repairs SP at a rate that is ~3-fold slower than that of the WT enzyme. Formation of TpTSO(2)(-) and TpT exhibits a V(max) deuterium kinetic isotope effect (KIE) of 1.7 ± 0.2, which is smaller than the (D)V(max) KIE of 2.8 ± 0.3 determined for the WT SPL reaction. These findings suggest that removing the intrinsic H atom donor disturbs the rate-limiting process during enzyme catalysis. As expected, the prereduced C141A mutant supports only ~0.4 turnover, which is in sharp contrast to the >5 turnovers exhibited by the WT SPL reaction, suggesting that the enzyme catalytic cycle (SAM regeneration) is disrupted by this single mutation.

Production of an extracellular matrix as an isotropic growth phase of Penicillium rubens on gypsum.

Indoor mold represents an important environmental concern, but a fundamental knowledge of fungal growth stages is needed to limit indoor fungal proliferation on finishing materials used in buildings. The present study focused on the succession of germination stages of the common indoor fungus Penicillium rubens on a gypsum substrate. This substrate is used as a model system representing porous materials that are widely used in indoor environments. Imaging with cryo-scanning electron microscopy showed that the formation of an extracellular matrix (ECM) is a phase of the isotropic growth of P. rubens that is uniquely related to germinating conidia. Furthermore, the ECM is observed only when a dry-state inoculation of the surface is applied, i.e., applying conidia directly from a 7-day-old colony, mimicking airborne contamination of the surface. When inoculation is done by spraying an aqueous conidial suspension, no ECM is observed. Moreover, it is concluded that the formation of an ECM requires active processes in the fungal cell. The porosity of the substrate proved that the ECM substance has high-viscosity characteristics. The present results stress that studies of indoor fungal growth should consider the method of inoculation, knowing that the common aqueous suspension may obscure specific stages in the initial phases of germination.

Effects of temperature, salinity and cold stratification on seed germination in halophytes

Salt tolerance of halophytes corresponds with the habitat requirement of the species. It is an important factor during the germination phase and it can determine successful establishment. This paper presents the effects of alternating temperature–light regimes (4/8°C, 10/20°C, 20/32°C; 12 h dark: 12 h light) and different salinity levels (0, 200, 400, 600 mmol l21 NaCl) on seed germination of five halophytes, Halimione pedunculata, Bupleurum tenuissimum, Aster tripolium, Triglochin maritimum and Armeria maritima. The five species differ with respect to family and life‐form and spatially correspond to a decreasing salt gradient (i.e. distance from salt water, with H. pedunculata being the most tolerant and A. maritima being the least). Armeria maritima, A. tripolium and T. maritimum seeds were additionally subjected to a cold stratification experiment. The results showed that Halimione pedunculata, an annual therophyte of year‐round heavily saline habitats, was dormant under all experimental conditions. Bupleurum tenuissimum, a species typical to sites of varying salinity prone to leaching during spring and autumn rainfall, germinated best under cold and warm temperatures, but only under non‐saline conditions. Aster tripolium and T. maritimum, close neighbours in salt marshes, showed very similar germination behaviour: seeds of both species tolerated high levels of salinity and germinated best in summer temperatures during periods of highest soil salinity, and germination was significantly promoted by cold. Armeria maritima, a species usually found on the marginal fringes of saline habitats, germinated only under low salt levels and maximum germination was under cold (spring) and warm (autumn) temperatures, with no significant effect of cold stratification.

Early carbon mobilization and radicle protrusion in maize germination

Considerable amounts of information is available on the complex carbohydrates that are mobilized and utilized by the seed to support early seedling development. These events occur after radicle has protruded from the seed. However, scarce information is available on the role of the endogenous soluble carbohydrates from the embryo in the first hours of germination. The present work analysed how the soluble carbohydrate reserves in isolated maize embryos are mobilized during 6–24 h of water imbibition, an interval that exclusively embraces the first two phases of the germination process. It was found that sucrose constitutes a very significant reserve in the scutellum and that it is efficiently consumed during the time in which the adjacent embryo axis is engaged in an active metabolism. Sucrose transporter was immunolocalized in the scutellum and in vascular elements. In parallel, a cell-wall invertase activity, which hydrolyses sucrose, developed in the embryo axis, which favoured higher glucose uptake. Sucrose and hexose transporters were active in the embryo tissues, together with the plasma membrane H+-ATPase, which was localized in all embryo regions involved in both nutrient transport and active cell elongation to support radicle extension. It is proposed that, during the initial maize germination phases, a net flow of sucrose takes place from the scutellum towards the embryo axis and regions that undergo elongation. During radicle extension, sucrose and hexose transporters, as well as H+-ATPase, become the fundamental proteins that orchestrate the transport of nutrients required for successful germination.

Proteomic Analysis of Embryonic Proteins Synthesized from Long-Lived mRNAs During Germination of Rice Seeds

Dry seeds contain translatable, long-lived mRNAs that are stored during seed maturation. Early studies using transcriptional inhibitors supported the view that protein synthesis during the initial phase of germination occurs on long-lived mRNA templates. Rice seeds were treated with the transcriptional inhibitor actinomycin D (Act D), and the embryonic proteins translated from long-lived mRNAs during germination were identified using a proteomic analysis. De novo transcription was not required for germination of rice seeds, since >80% of seeds germinated when transcription was prevented by treatment with Act D. In contrast, germination was completely inhibited in the presence of cycloheximide, an inhibitor of translation. Thus, de novo protein synthesis is necessary for germination of rice seeds. The proteomic analysis revealed that 20 proteins are up-regulated during germination, even after Act D treatment. Many of the up-regulated proteins are involved in carbohydrate metabolism and cytoskeleton formation. These results indicate that some of the germination-specific proteins involved in energy production and maintenance of cell structure in rice seeds are synthesized from long-lived mRNAs. The timing of translation of eight up-regulated proteins was clearly later than that of the other up-regulated proteins under conditions in which transcription was inhibited by Act D, suggesting that translation of long-lived mRNAs in rice seeds is regulated according to the germination phase.

The development of seedlings from fragments of monoembryonic seeds as an important survival strategy for Eugenia (Myrtaceae) tree species

The uncommon ability of monoembryonic seeds of Eugenia to develop seedlings after cutting or even after the removal of more than half of their storage tissue has recently been reported. Some Eugenia species are included among those at risk of disappearing, so the regeneration of plantlets from fragments of seeds could be an important mechanism both for the production of more than one normal seedling per seed and to understand the survival strategies of these species. Because the period in which this ability is present has not been studied, we analyzed the regenerative capacity of seeds of five Brazilian native species of Eugenia, considering two levels of maturity and three germination phases. Mature seeds have a greater potential to regenerate than immature ones; germinating seeds showed that the longer the root protrusion, the smaller the potential to regenerate new seedlings. However, our results clearly demonstrate that the ability to regenerate new roots or complete seedlings is present during a long period from the beginning of development until the late phases of germination. Cutting is potentially important to obtain plants on a larger scale. Because of intense predation of these seeds, this ability could be linked to an essential strategy for survival even after the removal of more than half of the storage tissues. As an alternative strategy to orthodox seeds, whose preservation is based on tolerance to desiccation, the recalcitrant seeds of Eugenia probably adopted the great reserve quantity and the regenerative capacity, allowing them to undergo successive germinations.

Nitric oxide and hydrogen cyanide as regulating factors of enzymatic antioxidant system in germinating apple embryos

Short-term (3 or 6 h) pre-treatment of apple (Malus domestica Borkh.) embryos with nitric oxide (NO) or hydrogen cyanide (HCN) induces transient accumulation of reactive oxygen species (ROS) leading to dormancy removal and germination. We demonstrated that enhanced NO emission by apple embryos during early phase of germination “sensu stricto” is required for seed transition from dormant into non-dormant state, and may be described by the model of “nitrosative door”, analogous to “oxidative window”. Cellular ROS concentration, resulting from NO or HCN embryo pre-treatment, seems to be under severe control of antioxidant system. Activity of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), glutathione peroxidase (GPX) and total peroxidases (Prxs) was determined during NO and HCN-mediated germination “sensu stricto” of embryos. CAT and SOD activity increased transiently 24 h after embryos pre-treatment, while GR and Prx activity was stimulated mainly after 96 h. The most evident alterations were detected in GPX activity, being more than threefold stimulated by NO or HCN. Based on this results, we conclude that these reactive molecules act simultaneously crossing their signaling pathways and we propose that ROS, reactive nitrogen species, HCN at accurate level are essential during seed germination as signaling factors.

Tolerance of arabica coffee cultivars for aluminum in nutritive solution

This work aimed to evaluate the Coffea arabica cultivars for aluminum toxicity tolerance, in modified Hoagland solution. A completely randomized design with five repetitions in a factorial 4 x 4 (cultivar x combinations of aluminum) was used. After 44 days of the sowing, were transferred ten seedlings each cultivar germinated in the absence of Al3+ to solution without Al3+, and ten for solution with Al3+; ten seedlings each cultivar germinated in presence of Al3+ to solution without Al3+, and ten for solution with Al3+. In the treatment with aluminum, the element was added to the nutritive solution in the concentration of 0.83 mmol L-1 as Al2(SO4)3.16H2O. The cultivars Catuaí Amarelo IAC 62 and Iapar 59 were tolerant to the aluminum; cultivar Oeiras presented intermediate tolerance, while cultivar Obatã IAC 1669-20 was sensitive. The tolerance of the coffee cultivars to the aluminum during the initial development of the seedlings did not depend on the presence of aluminum in the germination phase.

Critical periods of sorghum and palisadegrass in intercropped cultivation for climatic risk zoning

The objective of this work was to define critical periods for sorghum and palisadegrass cultivated on crop-livestock integrated systems under water deficit. An experiment was carried out in a completely random block design with four treatments (control and interruption of water supply in three periods) and three replicates. Water supply was interrupted until soil water humidity was close to permanent wilting point at the phases: germination of palisadegrass seeds; start of tillering of palisadegrass and initiation of panicles of shorghum; start of shorghum flowering. Water deficit starting at palisadegrass germination delayed intital development of the plants because of the reduction in tillering. Water restriction at panicle initiation phase and at sorghum flowering determined reduction of grain production. Critical periods for intercrop of sorghum and palisadegrass correspond to palisadegrass germination phase and flowering and panicle inititation phase of sorghum.

Probing the Reaction Mechanism of Spore Photoproduct Lyase (SPL) via Diastereoselectively Labeled Dinucleotide SP TpT Substrates

5-Thyminyl-5,6-dihydrothymine (commonly called spore photoproduct or SP) is the exclusive DNA photodamage product in bacterial endospores. It is generated in the bacterial sporulation phase and repaired by a radical SAM enzyme, spore photoproduct lyase (SPL), at the early germination phase. SPL utilizes a special [4Fe-4S] cluster to reductively cleave S-adenosylmethionine (SAM) to generate a reactive 5'-dA radical. The 5'-dA radical is proposed to abstract one of the two H-atoms at the C6 carbon of SP to initiate the repair process. Via organic synthesis and DNA photochemistry, we selectively labeled the 6-H(proS) or 6-H(proR) position with a deuterium in a dinucleotide SP TpT substrate. Monitoring the deuterium migration in enzyme catalysis (employing Bacillus subtilis SPL) revealed that it is the 6-H(proR) atom of SP that is abstracted by the 5'-dA radical. Surprisingly, the abstracted deuterium was not returned to the resulting TpT after enzymatic catalysis; an H-atom from the aqueous buffer was incorporated into TpT instead. This result questions the currently hypothesized SPL mechanism which excludes the involvement of protein residue(s) in SPL reaction, suggesting that some protein residue(s), which is capable of exchanging a proton with the aqueous buffer, is involved in the enzyme catalysis. Moreover, evidence has been obtained for a possible SAM regeneration after each catalytic cycle; however, such a regeneration process is more complex than currently thought, with one or even more protein residues involved as well. These observations have enabled us to propose a modified reaction mechanism for this intriguing DNA repair enzyme.

Testing the regeneration niche hypothesis with Gesneriaceae (tribe Sinningiae) in Brazil: Implications for the conservation of rare species

AbstractThe germination requirements of sexually reproducing plants are regulated by environmental factors such as temperature. Those factors acting at the germination phase are part of the regeneration niche, which is fundamental in the processes that contribute to habitat suitability and geographic distribution. We tested the hypothesis that rarity is associated with regeneration niche in three species of plants in the family Gesneriaceae (tribe Sinningieae), Sinningia rupicola (Mart.) Wiehler, Paliavana sericiflora Benth and Sinningia allagophylla (Mart.) Wiehler, which vary in their distribution and habitat specificity but share a small zone of sympatry in rocky fields south of Belo Horizonte in Minas Gerais, Brazil. The regeneration niche was tested using a seed germination experiment under controlled light conditions at seven fixed temperatures (10–40°C at 5°C intervals). Each of the three species germinated differently at the various temperatures. The species with the smallest geographic range, S. rupicola, also had the most restricted germination: germination peaked at 15°C when relatively few seeds germinated (45%), and even fewer germinated at other temperatures. The regeneration niche was wider in P. sericiflora and wider still in S. allagophylla, with germination greater than 90% between 15–25°C and greater than 80% between 15–30°C, respectively. Our germination results provide qualified support for the hypothesis of correlation of the regeneration niche with geographic distribution of related plant taxa, with important conservation implications for rare and endangered species.

Mid-Holocene pine woodland phases and mire development - significance of dendroecological data from subfossil trees from northwest Germany

Question: Can investigations of subfossil bog-pine woodlands contribute to the understanding of mire development, especially the influence of climate fluctuations on the fen‐bog transition? Location: Lowlands of northwest Germany. Methods: We investigated pine (Pinus sylvestris L.) tree remains (stumps and trunks) buried in peat deposits. Dendrochronology was used to date each sampled tree to calendar years and to reconstruct population dynamics of the pine woodlands. Ecological changes, especially changes in site hydrology during the pine woodland phases were inferred from peat stratigraphic analyses and investigations of stem and root morphology of the tree remains. Results: The subfossil pine woodlands occurred mostly during the transition from fen to raised bog conditions within the mire development. The population dynamics are strikingly wave-like whereas woodland phases of 100 to 250 years duration are separated by much shorter (10‐50 years) phases of high germination and dying-off rates (GDO phases). Such GDO phases are often synchronous at different sites and are also linked to growth depressions of the independent regional oak master chronology (LSBOC), indicating a climate trigger. Conclusions: The development of raised bogs started about 7000 BC and had a main phase between 5100 and 3600 BC in northwest Germany. The subfossil bog-pine woodlands document the transitional phase towards the onset of raised bog formation, as characterized by initial dry conditions that were followed by increasing wetness of the sites, whereas this development is at least partly the result of climate variations.

First off the mark: early seed germination

Most plant seeds are dispersed in a dry, mature state. If these seeds are non-dormant and the environmental conditions are favourable, they will pass through the complex process of germination. In this review, recent progress made with state-of-the-art techniques including genome-wide gene expression analyses that provided deeper insight into the early phase of seed germination, which includes imbibition and the subsequent plateau phase of water uptake in which metabolism is reactivated, is summarized. The physiological state of a seed is determined, at least in part, by the stored mRNAs that are translated upon imbibition. Very early upon imbibition massive transcriptome changes occur, which are regulated by ambient temperature, light conditions, and plant hormones. The hormones abscisic acid and gibberellins play a major role in regulating early seed germination. The early germination phase of Arabidopsis thaliana culminates in testa rupture, which is followed by the late germination phase and endosperm rupture. An integrated view on the early phase of seed germination is provided and it is shown that it is characterized by dynamic biomechanical changes together with very early alterations in transcript, protein, and hormone levels that set the stage for the later events. Early seed germination thereby contributes to seed and seedling performance important for plant establishment in the natural and agricultural ecosystem.

Fases da germinação de sementes de Annona emarginata (Schltdl.) H. Rainer em diferentes temperaturas

O presente trabalho foi realizado com o objetivo de identificar as fases da germinação de sementes de araticum-de-terra-fria (Annona emarginata (Schltdl.) H. Rainer) sob diferentes temperaturas. Para tanto, o trabalho foi dividido em dois experimentos: um para a determinação das fases I e II da germinação, e outro para a fase III. O delineamento experimental foi o inteiramente casualizado, com 3 tratamentos e 4 repetições de 25 sementes por parcela, em ambos os experimentos. Os tratamentos foram constituídos pelas temperaturas constantes de 25ºC e 30ºC e temperatura alternada de 20-30ºC (8-16 h, respectivamente), com luz constante. As variáveis analisadas foram a variação do grau de umidade ao longo do tempo (experimento 1), a porcentagem e a velocidade média de germinação (experimento 2). Os dados foram submetidos à análise de variância, as médias comparadas pelo teste Tukey, a 5% de probabilidade e regressão polinomial. A variação do grau de umidade ao longo do tempo foi estudada através da regressão não linear monomolecular, com os parâmetros das funções comparados conforme os tratamentos, ajustados, submetidos à análise de variância e teste Tukey, a 5% de probabilidade. Observou-se que as sementes mantidas a 30ºC apresentaram maiores valores para velocidade de aquisição de água, atingindo 27,85% de grau de umidade na mudança da fase I para a fase II. Quando as sementes foram submetidas a 25ºC, a fase I teve duração de 60 horas, atingindo 28,35% de grau de umidade e, sob 20/30ºC, a fase durou 72 horas, alcançando 28,33% de grau de umidade. Embora 30ºC tenha promovido a maior velocidade de embebição, não refletiu em maior porcentagem de germinação (fase III), que foi observada nas sementes mantidas a 20/30ºC.