Physical dormancy is the most common seed dormancy type seen in the tropics. Causes and types learning objectives importance of dormancy types of dormancy introduction dormancy is a condition where seeds will not germinate even when the environmental conditions such as water, temperature and air are favourable for germination. Seed dormancy is a complex adaptive trait of higher plants that allows embryos to survive extended periods of unfavorable environmental conditions. Seed dormancy or rest is the internal or innate inhibition of germination of otherwise normal or viable seed even when present under most favourable conditions required for its germination. Seed dormancy can be defined as the state or a condition in which seeds are prevented from germinating even under the favourable environmental conditions for germination including, temperature, water, light, gas, seed coats, and other mechanical restrictions. Dormancy is an important factor limiting production in many field crops. Water impermeability of the coat or in some species the fruit. The palisade layer in the seed coat of water impermeable seeds had no cracks in it, whereas cracks. Water impermeable seed dormancy 367 table i recovery of impermeable seeds fed to livestock adapted from harmon and keim 1934 11 2 germination % nonacid acid treated species recovery before after before after convolvuzus arvensis 23 9. In all cases, py is associated with a waterimpermeable layers of palisade cells macrosclereids baskin et al. Seeds that have hard, thick seedcoats that physi cally prevent water or oxygen movement into seeds have physical dormancy. So, under such circumstances it should be broken with proper method. Mechanisms underpinning the onset of seed coat impermeability. A hard seed coat prevents absorption of water, thus inhibiting germination and causing dormancy.
Dormancy caused by an impermeable seed coat is known as physical dormancy. Thus, dormancy is an adaptive trait that optimizes the distribution of germination over time in a population of seeds. In seeds with py, prevention of water uptake causes the seed to remain dormant until some factors render the covering layers permeable to water. Over the course of evolution, plant seeds have evolved diverse dormancy forms in response to various climates to maintain viability. Seeds germinate only when sufficient water is available to leach out inhibitors and soften the seed coats. Effects of warm water in breaking dormancy of rice seed 3 effect of breaking dormancy on respiration rate and ethylene production. Dormancy is based on hard seed coat impermeability or the lack of supply and activity of enzymes internal dormancy necessary for germination. Treatments allowing for the removal of physical dormancy needed to be markedly more severe with a. Pdf seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. Methods to break seed dormancy home guides sf gate. This impermeable layer prevents the seed from taking up water or gases. We 1 tested seeds of 15 species of caragana matured in the same site in 2014, 2016 andor 2017 for presence of py, 2 determined if dry storage decreased or. In some cases, a seed doesnt contain germination inhibitors, but it has a seed coat that is hard and impermeable to water.
Waterimpermeability of the coat or in some species the fruit. Seed dormancy is a trait acquired by the spermatophytes during evolution to subsist in adverse environments such as low or high temperature, salinity, and drought yildiz et al. This study aimed to investigate three main questions. Breaking physical dormancy in seeds focussing on the lens. This impermeability of the coat is caused by the presence of one or more palisade layers of lignified malphigian cells macrosclereids. Seed storage behaviour and seed germination in african and. The seed will not germinate until the seed coat is altered physically. These seeds have a nondormant embryo but do not germinate due to a water. Isabel ordizb, zhonglian huangb, mariko nonogakia, roger n. The treatments for dormancy breaking tegumentary seed were efficient because they promoted the rupture of the impermeable layer in the case of physical scarification or distributing pores in the integument when the sulfuric acid used, thus, enhancing the water absorption by seed and triggering the germination process. Seed coats were permeable to water in both species. In other cases regulation of the rate of water absorption by the seed covering slows the germination process which can be advantageous or disadvantageous depending on the circumstances.
A hard seed coat is capable of causing dormancy in three was. On the other hand some seeds are permeable to oxygen but impermeable to co 2 hence produced. The seed coat exerts its germinationrestrictive action most of the time by being impermeable to water and or oxygen or by its mechanical resistance to radicle protrusion linkies et. Water permeabilityimpermeability in seeds of 15 species. The physical dormancy of most seeds was not broken by dry heat or wet heat, but by alternating wet heat and ice water cycles. Seed dormancy in cultivation, have highly bred crops such.
However, some degree of dormancy is advan tageous, at least during seed development. Seeds with a waterimpermeable seed or fruit coat have physical dormancy py. Nonchemical methods to break seed dormancy of canafistula. However, various authors have reported that seeds of some caragana species are waterpermeable and thus nondormant. The ecological significance of hard seed includes the ability to rapidly recolonize burnt areas after fire and to withstand ingestion by animals and birds. The seed coats of many species are completely impermeable to water at the time of their maturity. Beachyb,1, and hiroyuki nonogakia,1 adepartment of horticulture, oregon state university, corvallis, or 97331. Convolvulaceae is the most advanced plant family asterid clade that produces seeds with physical dormancy waterimpermeable seed coat. Taxonomy, anatomy and evolution of physical dormancy in. Quiescence is the nongermination of viable and nondormant seeds, due to absence of favourable environmental conditions, e.
Seed dormancy caused by a waterimpermeable seed or fruit coat is called physical dormancy py, and it develops during maturation drying of the seed van staden et al. Pdf determining the phylogenetic and biogeographic distribution of physical dormancy remains a major challenge in germination ecology. Majority legumes in the temperate and arctic zones have waterimpermeable seeds physical dormancy, py. Freshly collected seeds with a moisture content of 9. In autumn, the seed fruit coat becomes water permeable and the seed germinates, as in geranium carolinianum l. Any process of breaking, scratching, or mechanically altering the seed coat to make it permeable to water and gases is known as scarification. Physical restriction of water and gas impermeable or relatively impermeable seed or fruit coat may prevent water or gas uptake by seed and prevent germination until physical damage occurs to this barrierexample. Embryogenesis, which is a morphogenesis phase, starts with the formation of a singlecell zygote and ends in the heart stage when all embryo structures have been formed mayer et al.
Thus, the structure andor chemical composition of the seed coat prevents the seeds water uptake, making germination not possible marcosfilho, 2005. Water absorption and dormancybreaking requirements of. A study with coronilla varia seeds showed that as the depth of a puncture to the seed coat increased, so did the permeability of the seed coat to water. A class ii knox gene, knox4, controls seed physical dormancy. Such seed coats develop during the last stages of seed development.
The present study aimed to determine the effect of. The present study was therefore conducted with the hypothesis of whether r. There can be various reasons behind the seed dormancy. Effect of the seed coat on dormancy and germination in. Occasionally the dormancy is caused by an inhibiting chemical in the epidermis or adjacent interior membranes.
Seed coats impermeable to water the seeds of certain plants especially those belonging to the familys leguminaceae, solanaceae, malvaceae, etc. As a result, the seed is prevented from germinating until dormancy is broken. This condition is very common in the seeds of many legumes, e. Pdf a new seed coat waterimpermeability mechanism in. Following baskin and baskin 2, the class of dormancy was assigned to each species with dormant seeds. It is followed by a growth phase during which the embryo fills the seed sac. Seed dormancy allows seeds to pass through drought, cold and other unfavourable conditions. Seed anatomy and water uptake in relation to seed dormancy. It is known, or highly suspected, to occur in nine orders and 15 families of angiosperms sensuangiosperm phylogeny group 1. Information on water impermeable seeds has been catalogued by barton. Seed coat dormancy in two species of grevillea oxford academic. Viable seeds that do not imbibe water and thus fail to germinate in an apparently favorable environment are commonly termed impermeable or hard seed. Thus, seed dormancy is a major cause of continuing weed problems, but no information is available about breaking seed dormancy of r.
Water permeabilityimpermeability in seeds of 15 species of. Some seeds are permeable to water but impermeable to gasses esp to oxygen eg xanthium. Most seeds had a waterimpermeable seed coat that became permeable after mechanical or acid scarification. Seed dormancy is generally an undesirable characteristic in agricultural crops, where rapid germination and growth are required.
External seed dormancy may be physical, physicalphysiological, chemical, or mechanical baskin and baskin 1998, 2004. Physical dormancy and soil seed bank dynamics in seeds of. Mechanical dormancy seed coats are too hard to allow the embryo to expand during germination. Seed dormancy causes, types, methods and its importance. Seed dormancy and germination characteristics of astragalus. This physical, exogenous dormancy is especially common in species of the fabaceae. However, a screen of literature fails to show experimental support to the assumption that c. Physical dormancy plays a key role in protecting seeds against microbial attacks and predators and maintaining seed banks in soils 35. The respiration rate of both treatments were increased and correlated with germination rate when the germination increased but slightly slow down after sowing 6 days at both treatments table 2 and fig. Breaking physical dormancy in seeds focussing on the. It should be mentioned that dry storage may, or may not, promote a dormancy break in seeds with only py. Py and 3 located the site on the seed coat of 11 species where water enters the seed. This type of dormancy has been found in at least 17 plant families and is very common in legume species 2, 6, 7.
The seeds remain dormant until the impermeable layer decay by the action of soil microorganisms. Seed coat external dormancy results from a seeds hard seed coat that is impervious to water and gases. Physical dormancy, also termed hardseededness, is an exogenous dormancy caused by a waterimpermeable. Dormancy is when there is a lack of germination in seeds or tubers even though the required conditions temperature, humidity, oxygen, and light are provided. However, upon water uptake, resumption of metabolism leads to the unavoidable release of vola. Summary historically, physical dormancy, or hard seededness, where seeds are prevented from germinating by a waterimpermeable seed coat, is viewed as a dormancy mechanism. Physical dormancy is the result of impermeable layers that develops during maturation and drying of the seed or fruit. Overcoming dormancy and enhancing germination of sphaeralcea. Taxonomy, anatomy and evolution of physical dormancy in seeds. The water gap can become permeable after exposure to temperature flux, drying, or scarification, thus allowing imbibition into an otherwise impermeable seed. Avena fatua wild oat seed dormancy broken easily by pricking pericarp. Majority legumes in the temperate and arctic zones have waterimpermeable seeds. In some species the regulation is extreme the seed covering is completely impermeable to water hard seed.
Break dormancy with scarification, hot water, acid, dormancy mechanisms seed coat impermeable to gases no exchange of oxygen or in some cases carbon dioxide slows respiration and metabolism coffee and cocklebur break dormancy by leaching with water, chilling, removing seed coat, scarification dormancy mechanisms. Hard seed coat these are impermeable to water, gases so restrict water uptake and oxygen exchange. In seeds with py, prevention of water uptake causes the seed to remain dormant until some factors render the. Morphology and anatomy of physical dormancy in ipomoea.