Physical Dormancy Research Articles

A function for the pleurogram in physically dormant seeds.

Different structures have been shown to act as a water gap in seeds with physical dormancy (PY), and in Fabaceae they are commonly located in the hilar region. However, the function of the pleurogram, a structure in the extra-hilar region that is common in legume seeds, remains unknown. Our aims were to review the literature for occurrence of the pleurogram in Fabaceae, determine if the pleurogram can open, and compare the functional morpho-anatomy of water gaps in seeds of 11 Senna species. Imbibition tests showed that all 11 species had PY. Structural features of the hilar and extra-hilar regions of the seeds were investigated using light and scanning electron microscopy, and dye-tracking was performed to trace the pathways of water through the seed coat. A pleurogram has been reported for 37 legume genera. Water gaps differed among Senna species, with lens, hilum, micropyle and pleurogram taking up water after PY was broken. In Senna alata seeds, only the pleurogram acted as a water gap, whereas in S. reniformis and S. silvestris water entered the seed through both the pleurogram and the hilar region. In the pleurogram of S. alata and S. reniformis, the palisade layer moved outward, exposing the hourglass cells, whereas in S. silvestris the palisade layer was broken. The pleurogram acts as a water gap in some of the 11 Senna species examined, but it is non-functional in others. Opening the pleurogram occurs due to the formation of a linear slit in the palisade layer. The pleurogram is of functional significance by creating a wide opening, whereby water can reach the embryo and start germination. This is the first report of the pleurogram functioning as a water gap. Because this structure is shared by at least 37 genera, it also may be a water gap in many other legume species.

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.

The role of the testa during the establishment of physical dormancy in the pea seed.

A water-impermeable testa acts as a barrier to a seed's imbibition, thereby imposing dormancy. The physical and functional properties of the macrosclereids are thought to be critical determinants of dormancy; however, the mechanisms underlying the maintenance of and release from dormancy in pea are not well understood. Seeds of six pea accessions of contrasting dormancy type were tested for their ability to imbibe and the permeability of their testa was evaluated. Release from dormancy was monitored following temperature oscillation, lipid removal and drying. Histochemical and microscopic approaches were used to characterize the structure of the testa. The strophiole was identified as representing the major site for the entry of water into non-dormant seeds, while water entry into dormant seeds was distributed rather than localized. The major barrier for water uptake in dormant seeds was the upper section of the macrosclereids, referred to as the 'light line'. Dormancy could be released by thermocycling, dehydration or chloroform treatment. Assays based on either periodic acid or ruthenium red were used to visualize penetration through the testa. Lipids were detected within a subcuticular waxy layer in both dormant and non-dormant seeds. The waxy layer and the light line both formed at the same time as the establishment of secondary cell walls at the tip of the macrosclereids. The light line was identified as the major barrier to water penetration in dormant seeds. Its outer border abuts a waxy subcuticular layer, which is consistent with the suggestion that the light line represents the interface between two distinct environments - the waxy subcuticular layer and the cellulose-rich secondary cell wall. The mechanistic basis of dormancy break includes changes in the testa's lipid layer, along with the mechanical disruption induced by oscillation in temperature and by a decreased moisture content of the embryo.

Seed dormancy in relation to predation and pathogenicity of four tropical Fabaceae species from Sri Lanka

According to the seed defense theory, predation and pathogenicity should be lower in physically dormant (PY) seeds than those of no-dormant (ND). Thus, a study was conducted to determine the predation and pathogenicity of four tropical Fabaceae species to test the above hypothesis. Imbibition and germination of manually scarified (MS) and untreated (UT) seeds were tested using locally collected seeds. Pathogenicity and predatory rates of MS and UT seeds of each species was observed by keeping them separately in a woodland and a grassland for four weeks. Nutrient content and production of volatile compounds in seeds were determined. Germination and imbibition experiments confirmed the physical dormancy of two Senna spp., non-dormancy of Bauhinia variegata and physiological dormancy in Pterocarpus indicus seeds. Predation and pathogenicity were higher in the woodland than in the grassland. Lowest seed predation and pathogenicity were recorded in P. indicus and the highest in B. variegata. Pathogenicity and predation of MS seeds of Senna spp. were higher than that of UT seeds. No correlation was revealed between nutrient content and pathogenicity or predation. MS seeds released more volatile compounds than intact seeds as they were in imbibed state. Pathogenicity and predation depended on seed dormancy classes.

PRIMING OF BRACHIARIA SEEDS WITH DIFFERENT SUGAR SOURCES AND CONCENTRATIONS

ABSTRACT Seed priming is a practice for improving the expression of seed physiological potential. Such technique consists of synchronizing and reducing the time of seed germination by controlled hydration. The aim of this study was to evaluate the effect of seed-priming with different sugar sources and concentrations on the physiological quality Urochloa brizantha seeds and initial seedling performance. Before treating, seeds were scarified chemically with concentrated sulphuric acid (H2SO4) for 5 minutes to overcome physical dormancy. The experimental design was completely randomized in a 3 x 6 factorial scheme consisting of priming using three sugar sources (glucose, sucrose, and maltose) and six concentrations (zero [water control], 2%, 5%, 10%, 15%, and 20%), with four replicates. The seeds were primed by direct immersion for 2 hours at 25 ºC and, after hydration, they were dried for moisture equilibrium recovery. Seed germination, vigor, viability, and initial seedling growth were evaluated. The results showed that glucose was the source able to promote beneficial effects on the germination of U. brizantha cv. MG-5 seeds. Moreover, the supply of glucose at the concentrations of 2 and 5% for physiological conditioning increased seedling dry phytomass.

Embryo rescue of Spondias tuberosa and exogenous application of gibberellic acid

The umbuzeiro (Spondias tuberosa Arruda) is a native fruit tree of Caatinga and has great economic importance. Its fruits are collected extractivistically due to lack of commercial plantations. Furthermore, the seeds have physical dormancy, which reduces germination rates. However, tissue culture can be used to overcome such problems by excision and in vitro culture of embryos with gibberellin. The objective of this study was to evaluate different disinfection times of zygotic embryos in sodium hypochlorite (NaOCl) and the effect of gibberellic acid (GA(3)) in the in vitro establishment of excised embryos. For aseptic test, zygotic embryos (15 replicates/treatment) were excised and immersed in NaOCl (1.25% active chlorine) for different times (5, 10, and 15 min) and rinsed three times with distilled and sterilized water. The explants were cultured on Murashige and Skoog medium (MS) supplemented with 2.88 µM GA(3). After 30 days we evaluated the contamination and germination percentages. The influence of different concentrations of GA(3) was also tested. Zygotic embryos (24 replicates/treatment) were inoculated onto MS medium with different concentrations of GA(3) (0.0, 0.72, 1.44, and 2.88 µM). After 30 days the germination and normal seedlings percentage, height, number of leaves, fresh and dry weight of shoot and root were evaluated. The experimental design of both trials was completely randomized. The immersion in NaOCl for more than 10 min reduced contamination as well as the germination percentage (p=0.0281). The effect of GA(3) was significant (p=0.0346) in stem elongation in a concentration of 2.88 µM compared to control without gibberellin. The 10-min time was effective in disinfection and gibberellic acid promotes increased height of seedlings.

Germination Biology of Sesbania (Sesbania cannabina): An Emerging Weed in the Australian Cotton Agro-environment

AbstractSesbania [Sesbania cannabina(Retz.) Pers.] is a problematic emerging weed species in Australian cotton-farming systems. However, globally, no information is available regarding its seed germination biology, and better understanding will help in devising superior management strategies to prevent further infestations. Laboratory and glasshouse studies were conducted to evaluate the impact of various environmental factors such as light, temperature, salt, osmotic and pH stress, and burial depth on germination and emergence of two Australian biotypes ofS. cannabina. Freshly harvested seeds of both biotypes possessed physical dormancy. A boiling-water scarification treatment (100±2 C) of 5-min duration was the optimum treatment to overcome this dormancy. Once dormancy was broken, the Dalby biotype exhibited a greater germination (93%) compared with the St George biotype (87%). The nondormant seeds of both biotypes showed a neutral photoblastic response to light and dark conditions, with germination marginally improved (6%) under illumination. Maximum germination of both biotypes occurred under an alternating temperature regime of 30/20 and 35/25 C and under constant temperatures of 32 or 35 C, with no germination at 8 or 11 C. Seed germination of both biotypes decreased linearly from 87% to 14% with an increase in moisture stress from 0.0 to −0.8 MPa, with no germination possible at −1.0 MPa. There was a gradual decline in germination for both biotypes when imbibed in a range of salt solutions of 25 to 250 mM, with a 50% reduction in germination occurring at 150 mM. Both biotypes germinated well under a wide range of pH values (4.0 to 10.0), with maximum germination (94%) at pH 9.0. The greatest emergence rate of the Dalby (87%) and St George (78%) biotypes was recorded at a burial depth of 1.0 cm, with no emergence at 16.0 cm. Deep tillage seems to be the best management strategy to stopS. cannabina’s emergence and further infestation of cotton (Gossypium hirsutumL.) fields. The findings of this study will be helpful to cotton agronomists in devising effective, sustainable, and efficient integrated weed management strategies for the control ofS. cannabinain cotton cropping lands.

Ecological role of physical dormancy in seeds of Oxytropis racemosa in a semiarid sandland with unpredictable rainfall

Seed dormancy and the soil seed bank are crucial to plant regeneration strategy, especially in semiarid ecosystems with unpredictable precipitation. The aim of this study was to investigate how seed dormancy is controlled by environmental factors and how it is correlated with the soil seed bank and regeneration of the perennial legume Oxytropis racemosa, a dominant perennial herb in Mu Us Sandland of semiarid China. Germination and imbibition experiments on fresh intact and scarified seeds of O. racemosa were used to identify physical dormancy (PY) in seeds of this species. Soil seed bank dynamics, timing of seedling emergence and the fate of buried seeds in the natural habitat were investigated. PY was broken by mechanical scarification or wet heat/ice water cycles but not solely by dry heat or wet heat treatment. The soil seed bank exhibited seasonal changes in the number of seeds, which was highest in September and lowest in July. Seeds buried at different sand depths gradually lost dormancy; 20–42% of the seeds remained dormant after 20 months of burial. Dormancy break occurs gradually throughout the year. Our results indicate that O. racemosa exhibits hardcoatedness heterogeneity that spreads germination of a seed cohort between seasons and years in the semiarid environment, where the amount of precipitation during the growing season is highly variable.

Aplikasi Trichoderma sp. dan Lama Penyimpanan Terhadap Dormansi Benih Oyong (Luffa acutangula (L.) Roxb.)

Benih Oyong (Luffa acutangula (L.) Roxb.) memiliki dormansi fisik, serta berkulit keras dan tebal. Salah satu upaya untuk mematahkan dormansi fisik adalah menggunakan mikroorganisme Trichoderma sp dan penyimpanan. Penelitian dilakukan selama 3 bulan, pada bulan Agustus hingga November 2017, bertempat di Laboratorium PT. Benih Citra Asia, Jl. Akmaludin No. 26 Jember Jawa Timur 64117. Percobaan ini menggunakan Rancangan Acak Lengkap (RAL) Faktorial dengan 2 faktor yang diulang sebanyak 3 kali. Faktor pertama yaitu lama perendaman yang terdiri dari 4 taraf, Tanpa Perendaman, direndam dengan larutan spora Trichoderma sp. selama 1 Hari, 2 Hari, dan 3 Hari. Faktor kedua adalah lama penyimpanan yang terdiri dari 2 taraf, yaitu disimpan selama 0 Minggu, dan 2 Minggu. Penelitian lama perendaman Trichoderma sp. memberikan hasil pengaruh nyata pada parameter Indeks Dormansi (ID) dan pengaruh sangat nyata pada parameter Kecepatan Tumbuh. Perlakuan perendaman selama 2 hari memberikan hasil. Sedangkan pengaruh penyimpanan (S) memberikan pengaruh berbeda sangat nyata pada parameter kecepatan tumbuh. Perlakuan penyimpanan setelah aplikasi memberikan hasil meningkatkan nilai KcT sampai 17,58%/etmal. Interaksi antara perlakuan Lama Perendaman Tichoderma sp dan penyimpanan memberikan pengaruh berbeda sangat nyata pada parameter kecepatan tumbuh dan pada parameter Indeks Dormansi serta benih abnormal. Benih Oyong yang yang direndam menggunakan larutan Trichoderma sp. selama 2 hari tanpa disimpan memiliki Kecepatan Tumbuh diatas 17%/ etmal, yaitu 17,65%/ etmal, dengan Indeks Dormansi paling rendah yaitu 13,33%.

Variability in hardseededness and seed coat thickness of three populations of Desmanthus virgatus (Fabaceae, Mimosoideae)

AbstractDesmanthus virgatus (L.) Willd. has significant fodder potential, but its seeds have high and persistent levels of physical dormancy which interfere with its field establishment. The purpose of this study was to analyse the variability of physical dormancy in three populations of seeds from humid and arid regions of Argentina, grown and collected in different years and locations. The hardseededness and the percentage of seeds with intact lenses after a softening treatment and the seed coat thicknesses were compared. The variability observed in the obtained percentages of hardseededness, intact lenses and seed coat thicknesses was related to the origin and the environment in which these seeds had developed. Although further work is needed, arid environments seem to favour the development of thicker seminal layers, being an important factor that determines the impermeable nature of the seeds in this species. This information is required for future genetic improvement programs and for understanding the evolutionary process of the populations of this species in different environments.

A meta-analysis of the effects of frugivory (endozoochory) on seed germination: role of seed size and kind of dormancy

Heretofore, no study has determined how germination of ingested seeds is affected by the kind (class) of dormancy nor by seed dormancy x seed size interaction. Thus, we aimed to determine the effects of seed size, kind of dormancy and their interaction on germination of defecated seeds using a meta-analysis. We collected data for 366 plant species in 97 plant families from 76 publications. In general, gut passage significantly increased germination percentage of defecated seeds by 5% compared with that of control seeds. Germination percentages of non-dormant, physiologically dormant, and morphologically/morphophysiologically dormant seeds (all water-permeable) significantly decreased after gut passage by 40, 18, and 14%, respectively, compared with control seeds (non-gut-passed). Changes in germination percentage of seeds with physical dormancy (water-impermeable) were positive, and gut passage increased germination by 69% compared with control seeds. Germination of small seeds decreased 8% after gut passage, whereas germination of both medium and large seeds increased by 18%. However, changes in germination percentage differed between categories of seed size in each class of dormancy. In physically dormant seeds, germination of all seed sizes improved after gut passage, and the magnitude of increase was higher for large than for medium and small seeds. Thus, gut passage increased germination of medium-size water-permeable seeds (physiologically dormant and morphologically/morphophysiologically dormant) more than it did for large and small seeds. However, gut-passage decreased or did not change the germination percentage of non-dormant seeds. Seed size and kind of dormancy should be included in studies on the effect of gut passage on germination.

Seed anatomy and water uptake and their relation to seed dormancy of Ormosia paraensis Ducke

Abstract: Ormosia paraensis Ducke has ornamental seeds widely used in the manufacture of bio-jewels and wood used in the furniture industry. For seedling production, the information on its seed physiology is scarce. Thus, the aim of this study was to assess methods for breaking dormancy and relate them to integument structure and water uptake by O. paraensis seeds. Seed dormancy-breaking was performed by mechanical scarification and soaking in sulfuric acid for 0, 15, 30, 60, 120, and 240 minutes. Dormancy‐broken seeds were compared with intact seeds. Seed integument is formed by a cuticle (hydrophobic substances), epidermis (macroesclereids of the palisade layer,), hypodermis (osteosclereids), and parenchyma cells. Intact seeds did not absorb water after 72 hours of soaking. The highest percentages and rates of seed germination were observed in treatments with mechanical scarification and soaking in sulfuric acid for 60 or 120 minutes. Seed soaking in sulfuric acid (H2SO4 p.a. 98.08%) for 60 or 120 minutes or mechanical scarification are adequate to overcome physical dormancy associated with O. paraensis seed integument impermeability to water or gases.

Effect of pre-sowing treatments and growing media on seed germination and seedling growth of Albizia lebbeck (L.) Benth

The seeds of Albizia lebbeck have been observed to exhibit physical dormancy due to presence of hard seed-coat. To overcome this problem, the seeds were subjected to seven pre-sowing treatments viz., T1-immersion of seeds in cold water for 12 h; T2-immersion of seeds in cold water for 24 h; T3-immersion of seeds in hot water (100 °C) and subsequent cooling at room temperature for 12 h; T4-immersion of seeds in hot water (100 °C) and subsequent cooling at room temperature for 24 h; T5-immersion of seeds in cold water for 12 h followed by immersion in hot water (100 °C) and allowed to cool for 1 h; T6-immersion of seeds in cold water for 24 h followed by immersion in hot water (100 °C) and allowed to cool for 1 h. Untreated seeds served as control (T0). Treatment T3 gave highest germination (96%) which was comparable with T5 (95 %), T4 (94 %) and T6 (93%). Nine growing media viz., T1: soil, T2: soil+sand (2:1), T3: soil+perlite (2:1), T4: soil+Farm Yard Manure (FYM) (2:1), T5: soil+vermicompost (2:1), T6: soil+sand+FYM (1:1:1), T7: soil+sand+vermicompost (1:1:1), T8: soil+perlite+FYM (1:1:1) and T9: soil+perlite+ vermicompost (1:1:1) were, also, studied for their effect on seedling growth of A. lebbeck. Among these media, maximum values of shoot length (23.82 cm), root length (21.14 cm), collar diameter (3.59 mm) and seedling quality index (0.350) were observed in T7.

Determination of the water gap and the germination ecology of Adenanthera pavonina (Fabaceae, Mimosoideae); the adaptive role of physical dormancy in mimetic seeds.

Dormancy caused by impermeable seed coats, i.e. physical dormancy (PY), regulates the timing of seed germination in species of several genera belonging to 18 angiosperm families. Physical dormancy also occurs in some mimetic species whose seeds mimic brightly coloured, fleshy fruits or arilled seeds. However, the conditions that break dormancy, as well as the location of water gaps in mimetic seeds, remain unclear. Here, we investigated the adaptive role of impermeable coats in the mimetic seeds of Adenanthera pavonina (Fabaceae: Mimosoideae). Specifically, we explored: (i) the conditions that break PY; (ii) the location of the primary water gap that forms during dormancy break; and (iii) the effect of seasonal temperature regimes on seed germination. Seeds were subjected to hot-water treatment, rapid temperature fluctuations and storage at temperatures mimicking summer and autumn conditions. Seed coat anatomy and water-gap regions were characterized using scanning electron microscopy (SEM) and light microscopy. Seeds were artificially buried in the field at 3 and 7 cm depths and exhumed every 6 months for 2 years to monitor germination. Adenanthera pavonina had impermeable seed coats, and thus PY. Seeds treated with hot water and exposed to summer–autumn temperature regimes broke dormancy. Water entered only through the lens (Type-II simple) due to dislodgement of the palisade layer. Seeds buried at 3 cm depth had significantly higher germination than those buried at 7 cm depth, with germination primarily occurring in autumn. Seeds required high summer temperatures followed by moderate autumn temperatures to become permeable to water and germinate in the field during the wet season. We conclude that the impermeable seed coat of A. pavonina is an adaptation that synchronizes germination with the growing season.

Why large seeds with physical dormancy become nondormant earlier than small ones.

Under natural conditions, large seeds with physical dormancy (PY) may become water permeable earlier than small ones. However, the mechanism for this difference has not been elucidated. Thus, our aim was to evaluate the traits associated with PY in seeds of Senna multijuga (Fabaceae) and to propose a mechanism for earlier dormancy-break in large than in small seeds. Two seedlots were collected and each separated into large and small seeds. Seed dry mass, water content, thickness of palisade layer in the hilar and distal regions and the ratio between palisade layer thickness (P) in the lens fissure and seed mass (M) were evaluated. Further, the correlation between seed mass and seed dimensions was investigated. Large seeds had higher dry mass and water content than small seeds. The absolute thickness of the palisade layer in the different regions did not show any trend with seed size; however, large seeds had a lower P:M ratio than small seeds. Seed mass correlated positively with all seed dimensions, providing evidence for a substantially higher volume in large seeds. Since wet, but not dry, high temperatures break PY in sensitive seeds of S. multijuga, the data support our prediction that internal pressure potential in the seed and palisade layer thickness in the water gap (lens), which is related to seed mass (i.e. P:M ratio), act together to modulate the second step (dormancy break) of the two-stage sensitivity cycling model for PY break. In which case, large seeds are predetermined to become water-permeable earlier than small ones.

Dormancy-defense syndromes and tradeoffs between physical and chemical defenses in seeds of pioneer species.

Seeds of tropical pioneer trees have chemical and physical characteristics that determine their capacity to persist in the soil seed bank. These traits allow seeds to survive in the soil despite diverse predators and pathogens, and to germinate and recruit even decades after dispersal. Defenses in seedlings and adult plants often are described in terms of tradeoffs between chemical and physical defense, but the interplay of defensive strategies has been evaluated only rarely for seeds. Here we evaluated whether classes of seed defenses were negatively correlated across species (consistent with tradeoffs in defense strategies), or whether groups of traits formed associations across species (consistent with seed defense syndromes). Using 16 of the most common pioneer tree species in a neotropical lowland forest in Panama we investigated relationships among four physical traits (seed fracture resistance, seed coat thickness, seed permeability, and seed mass) and two chemical traits (number of phenolic compounds and phenolic peak area), and their association with seed persistence. In addition, seed toxicity was assessed with bioassays in which we evaluated the activity of seed extracts against representative fungal pathogens and a model invertebrate. We did not find univariate tradeoffs between chemical and physical defenses. Instead, we found that seed permeability - a trait that distinguishes physical dormancy from other dormancy types - was positively associated with chemical defense traits and negatively associated with physical defense traits. Using a linear discriminant analysis and a hierarchical cluster analysis we found evidence to distinguish three distinct seed defense syndromes that correspond directly with seed dormancy classes (i.e., quiescent, physical, and physiological). Our data suggest that short and long-term persistence of seeds can be achieved via two strategies: having permeable seeds that are well defended chemically, corresponding to the physiologically dormant defense syndrome; or having impermeable seeds that are well defended physically, corresponding to the physically dormant defense syndrome. In turn, transient seeds appear to have a lower degree of chemical and physical defenses, corresponding to the quiescent defense syndrome. Overall, we find that seed defense and seed dormancy are linked, suggesting that environmental pressures on seed persistence and for delayed germination can select for trait combinations defining distinct dormancy-defense syndromes.

Seed germination of a South American forest tree described by linear thermal time models

Thermal time models may describe and compare seed germination, providing information useful to explain species distribution. However, the capacity of such threshold models to describe germination of tropical native species has been less studied. We evaluated seed germination of a legume tree species (Peltophorum dubium), typical from South American seasonal forests, as described by two linear thermal time models: probit model and graphic model. Seeds were provided from four different provenances in a latitudinal gradient in Brazil, and their physical dormancy mechanically released before the trials. Graphic model and probit regression were used to determine thermal parameters (cardinal temperatures and thermal time requirement) on sub- and supra-optimal ranges for the different seed provenances. Germination rate mainly followed linear relationship with temperature, and regression lines of different germination fractions converged base temperature in the x-axis. Therefore, probit model assumed a single-value of base temperature in the sub-optimal range and a normal distribution of thermal time. Base temperature tended to be higher in the Northern provenance, Porto Velho, showing slower germination under cooler temperatures. Supra-optimal temperatures have shown similar thermal time requirements and different values for ceiling temperature, according to model predictions. No clear patterns have been found between seed provenances and thermal time requirement. Both probit and graphic models have provided reasonable predictions of germination times (t10 and t50), except under the coolest temperatures. Probit regression always described at least 70% of seed germination. Thermal time assumptions, linear models applicability and their limitations are discussed.

Breaking seed coat dormancy of six tree species

Breaking physical dormancy in some forest seeds is a challenge for scientists and forest managers to obtain an homogeneous germination for larger seed samples. The role played by the seed coat in seed dormancy of six trees with great interest in agroforestry (Robinia pseudoacacia, Leucaena leucocephala, Erythrostemon gilliesii, Styphnolobium japonicum, Acacia dealbata and Brachychiton populneus) was tested by the effects of the pretreatment and its duration on the performance of seed germination, by considering the final germination percentage (FGP) and the mean germination time (MGT). These parameters are estimated at various times of incubation (5, 10 and 15 days) in Petri dishes and stored in darkness at (25 ± 2°C). The pretreatment consists of an immersion of seeds in concentrated sulphuric acid during 30, 60 and 90 minutes.
 Sowing without pretreatment (control) revealed no germination induction for B. populneus and A. dealbata, except for R. pseudoacacia, L. leucocephala and S. japonica where the germination does not exceed 12% for all the experience duration. Generally, pretreatments were very useful to improve seed germination. The time of immersion into sulphuric acid significantly affected (P < 0.0001) the FGP and the MGT in all studied species. A duration of 30 minutes of soaking was adequate to give a very high rate of germination for L. leucocephala, E. gilliesii, S. japonica and A. dealbata with respective FGP of 100%, 95%, 100% and 100%. However, an extended duration of pretreatment of 60 minutes was necessary for a maximal germination for R. pseudoacacia and B. populneus with FGP of 85% and 100%, respectively. A prolonged duration of 90 minutes of presowing was very fatal for L. leucocephala, A. dealbata and B. populneus. An excellent germinative strength is characterized by a higher FGP and a reduced MGT.

Tratamento térmico úmido em sementes de Acacia mangium

Acacia mangium Willd. is an arboreal and exotic species, which has potential for exploitation of timber products in Brazil. As it spreads mainly through sexual reproduction, it becomes relevant the phytosanitary treatment of seeds for the production of seedlings. Therefore, this study aimed to evaluate the effects of thermal treatment humid heat on health and physiological quality of A. mangium seeds. Thus, the seeds were thermal treated in water heated to 70° C during 0, 10, 15 and 20 minutes of exposure. Then, the seeds were subjected to germination test and seed health. The seed health analysis revealed that fungi of the genre Aspergillus sp., Penicillium sp., Pestalotia sp., Cladosporium sp., Alternaria sp., Fusarium sp. and Botrytis sp., were associated with seeds. The thermal treatment with humid heat at a temperature of 70°C for 10 to 15 minutes of exposure overcame the physical dormancy of seeds of A. mangium and reduced occurrence of fungal species.

Physical dormancy alleviation and soil seed bank establishment in Cassia roxburghii is determined by soil microsite characteristics

Seeds with an impermeable coat, i.e. physical dormancy (PY) persist in the soil for a long-time until suitable environmental cues break dormancy and germination is synchronized with the growing season. However, the dormancy loss of seeds at different microsites has not been well characterized. In this study, the effect of microsite affecting dormancy loss in Cassia roxburghii was studied by artificially burying the seeds in an open site and under canopy and exhuming the seeds at 6 month intervals for two years. The soil temperatures of the two microsites at seed vicinity were determined using a data-logger. The results showed that fresh intact seeds did not absorb water, but mechanically scarifying the seeds resulted in water absorption and germination at a wide range of temperatures both in light and darkness. Thus, the seeds of C. roxburghii have PY. Throughout the study period, impermeable seeds buried in the open site broke dormancy in significantly higher number compared with seeds buried under canopy cover (P < 0.05), presumably due to the temperature variation between microsites. Open sites recorded temperatures above 60 °C during summer with diurnal temperature fluctuations close to 20 °C, whereas under canopy cover temperature remained close to 40 °C during summer and much lower during other seasons. These results suggest that the ability of C. roxburghii seeds to break dormancy and establish soil seed banks is affected by the microsite variation.