By on. In my previous post, Plant Seed Basics , I described the germination process in very general terms. A seed lands on the ground, absorbs water, and germinates. It is all so simple — or is it? In this post I will have a closer look at something called seed dormancy. Seed dormancy — germinating Fritillaria seed, by Robert Pavlis. Notice the radicle growing through the seed and emerging from the top of the seed. Assume that a seed is from a plant that goes through a winter cold period — a time when it is too cold for the plant to grow properly.
What happens to our seed? The seed will be produced, land on soil, take up water and germinate. Just as things are getting going, winter sets in and kills the seedling. Any cold climate species that went through such a process would soon perish, so these plants must use a different process. They remain dormant until certain environmental and physiological changes take place. Some seed just waits for several months in the hope that spring will be there when it germinates. Other seed requires a drop in temperature to turn on certain chemical changes before germination starts.
Seed dormancy is a term that describes these delays in germination. Understanding the cause of seed dormancy is frustrating when you are trying to grow a plant, but it is also what makes seed germination so interesting and challenging. What about seeds produced in warm climates? They can also have a dormancy but it usually does not require fluctuation in temperature.
See Are Seeds Really Dormant , for a more in-depth description of seed dormancy. Yes, but most have simple forms of dormancy that are short in duration. If you are starting out growing plants from seed you will find that most things you try to grow, will grow just fine with water and room temperatures. As you get more adventurous, you will find that quite a few of the less common and more interesting plants have a more complicated dormancy.
Dormancy is both simple and complex. There are some fairly simple reasons why a seed remains dormant and they are easy to understand. At the same time the subject is complex for two reasons. The information that is available is mostly from amateurs and is anecdotal. Secondly, there can be more than one reason for a seed to be dormant. So what seems like a complicated system may actually be two or more simple dormancies, that just look very complicated.
Understanding the reasons for dormancy will help you understand the methods used to overcome dormancy — the subject matter for the next post in this series. Almost all seeds need to absorb water as part of the germination process and it is usually one of the first steps needed to break seed dormancy. Seeds are like freeze dried food that needs to be re-hydrated in order to make a meal.
Once you start adding water to seed, they should never dry out again. Because dormancy can be broken by most ideal growing conditions different and specific for each species , the seeds germinate when they are the most likely to flourish.
Species that have dormant seed have evolved dormancy because it is useful in survival. Plants utilize dormancy so that seed can endure unfavorable conditions and not all germinate at the same time and are killed by unfavorable weather Seed Dormancy.
While dormancy can enhance plant survival in the wild, it can prevent seeds from germinating uniformly and growing well in wildflower seed production fields.
The impact of moisture availability on germination has been extensively studied in the laboratory and can be described using hydro- and hydrothermal time analysis Fig.
Conditions in the soil can be very different from those in the Petri dish, and this has been described elsewhere Whalley and Finch-Savage, Seeds are not sensitive to the water content of soil per se , but the availability of water measured as water potential MPa —the sum of matric potential adhesion of water to soil structure and osmotic potential influence of solutes. It is this potential that is referred to in the hydrothermal time model for seed germination.
Thus all three act in a temporal pattern and appear to promote dormancy. Footitt, H. Further work will be required to resolve fully the observations made on seeds exhumed from field soil and results obtained in the laboratory, but we consider current understanding of these signals and the responses to them below.
Light is a key spatial signal, and phytochromes play a dominant role in its perception in seeds. As the signal declines, PHYA in the embryo removes the final layer of dormancy, enabling germination Lee et al. PHYA is the most abundant phytochrome in seeds with high protein levels accumulating in the dark Sharrock and Clack, that photo-irreversibly result in germination in monochromatic light from nm to nm Shinomura et al.
However, in tomato, PHYA can both positively and negatively regulate germination depending on the fluence rate of red light; in a low fluence rate, PHYA can relieve dormancy, whereas at a high fluence rate PHYA maintains dormancy Appenroth et al. During the spatial sensing phase, the final layer of dormancy can be removed by millisecond flashes of low fluence sunlight as the soil is disturbed the very low fluence response: VLFR.
Seeds therefore are extremely light sensitive. Dark incubation of seeds sensitized them to dormancy breaking by PHYA-mediated low fluence red light in the range 1— nmol m —2 s —1 at wavelengths from nm to nm Shinomura et al.
Finally, the potential involvement of heterotrimeric G-proteins in PHYA-mediated signalling and germination Botto et al. PHYA is implicated in the positive regulation of dormancy in seeds matured at low, but not warm temperature Donohue et al. However, the response was dependent upon the conditions under which seeds were produced Donohue et al.
Furthermore, regulation by PHYA could appear positive or negative depending on the wavelength and fluence rate used in experiments Appenroth et al.
For dormancy cycling, it should also be considered that such differences probably occur during the continuous process of change in dormancy level in the soil seed bank. The response can also differ with ecotype Dechaine et al.
Such differences in PHYA expression may represent adaptations to climate affecting fitness, as found by Donohue et al. Nitrate, especially in conjunction with light, is another important spatial signal that has been studied in both the laboratory and field. Nitrate concentration in soil solution fluctuates and can vary from almost 0 to 50 mmol l —1 Bouwmeester et al. However, although annual variations in soil nitrate Bouwmeester and Karssen, , Derkx and Karssen, and Sysmbrium officinale seed nitrate content Derkx and Karssen, were observed, changes in dormancy appeared driven by temperature, and not influenced by soil moisture or soil nitrate.
In Arabidopsis, similar conclusions were reached, and temperature-driven seasonal dormancy patterns appeared to be regulated by changes in sensitivity to light Derkx and Karssen, Nevertheless, seed nitrate content in Arabidopsis affected the maintenance of dormancy in the laboratory Alboresi et al.
A reason for this apparent contradiction is provided by Hilhorst who showed that most endogenous nitrate is leached from seeds in the first 24 h of imbibition on water in the laboratory. Thus high nitrate content will relieve dormancy, but only temporally when placed in soil, and therefore nitrate concentration may have little ecological importance Bouwmeester et al.
In contrast, seed sensitivity to nitrate is likely to have a significant ecological role in response to soil nitrate that varies both spatially and temporally. In Arabidopsis, nitrate is thought to have a direct regulatory role and promotes germination by reducing the light requirement Hilhorst and Karssen, Based on field studies, Derkx and Karssen suggested a model where temperature results in reversible changes in sensitivity to light and nitrate, which occur at the level of receptors.
This was consistent with the model and earlier conclusions of Hilhorst in the laboratory studying secondary dormancy. It was later suggested that the nitrate receptor could be NRT1.
Furthermore, nitrate release of seed dormancy acts by accelerating the decrease in ABA during germination Ali-Rachedi et al. This response is therefore separate from the GA response to light, consistent with nitrate acting to enhance the effect of light. Alboresi et al. In seeds this would be expected to relieve dormancy, leading to germination. However, nitrate signalling via NRT1. The resulting decrease in ABA levels results in the removal of the final level of dormancy proportional to the external nitrate concentration Yan et al.
In the field, during the spatial sensing phase, there is a transient increase in NRT1. Collectively this suggests that the level of NRT1. At this time, a switch between high and low affinity forms of the transceptor will further increase sensitivity to nitrate. This switch may also be linked to the control of the primary nitrate response, known to regulate downstream expression of genes Krapp et al. There can be substantial variation in both genetic and phenotypic plasticity for seed dormancy and germination within Arabidopsis and other species over elevational and latitudinal gradients Baskin and Baskin, ; Cavieres and Arroyo, ; Chiang et al.
Genetically identical cohorts of seeds can adapt to contrasting life cycles Montesinos-Navarro et al. DOG1 is thought to have an important role in the adaptation of dormancy to climate Kronholm et al. When Cvi winter annual and Bur summer annual were put through a summer annual dormancy cycle Fig.
In the case of DOG1 , transcription profiles were negatively correlated with the soil temperature cycle in both ecotypes. This may reflect differences between transcript and protein profiles, but also suggests that the relationship between thermal sensing and dormancy is plastic as a result of allelic variation in DOG1 ; hence contributing to adaptation e.
Chiang et al. Differences in the spatial sensing phase also become apparent, with the transcript profiles of genes associated with spatial sensing being highly correlated with one another in the shallow dormant Bur ecotype compared with Cvi Footitt et al. Dormancy and gene expression patterns in winter Cvi and summer Bur annual ecotypes. The height of the bars indicates the relative levels of gene expression.
A Data are shown for seeds buried in the autumn to mimic Cvi in the persistent seed bank i. In A data are also shown for Cvi seeds buried in spring to mimic its natural winter annual dormancy cycle following shedding. In this case, depth of dormancy, germination timing, and DOG1 expression are the same as autumn buried seeds; however, MFT expression is significantly different as shown.
Of the genes examined, two had reversed transcript profiles in relation to temperature, highlighting this enhanced role Fig. MFT transcription is high in Bur during the spatial sensing phase of the cycle prior to seedling emergence, indicating that MFT contributes to shallow dormancy maintenance Fig. On the other hand, in Cvi it positively correlates with DOG1 and dormancy level, but has low expression during the spatial sensing phase Fig.
Crucially, this changes when the deeply dormant Cvi ecotype undergoes its natural winter annual dormancy cycle with newly shed seed in spring spending the summer in the soil seed bank compare Autumn and Spring burial in Fig. Here, in the absence of a low temperature winter phase, DOG1 is not highly induced therefore bypassing induction of deep dormancy. Possibly as a result, MFT transcription increases in the spatial sensing phase, implying that MFT now has a more dominant role in dormancy maintenance in this phase similar to that seen in the summer annual Bur.
Nevertheless, in both situations, maximum germination in Cvi coincides with the lowest MFT transcription. The implication is that when seeds are shed to the soil seed bank at their natural time, only a shallow dormancy cycle is required to position the spatial sensing phase at the appropriate time of year for seedling emergence. If seeds are shed outside of this period or do not receive appropriate spatial signals to remove the final layer of dormancy, they enter the persistent soil seed bank Figs 2 , 6.
Then seeds enter a DOG1 -dominated deep dormancy phase in order to position the spatial sensing phase correctly in the following year. This may represent events in the persistent seed bank and highlights the innate plasticity of dormancy cycling. The natural variation of Arabidopsis exploited by mapping populations has led to the identification of DOG1 and showed its apparently overarching dominance of dormancy, germination timing dormancy cycling , and seedling establishment.
Natural variation has also led to advances in understanding of adaptation to climate and how dormancy and flowering times are linked to determine life cycle patterns. Nevertheless, we need a more detailed understanding of the regulation of dormancy cycling, in particular interaction at the molecular level between deep and shallow dormancy.
Studying dormancy cycling in the field is a long-term undertaking, and ethical and regulatory reasons can preclude the use of seeds from genetically modified plants to dissect the role of individual genes. Progress in understanding is therefore likely to be slow. However, recent laboratory studies show that cycling can be simulated in Col-0 and Ler by enhancing their primary dormancy during production and by manipulating temperature and water stress to cycle them through secondary dormancy S.
Footitt and W. Future use of such dormancy cycling screens to compare ecotypes and mutants should more rapidly enhance understanding.
Climate of origin of the winter and summer annual Arabidopsis ecotypes Cvi and Bur, respectively. The seed literature is vast; we apologize to the authors of the many excellent publications it was not possible to include due to limited space. Nitrate, a signal relieving seed dormancy in Arabidopsis. Plant, Cell and Environment 28 , — Google Scholar. Changes in endogenous abscisic acid levels during dormancy release and maintenance of mature seeds: studies with the Cape Verde Islands ecotype, the dormant model of Arabidopsis thaliana.
Planta , — Alvarado V Bradford KJ. Hydrothermal time analysis of seed dormancy in true botanical potato seeds. Seed Science Research 15 , 77 — Tomato seed germination: regulation of different response modes by phytochrome B2 and phytochrome A.
Plant, Cell and Environment 29 , — From intracellular signaling networks to cell death: the dual role of reactive oxygen species in seed physiology.
Biologies , — Seeds—ecology, biogeography, and evolution of dormancy and germination. San Diego : Academic Press. Google Preview. A classification system for seed dormancy. Seed Science Research 14 , 1 — Bassel GW. To grow or not to grow? Trends in Plant Science 21 , — Genome-wide network model capturing seed germination reveals coordinated regulation of plant cellular phase transitions.
Plant Physiology , — Weed seed germination and the light environment: implications for weed management. Weed Biology and Management 14 , 77 — A framework for the interpretation of temperature effects on dormancy and germination in seed populations showing dormancy. Seed Science Research 25 , — Journal of Experimental Botany 62 , — Cloning of DOG1, a quantitative trait locus controlling seed dormancy in Arabidopsis. Benvenuti S Macchia M. Effect of hypoxia on buried weed seed germination.
Weed Research 35 , — The heterotrimeric G-protein complex modulates light sensitivity in Arabidopsis thaliana seed germination. Photochemistry and Photobiology 85 , — Effects of endogenous nitrate content of Sisymbrium officinale seeds on germination and dormancy. Acta Botanica Neerlandica 43 , 39 — Annual changes in dormancy and germination in seeds of Sisymbrium officinale L scop.
New Phytologist , — Bradford KJ. Water relations in seed germination. In: Kigel J Galili G , eds. Seed development and germination. New York : Marcel Dekker , — Applications of hydrothermal time to quantifying and modeling seed germination and dormancy.
Weed Science 50 , — Threshold models applied to seed germination ecology. Gene expression profiles of Arabidopsis Cvi seeds during dormancy cycling indicate a common underlying dormancy control mechanism. The Plant Journal 46 , — Gibberellin mobilizes distinct DELLA-dependent transcriptomes to regulate seed germination and floral development in Arabidopsis. Seed germination response to cold stratification period and thermal regime in Phacelia secunda Hydrophyllaceae : altitudinal variation in the mediterranean Andes of central Chile.
Plant Ecology , 1 — 8. Distribution of spring and winter types in the local populations of Arabidopsis thaliana L. Arabidopsis Information Service 2. DOG1 expression is predicted by the seed-maturation environment and contributes to geographical variation in germination in Arabidopsis thaliana.
The highest seedling fresh and dry weights and tissue water content were recorded when seeds were treated with a 3. The fresh weight increase could be attributed to cell enlargement [ 55 ]. The increase in dry weight was due to cell division and new material synthesis [ 56 ]. Higher results in seedlings grown were from seeds treated with 3. In the study, the highest chlorophyll a, chlorophyll b, and total chlorophyll contents were seen with a 3.
Gamma rays have an ionizing radiation effect on plant growth and development by inducing cytological, biochemical, physiological, and morphological changes in cells and tissues by producing free radicals in cells [ 58 — 60 ]. Higher doses of gamma radiation have been reported to be inhibitory [ 61 , 62 ], whereas lower doses are stimulatory.
Low doses of gamma rays have been reported to increase seed germination and plant growth, cell proliferation, germination, cell growth, enzyme activity, stress resistance, and crop yields [ 63 — 69 ]. Stimulation of plant growth at low gamma radiation doses is known as hormesis [ 70 ]. The hormesis phenomenon is described as a stimulating effect on any factor in the growth of an organism [ 71 ]. In the study conducted by Beyaz et al.
Seeds were surface-sterilized with a 3. The seed germination percentage was determined at the end of the 7th day, while seedling growth percentage, seedling height, and root length were recorded 14 days after culture initiation [ 20 ]. Three replicates were tested, and there were 30 seeds per replication.
One-way Analysis of Variance ANOVA was used to test the effect of different doses of gamma radiation on seed germination and seedling growth. The stimulatory effect of low gamma doses was observed in the study at a radiation dose of Gy. The best results in seed germination percentage at the end of the 7th day and in seedling growth percentage, seedling height, and root length at the end of the 14th day were observed at a dose of Gy of gamma radiation Table 6 and Figure 5. In doses over Gy, the inhibitory effect of gamma radiation was seen.
Seed germination percentage was The highest seedling growth percentage, seedling height, and root length were again recorded for a Gy gamma radiation dose as The root length obtained from seeds irradiated with Gy of gamma radiation was significantly increased by Effects of different gamma doses on in vitro seed germination, seedling growth, seedling height, and root length in L.
Values in a column followed by the different letters are significantly different at the 0. In vitro seed germination and seedling growth in L. This chapter focuses on four different methods that have not been reported elsewhere for overcoming dormancy. We think that these newly described methods will help growers and researchers to overcome dormancy problem in plant production.
Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by Jose Carlos Jimenez-Lopez. By Timothy L. Grey, Charles Y. By Arindam Barman, Chinky M. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals.
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