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GERMINATION AND SEEDLING ESTABLISHMENT
130 GERMINATION AND SEEDLING ESTABLISHMENT
GERMINATION AND SEEDLING ESTABLISHMENT A Hadas, The Volcani Center, Bet Dagan, Israel ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Seed germination and seedling establishment are critical phases in the plant growth cycle, since they influence and determine species survival in natural habitats and the onset and yields of agricultural crops. A dormant seed combines an embryonic plant and stored materials to be used during germination and seedling establishment until the onset of photosynthesis by the established seedling. ‘Germination’ is a general term describing the sequences of complex processes involved in initiating an array of metabolic activities that lead eventually to the renewal of growth of the dormant seed embryo and, ultimately, to seedling establishment. The various processes known to occur during these two phases take place in parallel or in serial sequences. Their times of initiation and of transition from one sequence to another are triggered by events endogenous to the seeds (seed-development processes of the parent plants) and depend greatly on the prevailing environmental conditions (aeration, temperature, available water and nutrients, soil mechanical impedance) and seed-development conditions.
Seed Germination and Environmental Conditions Proper seed germination and stand establishment depend strongly on the environmental conditions (moisture, thermal and aeration regimes in the soil, light). Where favorable environmental conditions prevail, other factors may decide the success or failure of seed germination and stand establishment (seed-development processes on the parent plants, seed dispersion, and depth of seed burial). Seeds are self-contained units, owing to the storage materials contained in the seeds. Therefore, the environmental requirements for germination and seedling establishment are considered to be fewer and simpler than those required by full-sized plants. Water Requirements
Water uptake by seeds is a prerequisite for proper germination and seedling establishment. The amount of water required by a seed for germination itself is considered to be very small, but the rate of uptake and the total amounts taken up depend greatly on the seed
and soil properties with respect to water – the water potential differences between the seed and the soil – and are controlled by the water conductivity of the soil and the soil–seed contact zone. Whether or not the amount of water taken up will suffice for germination and seedling development depends on the water energy status of the seed, and on the soil-water potential. There is a certain minimal level of seed hydration, termed the ‘critical hydration level,’ below which the seed will not germinate; instead it may enter secondary dormancy. Seeds that enter secondary dormancy become more susceptible to pests and diseases. These seeds may germinate later in the season or during the coming seasons. Thus the uniform germination, establishment, and stand of crops are impaired. The viability of the seeds that reenter secondary dormancy diminishes with time. Species and cultivars differ markedly in conditions they require for germination, and these differences are attributed to the differences among the water regimes and other soil physical conditions of the soil to which the plants were adapted or which they encountered during germination. Temperature Requirements
Temperature affects the soil properties, with respect to water, that influence seeds, air and water regimes, and the biological activity of seeds themselves. Soil temperature varies greatly, because of both diurnal and seasonal thermal processes, and is dependent on the constituents of the soil (water, solids, organic materials, and air), its texture, structure, color, and layering. For germination to occur, the temperature of the seed environment should fall within a favorable, species-specific range. Within that interval lies the optimal temperature at which optimal germination is observed. The temperature limits (minimum and maximum temperatures) below or above which no germination will occur are 3–5 and 30–43 for wheat cultivars; and 8–12 and 40–44 for corn cultivars. The corresponding optimal temperatures are 15–30 and 26–32 for wheat and corn cultivars, respectfully. Planting during spring depends on the soil temperature and, as an example, corn will be planted only when the soil temperature at the planting depth, during most of the diurnal cycle, is slightly above its minimal temperature for germination. Favorable temperature ranges, specific germination-enhancing periodicities of diurnal or seasonal temperature variation, induction of secondary dormancy, and the combined effects of water stress and temperature vary among
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species. Germination is greatly affected by the interactions among temperature, soil-water potential, and water flow in the soil, and by variations in the Q10 factors of effective rates of seed biological activity. Each of the processes that occur in plants has its own rate response to temperature, commonly designated as a coefficient, Q10, which is defined as the number of times that process rate increases with a 10 rise in temperature. The plant response to interactions between temperature and processes involved in water uptake, respiration, or cell division and growth are extremely complicated and unpredictable. Adverse effects of soil-water stress on germinating seeds intensify as temperature rises and may persist beyond the germination stages, into the seedling growth and emergence stages. Under field conditions, soil temperature is characterized by a diurnal temperature fluctuation, with amplitudes as great as 10 or more. If seeds respond to a single process, the observed rate will differ from that measured under constant temperature conditions. Aeration Requirements
The total air content in the soil and the rate of gaseous exchange greatly affect the soil biological activity and the availability of oxygen to germinating seeds and developing seedlings. However, the effects of the interactions between the total air content, its constituents and composition in the soil, and the rate of gaseous exchange are complex and difficult to define, unless a definite knowledge of the interrelationships among complex diffusion processes (in air-filled pores and across water films) that control the oxygen supply and the dissipation of respiratory and decomposition by-products (CO2, N2, NO2, H2S, ethylene, methane) is attained. Low oxygen availability reduces or even prevents germination in most species. Oxygen supply to support metabolic activity in germinating seeds becomes decisive at a very early stage of germination, namely at the beginning of respiration and utilization of storage materials, and especially when growth is initiated during seedling development. Oxygen requirements increase with soil temperature, and under light and/or water stress. Very often a conflict develops between oxygen supply and water supply to germinating seeds, because of the very low solubility and diffusivity of oxygen in water. Thus, oxygen supply is greatly impaired as the thickness of the water films around the seeds and the hydrated seed coat increases, especially with seeds that have a swollen, hydrated mucilaginous cover. Low CO2 concentrations may stimulate germination, either solely or at times in combination with ethylene. Soil seals or crusts and compacted soil layers may have deleterious effects on gas exchange and, in turn on
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seed germination, and seedling development and establishment. Light Requirements
Sensitivity to light is observed in germinating seeds of various species, which fall into four classes: those that are insensitive to light, like most agricultural crops (e.g., anemone, oregano); those that require a short exposure to light during their germination (e.g., nasturtium, compass lettuce); those that will germinate in total darkness (tulip, gladiolus, Nigella); and those that will germinate under full exposure to light. The sensitivity of light-sensitive seeds increases as they take up more water. Germination of light-sensitive seeds is impaired when they are shaded by a dense plant canopy or when light is prevented from reaching the seeds because of burial under the soil surface or in cracks in the soil. Soil Mechanical Constraints
Soil particles of various sizes and origins form a matrix that exhibits a degree of resistance under mechanical stress, described as ‘soil mechanical strength.’ Soil strength depends on the soil constituents, density, moisture content, and structure; it increases with increasing soil bulk density, and decreases with soil-water and organic matter contents. Silty soils with low organic matter content tend to deform plastically and to compress easily, and to form surface seals under the impact and slaking action of raindrops or under instantaneous flooding by rain or irrigation. These thin, rather dense seals may restrict gaseous exchange and water infiltration into the soil around the seeds, mechanically impede or obstruct germination and seedling emergence, or there can be a combination of these effects. Wheat seedlings are affected by the interaction between crust strength, water content, and rainfall. Adverse effects on seed germination and seedling establishment, similar to those of soil seals, are caused by mechanical compaction of the soil surface.
Water Uptake by Seeds and Seedlings Water uptake by a germinating seed is an essential step toward rehydration of its tissues. The initiation of the array of metabolic processes in the seed and the minute amounts of water required for germination depend on the seed genome and the constituents of its individual parts. The various organs (embryo, cotyledons) and tissues differ in their internal physical structure, biochemical properties, and composition; therefore, they differ in their water retention, distribution, and swelling. In most seeds the seed embryos are minute (the embryo weight varies between 0.005
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and 0.02 of the total dry seed weight). Storage tissues constitute the major volume of the seeds, and the observed amount of water taken up by those seeds is that which is taken into these tissues under the water regime that exists during the germination period of these seeds. Water uptake by dry seeds during germination is characterized by three phases: the initial phase – the imbibition phase – is characterized by a saturation kinetics pattern; the second phase – the transition phase – is characterized by a low-to-negligible water uptake rate; and the third phase – the growth phase – is characterized by a rapid, exponential increase in the water uptake rate, accompanied by the emergence of the radicle. The first two phases are observed in dead, inert, and viable seeds alike, whereas the growth phase is unique to viable, germinating seeds. Water uptake rates during these phases are controlled by one or more of the following factors: (1) the seed properties with respect to water (the seed’s water content–water potential characteristics; the seed’s diffusivities to water, which range between 1.7 102 to 1.5 106 m2 day1); (2) the soil-water properties (soil-water characteristics, diffusivities to water of the soil around the seed, which range from 4.0 104 to 5.0 107 m2 day1); and (3) the seed–soil interface properties with respect to water (seed–soil contact area, impedance to water flow across the contact zone). Once the third phase starts, the radicle emerges, the roots elongate, and the seedling starts to take up water with its radicle and developing roots, while the hypocotyl or mesocotyl emerges and elongates toward the soil surface. Water uptake by the radicle or the roots is controlled by one or more of the following factors: (1) the soil-water properties; (2) the root anatomy (root and/or radicle structure and tissue arrangement); (3) rate of elongation, radicle or root–soil contact, and hydraulic impedance to water flow across the seed–root contact zone. The Imbibition Phase
The imbibition phase starts with the entry of water into the seed. This water is distributed in crevices, cracks, and flaws in the seed cover, and is absorbed by the seed tissues. Water uptake rate measurements taken during this phase have shown these rates to be: (1) temperature-dependent; and (2) accompanied by increases in respiration rate and in light sensitivity in some seed species. These observations suggest that water uptake during imbibition is not a ‘passive’ process, as it is usually taken to be, but becomes an active one at an early stage of this phase. The end of this phase is marked by an asymptotic approach to a final water gain, or hydration level, which depends on
ambient soil-water potential, soil conductivity to water, seed–soil contact, and seed composition. The Transition Phase
During the transition phase, known also as the ‘pause phase,’ the seed-water content, respiration rate, and apparent morphology remain almost unchanged. Nevertheless, a variety of metabolic processes are activated, and differences in the activity levels of these processes and in their order of occurrence have been observed among seeds of different species and among seeds that have reached different hydration levels. Therefore, any adverse environmental conditions that lead to redrying of the seeds, e.g., by subjecting them to water stress, or reduce the seed–soil contact area, and thus influence their hydration levels, may impair, retard, or even inhibit germination. If no damage had resulted, no secondary dormancy been induced, and no inhibitory processes been blocked, the germination of these seeds upon rewetting would be enhanced because of the high concentration of unused metabolites accumulated prior to drying. The duration of the transition phase influences the initiation time and the extent of radicle growth. Seeds have been observed to reach the transition phase and to remain in it for long periods that may extends for days, weeks, or more before germination. Toward the end of the transition phase, the embryonic cells of the radicle start to divide. The Growth Phase
The growth phase starts with an increase in the respiratory rate, initiation of cell division, and extension of the embryonic radicle and ends with radicle protrusion. The common definition of germination states that a seed has germination once its radicle has broken out of the seed coat and started to elongate. From that stage, with germination completed, the seedling development and growth have commenced, and water is taken up solely by the elongating roots. Differentiation Between the Seed Germination Phases
The definition of and the differentiation among the phases are based on an arbitrary partitioning of the continuous sequential order of processes in the germinating seed and the developing seedling. In reality, all the phases are interdependent, and the interrelationships among them suggest that each phase depends greatly on the preceding phases. The end of the third phase (marked by radicle protrusion), termed ‘the growth phase,’ cannot logically be separated from
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the subsequent seedling growth toward its protrusion through the soil surface and its establishment. The end of the transition phase depends on whether the amount of water finally taken up at the end of imbibition has reached the required minimal level. A generally accepted concept is that to germinate a seed must reach a minimal water content – the ‘critical hydration level.’ That ‘hydration level’ does not reflect the water distribution among the seed components, nor is it an absolute value, since it depends on the water uptake rate, variations in soil-water potential, conductivity to water and temperature, and seed adaptability to variations in these environmental factors. The concept of critical hydration level fails in cases of partial seed wetting, especially when the wetted seed volume includes only the embryo and the adjacent storage tissues, and an affirmative concept is the ‘critical water potential,’ taken as the external water potential at or below which seeds cannot reach their critical hydration level. These concepts are applied respectively, when water uptake is evaluated according to its final amount of water taken up or in terms of seed–soil or seed–substrate water potential equilibrium states.
Physical Principles of Water Uptake by Seeds and Seedlings Water movement within the soil toward the seed or radicle–soil contact interfaces, across them and into the seed or into the root, can be described by the general flow equation combining the Darcy flow equation and the continuity equation based on the principle of conservation of matter: @ @ @ @ @ ¼ kð Þ Dð Þ ¼ ½1 @t @S @S @S @S
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Dseed ð@seed =@rÞ ¼ ðLDseed =aÞ½final t t 0 r¼a where , initial, final, t, seed, are the volumetric water contents of the soil, the initial and final values (cubic centimeters per cubic centimeter), at time t, respectively; seed is the volumetric water content of the seed (cubic centimeters per cubic centimeter); is the soil-water potential (kilopascals); k( ) is the soil capillary conductivity (centimeters per day); D(), Dseed are the diffusivity coefficients of water in the soil and the seed, respectively (centimeters squared per day); r, a are the radial distance from the center of the seed and the seed radius, respectively (centimeters); and L is the seed–soil interface impedance to water flow (per day) (Figure 1). The rate of flow depends on: the water potential gradients (the driving force that induces water movement) in the soil, across the seed–soil contact zone, and in the seed tissues; and on the respective conductivities to water of the soil, the seed and radicle–soil contact zone and the seed or radicle tissues. In order to simplify the solutions to the above equation and the initial and boundary conditions, the number of variables has to be reduced. The choice to use the right side of the equation given above (where use of diffusivity to water in the soil is made) simplifies the calculations by reducing the number of variables and makes the measurements required to determine the flow parameters for use in the equation practicable. Under normal conditions water will tend to move from the higher (in the soils) to the lower (in the seed
For water movement in the soil and to the seed, it is simpler to use the right and the left sides of this equation, and the proper initial and boundary conditions. The initial conditions describe the initial state of the system under consideration, whereas the boundary conditions express the conditions that are imposed on or those that will exist once the processes to be analyzed are started. For a planted seed that starts to imbibe water from the soil around it, the initial conditions are: ¼ initial t<0 0
Figure 1 Seed–soil water contact impedance factor L as a function of the relative wetted seed surface area.
134 GERMINATION AND SEEDLING ESTABLISHMENT
or radicle) water potential. The energy status of soil water termed ‘soil-water potential’ depends on the soil texture, structure, water content, soil solution composition, and solutes concentration. The matric potential is the soil-water potential component that arises from the interaction of soil water with the matrix of the solid particles in which it is contained. The osmotic potential is the soil-water potential component that arises from the interaction of soil-water solutes’ composition and concentrations with the active membranes of the seed cells. Soil-water potential per unit of soil is given as work per unit of soil volume, or soil weight. At the beginning of the imbibition phase, these gradients are very large and the uptake rate is high, but, as the seed gains water, the gradient diminishes and the rate of water uptake decreases. As water moves into the seed, the water content in the soil next to the seed surface decreases; the matric and the osmotic components of the soil water decrease also (due to reduction of water content and an increase in solute concentration caused by ion exclusion). Consequently, the conductivity and diffusivity to water, through the soil and across the seed–soil contact, decrease, which tends to make the impedance to water flow increase. But, on the other hand, as the seed water content increases, the seed swells and the seed–soil contact improves, which tends to cause a decrease in the flow impedance. The combined effect leads to the net water flow rate into the seed. Specific Effects of Matric and Osmotic Soil-Water Potential Components
Both the matric and the osmotic components of the soil-water potential are directly involved in soil-water movement to germinating seeds. Seeds immersed in solutions and in saturated soils respond equally to equal changes in these two components, provided their membranes are intact and fully active. But in soils, small reductions in soil-water matric potential have been observed to affect germination, and seedling development and establishment to a greater extent than equal or even greater reductions in the soilwater osmotic potential (Figure 2). This difference in response is because the changes in the seed–soil contact area and soil-water content that accompany small changes in the soil-water matric potential lead to proportionally greater changes in the soil conductivity and diffusivity to water. The matric potential may affect seed swelling during germination and radicle or hypocotyl elongation, by its direct contribution to the effective soil mechanical stress. Under normal conditions the stresses induced in the soil are too small to confine seeds, to impair their swelling and germination, or to restrict
seedling development. But poor germination observed in or next to compacted soils, or when seeds are entrapped in shrinking soil, can be attributed to greater mechanical constraints imposed by the soil, which are partly due to the matric potential. Seed-Water Potential
The seed-water potential results from the osmotic water potential derived from the composition of the seed cells and the concentration of their constituents, and the turgor water potential component derived from the cell membranes and elasticity of the wall structure. In dry seeds the water potential is very low (negative in relation to free water), but as the seed imbibes water its water potential increases and becomes less negative. By changing the concentrations of their constituents and by modifying the activity and selectivity of their membranes, the seed cells can regulate their water potential relative to that of the soil in contact with the seed. These changes require energy inputs that develop from respiration and use of storage material. The lower the soil-water potential becomes, the greater effort and the faster the rate of depletion of the material stored in the seed for growth and establishment. Consequently, germination rate and final stand establishment will be impaired. Seed and Soil Conductivities and Diffusivities to Water
Reported values of seed diffusivity to water (1.5 105 to 1.6 103 m2 day1) are lower than those reported for soils (4.0 104 to 5 107 m2 day1) by several orders of magnitude. These observations suggest that under moist or wet soil conditions the rates of water uptake by seeds are not limited by soil-water movement rates toward them. Observed and calculated rates of water movement toward seeds indicate that for seeds that maintain lower water potentials than those in the soil, the soil could provide water to the seeds faster than the experimentally observed rates. These calculations strongly suggest that water uptake is controlled by the low permeability of the seed coat as well as the low seed diffusivity to water. Seed-Coat Diffusivity to Water
In general, seed coats are nonuniform in shape and roughness, and in their ability to transmit water. Most seed coats are semi- or impermeable to water but they may have an opening (micropyle, hilum, chalaza). The few measured values of diffusivity to water of permeable, saturated seed coats (ranging from 9 102 to 3 102 m2 day1) were found to be equal to or lower than those for the wet seed bulk (2.4 102 to 1.6 103 m2 day1). These values of seed-coat
GERMINATION AND SEEDLING ESTABLISHMENT
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Figure 2 (a) Total germination of chickpea seeds as a function of time and matric potential: rhomboids, 100 J kg1; squares 500 J kg1; triangles 1000 J kg1. Seeds imbedded in aggregates of 0.25–0.50 mm diameter. (b) Total germination of chickpea seeds as a function of time and osmotic soil-water potential: full rhomboids 100 J kg1; triangles 330 J kg1; circles 1000 J kg1; and open rhomboids 2000 J kg1. Seeds imbedded in osmotic solutions.
diffusivity to water may affect water uptake rates and if there is also decreasing soil–seed interface contact the uptake rates could be even more strongly affected.
void found between the seed coat and the soil water by the mat of hairs).
Seed-Coat and Seed–Soil Interface Geometrical Configuration
The hydraulic properties of the seed–soil contact zone with respect to water vary during imbibition, because the water content at and next to the contact zone diminishes with water uptake. Since the seed–soil contact zone configuration is difficult to define and its water retention and conduction properties cannot be determined, a compound parameter (impedance to water flow) is used to describe water transfer across the contact zone. The seed–soil contact impedance to flow increases with decreasing water content in the seed–soil contact zone, seed–soil contact area, soil conductivity to water, and coarseness of soil texture and/or structure. The greater the impedance to water flow, the more restricted is the water flow from the soil toward and across the seed–soil contact interface and, consequently, the lower are the imbibition and water uptake
The geometrical configuration of the seed–soil interface zone depends on: the seed-coat surface composition and roughness; the ratio of seed size to sizes of the soil structural units in contact with it; and their spatial arrangement in the contact zone with the seed. The smaller the soil aggregates are, relative to the seed, the greater are the number of contact points and the total contact area. The contact area increases further when those contact points are wetted. Seeds with mucilaginous cover (e.g., like those of some species of the flax family) may achieve complete contact with the soil when they swell while being wetted. Hairy seeds may fail to form any direct seed–soil contact until their hairs either collapse or decay (due to the
Impedance to Water Flow Across the Seed–Soil Interface
136 GERMINATION AND SEEDLING ESTABLISHMENT
Figure 3 Total germination of chickpea seeds as a function of time and relative wetted seed surface area: filled squares 17%; open squares 6%; and rhomboids 3%. Matric potential range 10 to 33 J kg1.
rates (Figure 3). Most seeds swell during imbibition, which results in concurrent improvement of the seed– soil contact and increase in the soil mechanical stress on the seed. These changes may affect imbibition and final germination, especially in dense and compacted soils. In saline soils, seed–soil contact impedance effects may be partly obscured by the increase in the soil water osmotic potential caused by accumulation of salts excluded from entering through active membranes of the seed-surface cells during imbibition.
Seedling Emergence and Establishment The germinated seed extends its radicle or seminal rootlets deeper into the soil to adsorb the water needed for the extension of the rootlets downward, and of the hypocotyl toward the soil surface. Water in ample quantities is needed by the rootlet and hypocotyl, to maintain the pressure required for their elongation within the soil and to overcome the weight of the overlying soil and the tensile strength of the soil surface layer, in order to emerge through it. The emerging seedling starts to transpire and to photosynthesize, and under a favorable soil-water regime, its radicle or roots elongate and start to explore the deeper soil layers in order to draw enough water to sustain the transpirational demands. It is obvious how difficult seed germination, emergence, and establishment become under arid conditions, under which the soil surface is wetted infrequently, the evaporative demands are high, and often the soils tend to slake and form a surface crust. Under such circumstances the germinating seeds and seedlings must cope with scarce and rapidly diminishing supplies of available soil water. Furthermore, soil-water depletion is too rapid either for the seedlings to extend their radicle or rootlets down into deeper, wetter soil layers
where water is still available or for the seedling to break through the rapidly hardening soil crust.
Future Research Timely, fast, and uniform seed germination, emergence, and seedling establishment are as crucial for successful crops as they are for survival of wild plants. Each seed reacts individually to the varying conditions prevailing in its microenvironment. The various aspects of seed germination and the interactions between seeds and the various environmental factors affecting their germination have been discussed above. The information presented, based on many studies carried out to resolve the complex system of seed germination and seedling establishment, is still incomplete and does not furnish a full understanding of the processes and interactions between germinating seeds and their microenvironment.
List of Technical Nomenclature Critical hydration level (cm3 cm3)
The minimal seed volumetric water content below which the seed will not germinate
Critical water potential (kPa)
A given soil-water potential value below which the seed will not germinate
Effective soil mechanical stress (kPa)
The stress transmitted through the soil by intergranular and water menisci in the soil voids
Envelope soilwater pressure (kPa)
The magnitude of change in the total soil-water potential at a given position caused by the mechanical stress imposed by the soil layers above it
Growth
The last phase of seed germination, during which the radicles and rootlets are protruding and elongating into the soil
GERMINATION AND SEEDLING ESTABLISHMENT Imbibition
A first phase of seed germination, during which the seed absorbs water and its tissues are rehydrated
Impedance to flow, soil–seed interface impedance to water flow (day1)
A condition which hinders the movement of water through the seed-soil contact zone under the influence of water potential gradient
Matric potential The difference between the water poten(kPa) tial of source pool of soil solution at a given elevation and air pressure and a pool identical to the source but at the elevation under consideration (above water table) Mechanical impedance
Soil strength conditions which hinder or inhibit the devleopment and growth of plant organs in the soil
Osmotic soilwater potential (kPa)
The magnitude of change in soil-water potential due to differences in the chemical composition of the soil solution related to free pure water at the same elevation and air pressure
Seed diffusivity to water (cm2 day1)
The flux of water per unit gradient of volumetric water content in the seed
Seed germination
A general term describing the sequence of processes involved in starting metabolic activities and leading to the initiation of growth in the quiescent embryo in the seed
Seedling emergence
A term stating that a growing seedling has broken through the soil surface
Seedling establishment
A term describing the stable development and growth of a seedling that has recently emerged
Seed’s volumetric water content (cm3 cm3)
The volume of water per unit bulk volume of the seed
Soil conductivity to water (cm day1)
The flux of water per unit gradient of matric potential
Soil diffusivity to water (cm2 day1)
The flux of water per unit of gradient of volumetric water content in the soil
Soil strength
A transient, localized soil property which is a combined measure of a soil to resist deformation under external or internal stress imposed on it
Soil structure
The spatial arrangement of soil primary particles into secondary particles or units
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Soil texture
The relative proportions of the various size classed separates in the soil
Soil-water potential (kPa)
The amount of work that must be done in order to transport reversibly and isothermally an infinitesimal quantity of pure water from a specified source to a specified destination
Transition
The second phase in seed germination
See also: Crusts: Structural; Cultivation and Tillage; Nutrient Availability; Plant–Water Relations; Rhizosphere; Root Architecture and Growth; Structure
Further Reading Arndt W (1965) The nature of the mechanical impedance to seedlings by soil surface seals. Australian Journal of Soil Research 3: 45–54. Benesch-Arnold RL and Sanchez RA (1995) Modeling weed seed germination. In: Kigel J and Galili G (eds) Seed Germination and Development, pp. 545–566. New York: Marcel Dekker. Bewley JD and Black M (1982) Physiology and Biochemistry of Seeds. Berlin, Germany: Springer-Verlag. Bradford KL (1995) Water relations in seed germination. In: Kigel J and Galili J (eds) Seed Development and Germination, pp. 351–396. New York: Marcel Dekker. Corbineau F and Come D (1995) Control of seed germination by dormancy and by the gaseous environment. In: Kigel J and Galili J (eds) Seed Development and Germination, pp. 397–424. New York: Marcel Dekker. Hadas A (1982) Seed soil contact and germination. In: Khan AA (ed.) The Physiology and Biochemistry of Seed Development, Dormancy and Germination, pp. 507–527. Amsterdam, the Netherlands: Elsevier. Hadas A (1997) Soil tilth – the desired soil structural state obtained through proper soil fragmentation and reorientation processes. Soil Tillage Research 43: 7–40. Hadas A and Stibbe E (1977) Soil crusting and emergence of wheat seedlings. Agronomy Journal 69: 547–550. Koller D and Hadas A (1982) Water relations in the germination of seeds. In: Lange OL, Nobel PS, Osmond CB, and Zigler H (eds) Encyclopedia of Plant Physiology, vol. 12B, pp. 410–431. Berlin, Germany: Springer– Verlag. Kutilek M and Nielsen DR (1994) Soil Hydrology. Cremlingen-Destedt, Germany: Catena-Verlag. Marshal TJ, Holmes JW, and Rose CW (1996) Soil Physics. Cambridge, UK: Cambridge University Press. Mayer AM and Poljakoff-Mayber A (1989) The Germination of Seeds, 4th edn. London, UK: Pergamon Press. Van Duin RHA (1956) On the Influence of Heat, Diffusion of Air and Infiltration of Water in the Soil, Versl. Landb. K. Onderz. 62, pp. 1–82. Pudoc: Wageningen, the Netherlands.
GERMINATION AND SEEDLING ESTABLISHMENT A Hadas, The Volcani Center, Bet Dagan, Israel ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Seed germination and seedling establishment are critical phases in the plant growth cycle, since they influence and determine species survival in natural habitats and the onset and yields of agricultural crops. A dormant seed combines an embryonic plant and stored materials to be used during germination and seedling establishment until the onset of photosynthesis by the established seedling. ‘Germination’ is a general term describing the sequences of complex processes involved in initiating an array of metabolic activities that lead eventually to the renewal of growth of the dormant seed embryo and, ultimately, to seedling establishment. The various processes known to occur during these two phases take place in parallel or in serial sequences. Their times of initiation and of transition from one sequence to another are triggered by events endogenous to the seeds (seed-development processes of the parent plants) and depend greatly on the prevailing environmental conditions (aeration, temperature, available water and nutrients, soil mechanical impedance) and seed-development conditions.
Seed Germination and Environmental Conditions Proper seed germination and stand establishment depend strongly on the environmental conditions (moisture, thermal and aeration regimes in the soil, light). Where favorable environmental conditions prevail, other factors may decide the success or failure of seed germination and stand establishment (seed-development processes on the parent plants, seed dispersion, and depth of seed burial). Seeds are self-contained units, owing to the storage materials contained in the seeds. Therefore, the environmental requirements for germination and seedling establishment are considered to be fewer and simpler than those required by full-sized plants. Water Requirements
Water uptake by seeds is a prerequisite for proper germination and seedling establishment. The amount of water required by a seed for germination itself is considered to be very small, but the rate of uptake and the total amounts taken up depend greatly on the seed
and soil properties with respect to water – the water potential differences between the seed and the soil – and are controlled by the water conductivity of the soil and the soil–seed contact zone. Whether or not the amount of water taken up will suffice for germination and seedling development depends on the water energy status of the seed, and on the soil-water potential. There is a certain minimal level of seed hydration, termed the ‘critical hydration level,’ below which the seed will not germinate; instead it may enter secondary dormancy. Seeds that enter secondary dormancy become more susceptible to pests and diseases. These seeds may germinate later in the season or during the coming seasons. Thus the uniform germination, establishment, and stand of crops are impaired. The viability of the seeds that reenter secondary dormancy diminishes with time. Species and cultivars differ markedly in conditions they require for germination, and these differences are attributed to the differences among the water regimes and other soil physical conditions of the soil to which the plants were adapted or which they encountered during germination. Temperature Requirements
Temperature affects the soil properties, with respect to water, that influence seeds, air and water regimes, and the biological activity of seeds themselves. Soil temperature varies greatly, because of both diurnal and seasonal thermal processes, and is dependent on the constituents of the soil (water, solids, organic materials, and air), its texture, structure, color, and layering. For germination to occur, the temperature of the seed environment should fall within a favorable, species-specific range. Within that interval lies the optimal temperature at which optimal germination is observed. The temperature limits (minimum and maximum temperatures) below or above which no germination will occur are 3–5 and 30–43 for wheat cultivars; and 8–12 and 40–44 for corn cultivars. The corresponding optimal temperatures are 15–30 and 26–32 for wheat and corn cultivars, respectfully. Planting during spring depends on the soil temperature and, as an example, corn will be planted only when the soil temperature at the planting depth, during most of the diurnal cycle, is slightly above its minimal temperature for germination. Favorable temperature ranges, specific germination-enhancing periodicities of diurnal or seasonal temperature variation, induction of secondary dormancy, and the combined effects of water stress and temperature vary among
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species. Germination is greatly affected by the interactions among temperature, soil-water potential, and water flow in the soil, and by variations in the Q10 factors of effective rates of seed biological activity. Each of the processes that occur in plants has its own rate response to temperature, commonly designated as a coefficient, Q10, which is defined as the number of times that process rate increases with a 10 rise in temperature. The plant response to interactions between temperature and processes involved in water uptake, respiration, or cell division and growth are extremely complicated and unpredictable. Adverse effects of soil-water stress on germinating seeds intensify as temperature rises and may persist beyond the germination stages, into the seedling growth and emergence stages. Under field conditions, soil temperature is characterized by a diurnal temperature fluctuation, with amplitudes as great as 10 or more. If seeds respond to a single process, the observed rate will differ from that measured under constant temperature conditions. Aeration Requirements
The total air content in the soil and the rate of gaseous exchange greatly affect the soil biological activity and the availability of oxygen to germinating seeds and developing seedlings. However, the effects of the interactions between the total air content, its constituents and composition in the soil, and the rate of gaseous exchange are complex and difficult to define, unless a definite knowledge of the interrelationships among complex diffusion processes (in air-filled pores and across water films) that control the oxygen supply and the dissipation of respiratory and decomposition by-products (CO2, N2, NO2, H2S, ethylene, methane) is attained. Low oxygen availability reduces or even prevents germination in most species. Oxygen supply to support metabolic activity in germinating seeds becomes decisive at a very early stage of germination, namely at the beginning of respiration and utilization of storage materials, and especially when growth is initiated during seedling development. Oxygen requirements increase with soil temperature, and under light and/or water stress. Very often a conflict develops between oxygen supply and water supply to germinating seeds, because of the very low solubility and diffusivity of oxygen in water. Thus, oxygen supply is greatly impaired as the thickness of the water films around the seeds and the hydrated seed coat increases, especially with seeds that have a swollen, hydrated mucilaginous cover. Low CO2 concentrations may stimulate germination, either solely or at times in combination with ethylene. Soil seals or crusts and compacted soil layers may have deleterious effects on gas exchange and, in turn on
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seed germination, and seedling development and establishment. Light Requirements
Sensitivity to light is observed in germinating seeds of various species, which fall into four classes: those that are insensitive to light, like most agricultural crops (e.g., anemone, oregano); those that require a short exposure to light during their germination (e.g., nasturtium, compass lettuce); those that will germinate in total darkness (tulip, gladiolus, Nigella); and those that will germinate under full exposure to light. The sensitivity of light-sensitive seeds increases as they take up more water. Germination of light-sensitive seeds is impaired when they are shaded by a dense plant canopy or when light is prevented from reaching the seeds because of burial under the soil surface or in cracks in the soil. Soil Mechanical Constraints
Soil particles of various sizes and origins form a matrix that exhibits a degree of resistance under mechanical stress, described as ‘soil mechanical strength.’ Soil strength depends on the soil constituents, density, moisture content, and structure; it increases with increasing soil bulk density, and decreases with soil-water and organic matter contents. Silty soils with low organic matter content tend to deform plastically and to compress easily, and to form surface seals under the impact and slaking action of raindrops or under instantaneous flooding by rain or irrigation. These thin, rather dense seals may restrict gaseous exchange and water infiltration into the soil around the seeds, mechanically impede or obstruct germination and seedling emergence, or there can be a combination of these effects. Wheat seedlings are affected by the interaction between crust strength, water content, and rainfall. Adverse effects on seed germination and seedling establishment, similar to those of soil seals, are caused by mechanical compaction of the soil surface.
Water Uptake by Seeds and Seedlings Water uptake by a germinating seed is an essential step toward rehydration of its tissues. The initiation of the array of metabolic processes in the seed and the minute amounts of water required for germination depend on the seed genome and the constituents of its individual parts. The various organs (embryo, cotyledons) and tissues differ in their internal physical structure, biochemical properties, and composition; therefore, they differ in their water retention, distribution, and swelling. In most seeds the seed embryos are minute (the embryo weight varies between 0.005
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and 0.02 of the total dry seed weight). Storage tissues constitute the major volume of the seeds, and the observed amount of water taken up by those seeds is that which is taken into these tissues under the water regime that exists during the germination period of these seeds. Water uptake by dry seeds during germination is characterized by three phases: the initial phase – the imbibition phase – is characterized by a saturation kinetics pattern; the second phase – the transition phase – is characterized by a low-to-negligible water uptake rate; and the third phase – the growth phase – is characterized by a rapid, exponential increase in the water uptake rate, accompanied by the emergence of the radicle. The first two phases are observed in dead, inert, and viable seeds alike, whereas the growth phase is unique to viable, germinating seeds. Water uptake rates during these phases are controlled by one or more of the following factors: (1) the seed properties with respect to water (the seed’s water content–water potential characteristics; the seed’s diffusivities to water, which range between 1.7 102 to 1.5 106 m2 day1); (2) the soil-water properties (soil-water characteristics, diffusivities to water of the soil around the seed, which range from 4.0 104 to 5.0 107 m2 day1); and (3) the seed–soil interface properties with respect to water (seed–soil contact area, impedance to water flow across the contact zone). Once the third phase starts, the radicle emerges, the roots elongate, and the seedling starts to take up water with its radicle and developing roots, while the hypocotyl or mesocotyl emerges and elongates toward the soil surface. Water uptake by the radicle or the roots is controlled by one or more of the following factors: (1) the soil-water properties; (2) the root anatomy (root and/or radicle structure and tissue arrangement); (3) rate of elongation, radicle or root–soil contact, and hydraulic impedance to water flow across the seed–root contact zone. The Imbibition Phase
The imbibition phase starts with the entry of water into the seed. This water is distributed in crevices, cracks, and flaws in the seed cover, and is absorbed by the seed tissues. Water uptake rate measurements taken during this phase have shown these rates to be: (1) temperature-dependent; and (2) accompanied by increases in respiration rate and in light sensitivity in some seed species. These observations suggest that water uptake during imbibition is not a ‘passive’ process, as it is usually taken to be, but becomes an active one at an early stage of this phase. The end of this phase is marked by an asymptotic approach to a final water gain, or hydration level, which depends on
ambient soil-water potential, soil conductivity to water, seed–soil contact, and seed composition. The Transition Phase
During the transition phase, known also as the ‘pause phase,’ the seed-water content, respiration rate, and apparent morphology remain almost unchanged. Nevertheless, a variety of metabolic processes are activated, and differences in the activity levels of these processes and in their order of occurrence have been observed among seeds of different species and among seeds that have reached different hydration levels. Therefore, any adverse environmental conditions that lead to redrying of the seeds, e.g., by subjecting them to water stress, or reduce the seed–soil contact area, and thus influence their hydration levels, may impair, retard, or even inhibit germination. If no damage had resulted, no secondary dormancy been induced, and no inhibitory processes been blocked, the germination of these seeds upon rewetting would be enhanced because of the high concentration of unused metabolites accumulated prior to drying. The duration of the transition phase influences the initiation time and the extent of radicle growth. Seeds have been observed to reach the transition phase and to remain in it for long periods that may extends for days, weeks, or more before germination. Toward the end of the transition phase, the embryonic cells of the radicle start to divide. The Growth Phase
The growth phase starts with an increase in the respiratory rate, initiation of cell division, and extension of the embryonic radicle and ends with radicle protrusion. The common definition of germination states that a seed has germination once its radicle has broken out of the seed coat and started to elongate. From that stage, with germination completed, the seedling development and growth have commenced, and water is taken up solely by the elongating roots. Differentiation Between the Seed Germination Phases
The definition of and the differentiation among the phases are based on an arbitrary partitioning of the continuous sequential order of processes in the germinating seed and the developing seedling. In reality, all the phases are interdependent, and the interrelationships among them suggest that each phase depends greatly on the preceding phases. The end of the third phase (marked by radicle protrusion), termed ‘the growth phase,’ cannot logically be separated from
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the subsequent seedling growth toward its protrusion through the soil surface and its establishment. The end of the transition phase depends on whether the amount of water finally taken up at the end of imbibition has reached the required minimal level. A generally accepted concept is that to germinate a seed must reach a minimal water content – the ‘critical hydration level.’ That ‘hydration level’ does not reflect the water distribution among the seed components, nor is it an absolute value, since it depends on the water uptake rate, variations in soil-water potential, conductivity to water and temperature, and seed adaptability to variations in these environmental factors. The concept of critical hydration level fails in cases of partial seed wetting, especially when the wetted seed volume includes only the embryo and the adjacent storage tissues, and an affirmative concept is the ‘critical water potential,’ taken as the external water potential at or below which seeds cannot reach their critical hydration level. These concepts are applied respectively, when water uptake is evaluated according to its final amount of water taken up or in terms of seed–soil or seed–substrate water potential equilibrium states.
Physical Principles of Water Uptake by Seeds and Seedlings Water movement within the soil toward the seed or radicle–soil contact interfaces, across them and into the seed or into the root, can be described by the general flow equation combining the Darcy flow equation and the continuity equation based on the principle of conservation of matter: @ @ @ @ @ ¼ kð Þ Dð Þ ¼ ½1 @t @S @S @S @S
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Dseed ð@seed =@rÞ ¼ ðLDseed =aÞ½final t t 0 r¼a where , initial, final, t, seed, are the volumetric water contents of the soil, the initial and final values (cubic centimeters per cubic centimeter), at time t, respectively; seed is the volumetric water content of the seed (cubic centimeters per cubic centimeter); is the soil-water potential (kilopascals); k( ) is the soil capillary conductivity (centimeters per day); D(), Dseed are the diffusivity coefficients of water in the soil and the seed, respectively (centimeters squared per day); r, a are the radial distance from the center of the seed and the seed radius, respectively (centimeters); and L is the seed–soil interface impedance to water flow (per day) (Figure 1). The rate of flow depends on: the water potential gradients (the driving force that induces water movement) in the soil, across the seed–soil contact zone, and in the seed tissues; and on the respective conductivities to water of the soil, the seed and radicle–soil contact zone and the seed or radicle tissues. In order to simplify the solutions to the above equation and the initial and boundary conditions, the number of variables has to be reduced. The choice to use the right side of the equation given above (where use of diffusivity to water in the soil is made) simplifies the calculations by reducing the number of variables and makes the measurements required to determine the flow parameters for use in the equation practicable. Under normal conditions water will tend to move from the higher (in the soils) to the lower (in the seed
For water movement in the soil and to the seed, it is simpler to use the right and the left sides of this equation, and the proper initial and boundary conditions. The initial conditions describe the initial state of the system under consideration, whereas the boundary conditions express the conditions that are imposed on or those that will exist once the processes to be analyzed are started. For a planted seed that starts to imbibe water from the soil around it, the initial conditions are: ¼ initial t<0 0
Figure 1 Seed–soil water contact impedance factor L as a function of the relative wetted seed surface area.
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or radicle) water potential. The energy status of soil water termed ‘soil-water potential’ depends on the soil texture, structure, water content, soil solution composition, and solutes concentration. The matric potential is the soil-water potential component that arises from the interaction of soil water with the matrix of the solid particles in which it is contained. The osmotic potential is the soil-water potential component that arises from the interaction of soil-water solutes’ composition and concentrations with the active membranes of the seed cells. Soil-water potential per unit of soil is given as work per unit of soil volume, or soil weight. At the beginning of the imbibition phase, these gradients are very large and the uptake rate is high, but, as the seed gains water, the gradient diminishes and the rate of water uptake decreases. As water moves into the seed, the water content in the soil next to the seed surface decreases; the matric and the osmotic components of the soil water decrease also (due to reduction of water content and an increase in solute concentration caused by ion exclusion). Consequently, the conductivity and diffusivity to water, through the soil and across the seed–soil contact, decrease, which tends to make the impedance to water flow increase. But, on the other hand, as the seed water content increases, the seed swells and the seed–soil contact improves, which tends to cause a decrease in the flow impedance. The combined effect leads to the net water flow rate into the seed. Specific Effects of Matric and Osmotic Soil-Water Potential Components
Both the matric and the osmotic components of the soil-water potential are directly involved in soil-water movement to germinating seeds. Seeds immersed in solutions and in saturated soils respond equally to equal changes in these two components, provided their membranes are intact and fully active. But in soils, small reductions in soil-water matric potential have been observed to affect germination, and seedling development and establishment to a greater extent than equal or even greater reductions in the soilwater osmotic potential (Figure 2). This difference in response is because the changes in the seed–soil contact area and soil-water content that accompany small changes in the soil-water matric potential lead to proportionally greater changes in the soil conductivity and diffusivity to water. The matric potential may affect seed swelling during germination and radicle or hypocotyl elongation, by its direct contribution to the effective soil mechanical stress. Under normal conditions the stresses induced in the soil are too small to confine seeds, to impair their swelling and germination, or to restrict
seedling development. But poor germination observed in or next to compacted soils, or when seeds are entrapped in shrinking soil, can be attributed to greater mechanical constraints imposed by the soil, which are partly due to the matric potential. Seed-Water Potential
The seed-water potential results from the osmotic water potential derived from the composition of the seed cells and the concentration of their constituents, and the turgor water potential component derived from the cell membranes and elasticity of the wall structure. In dry seeds the water potential is very low (negative in relation to free water), but as the seed imbibes water its water potential increases and becomes less negative. By changing the concentrations of their constituents and by modifying the activity and selectivity of their membranes, the seed cells can regulate their water potential relative to that of the soil in contact with the seed. These changes require energy inputs that develop from respiration and use of storage material. The lower the soil-water potential becomes, the greater effort and the faster the rate of depletion of the material stored in the seed for growth and establishment. Consequently, germination rate and final stand establishment will be impaired. Seed and Soil Conductivities and Diffusivities to Water
Reported values of seed diffusivity to water (1.5 105 to 1.6 103 m2 day1) are lower than those reported for soils (4.0 104 to 5 107 m2 day1) by several orders of magnitude. These observations suggest that under moist or wet soil conditions the rates of water uptake by seeds are not limited by soil-water movement rates toward them. Observed and calculated rates of water movement toward seeds indicate that for seeds that maintain lower water potentials than those in the soil, the soil could provide water to the seeds faster than the experimentally observed rates. These calculations strongly suggest that water uptake is controlled by the low permeability of the seed coat as well as the low seed diffusivity to water. Seed-Coat Diffusivity to Water
In general, seed coats are nonuniform in shape and roughness, and in their ability to transmit water. Most seed coats are semi- or impermeable to water but they may have an opening (micropyle, hilum, chalaza). The few measured values of diffusivity to water of permeable, saturated seed coats (ranging from 9 102 to 3 102 m2 day1) were found to be equal to or lower than those for the wet seed bulk (2.4 102 to 1.6 103 m2 day1). These values of seed-coat
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Figure 2 (a) Total germination of chickpea seeds as a function of time and matric potential: rhomboids, 100 J kg1; squares 500 J kg1; triangles 1000 J kg1. Seeds imbedded in aggregates of 0.25–0.50 mm diameter. (b) Total germination of chickpea seeds as a function of time and osmotic soil-water potential: full rhomboids 100 J kg1; triangles 330 J kg1; circles 1000 J kg1; and open rhomboids 2000 J kg1. Seeds imbedded in osmotic solutions.
diffusivity to water may affect water uptake rates and if there is also decreasing soil–seed interface contact the uptake rates could be even more strongly affected.
void found between the seed coat and the soil water by the mat of hairs).
Seed-Coat and Seed–Soil Interface Geometrical Configuration
The hydraulic properties of the seed–soil contact zone with respect to water vary during imbibition, because the water content at and next to the contact zone diminishes with water uptake. Since the seed–soil contact zone configuration is difficult to define and its water retention and conduction properties cannot be determined, a compound parameter (impedance to water flow) is used to describe water transfer across the contact zone. The seed–soil contact impedance to flow increases with decreasing water content in the seed–soil contact zone, seed–soil contact area, soil conductivity to water, and coarseness of soil texture and/or structure. The greater the impedance to water flow, the more restricted is the water flow from the soil toward and across the seed–soil contact interface and, consequently, the lower are the imbibition and water uptake
The geometrical configuration of the seed–soil interface zone depends on: the seed-coat surface composition and roughness; the ratio of seed size to sizes of the soil structural units in contact with it; and their spatial arrangement in the contact zone with the seed. The smaller the soil aggregates are, relative to the seed, the greater are the number of contact points and the total contact area. The contact area increases further when those contact points are wetted. Seeds with mucilaginous cover (e.g., like those of some species of the flax family) may achieve complete contact with the soil when they swell while being wetted. Hairy seeds may fail to form any direct seed–soil contact until their hairs either collapse or decay (due to the
Impedance to Water Flow Across the Seed–Soil Interface
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Figure 3 Total germination of chickpea seeds as a function of time and relative wetted seed surface area: filled squares 17%; open squares 6%; and rhomboids 3%. Matric potential range 10 to 33 J kg1.
rates (Figure 3). Most seeds swell during imbibition, which results in concurrent improvement of the seed– soil contact and increase in the soil mechanical stress on the seed. These changes may affect imbibition and final germination, especially in dense and compacted soils. In saline soils, seed–soil contact impedance effects may be partly obscured by the increase in the soil water osmotic potential caused by accumulation of salts excluded from entering through active membranes of the seed-surface cells during imbibition.
Seedling Emergence and Establishment The germinated seed extends its radicle or seminal rootlets deeper into the soil to adsorb the water needed for the extension of the rootlets downward, and of the hypocotyl toward the soil surface. Water in ample quantities is needed by the rootlet and hypocotyl, to maintain the pressure required for their elongation within the soil and to overcome the weight of the overlying soil and the tensile strength of the soil surface layer, in order to emerge through it. The emerging seedling starts to transpire and to photosynthesize, and under a favorable soil-water regime, its radicle or roots elongate and start to explore the deeper soil layers in order to draw enough water to sustain the transpirational demands. It is obvious how difficult seed germination, emergence, and establishment become under arid conditions, under which the soil surface is wetted infrequently, the evaporative demands are high, and often the soils tend to slake and form a surface crust. Under such circumstances the germinating seeds and seedlings must cope with scarce and rapidly diminishing supplies of available soil water. Furthermore, soil-water depletion is too rapid either for the seedlings to extend their radicle or rootlets down into deeper, wetter soil layers
where water is still available or for the seedling to break through the rapidly hardening soil crust.
Future Research Timely, fast, and uniform seed germination, emergence, and seedling establishment are as crucial for successful crops as they are for survival of wild plants. Each seed reacts individually to the varying conditions prevailing in its microenvironment. The various aspects of seed germination and the interactions between seeds and the various environmental factors affecting their germination have been discussed above. The information presented, based on many studies carried out to resolve the complex system of seed germination and seedling establishment, is still incomplete and does not furnish a full understanding of the processes and interactions between germinating seeds and their microenvironment.
List of Technical Nomenclature Critical hydration level (cm3 cm3)
The minimal seed volumetric water content below which the seed will not germinate
Critical water potential (kPa)
A given soil-water potential value below which the seed will not germinate
Effective soil mechanical stress (kPa)
The stress transmitted through the soil by intergranular and water menisci in the soil voids
Envelope soilwater pressure (kPa)
The magnitude of change in the total soil-water potential at a given position caused by the mechanical stress imposed by the soil layers above it
Growth
The last phase of seed germination, during which the radicles and rootlets are protruding and elongating into the soil
GERMINATION AND SEEDLING ESTABLISHMENT Imbibition
A first phase of seed germination, during which the seed absorbs water and its tissues are rehydrated
Impedance to flow, soil–seed interface impedance to water flow (day1)
A condition which hinders the movement of water through the seed-soil contact zone under the influence of water potential gradient
Matric potential The difference between the water poten(kPa) tial of source pool of soil solution at a given elevation and air pressure and a pool identical to the source but at the elevation under consideration (above water table) Mechanical impedance
Soil strength conditions which hinder or inhibit the devleopment and growth of plant organs in the soil
Osmotic soilwater potential (kPa)
The magnitude of change in soil-water potential due to differences in the chemical composition of the soil solution related to free pure water at the same elevation and air pressure
Seed diffusivity to water (cm2 day1)
The flux of water per unit gradient of volumetric water content in the seed
Seed germination
A general term describing the sequence of processes involved in starting metabolic activities and leading to the initiation of growth in the quiescent embryo in the seed
Seedling emergence
A term stating that a growing seedling has broken through the soil surface
Seedling establishment
A term describing the stable development and growth of a seedling that has recently emerged
Seed’s volumetric water content (cm3 cm3)
The volume of water per unit bulk volume of the seed
Soil conductivity to water (cm day1)
The flux of water per unit gradient of matric potential
Soil diffusivity to water (cm2 day1)
The flux of water per unit of gradient of volumetric water content in the soil
Soil strength
A transient, localized soil property which is a combined measure of a soil to resist deformation under external or internal stress imposed on it
Soil structure
The spatial arrangement of soil primary particles into secondary particles or units
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Soil texture
The relative proportions of the various size classed separates in the soil
Soil-water potential (kPa)
The amount of work that must be done in order to transport reversibly and isothermally an infinitesimal quantity of pure water from a specified source to a specified destination
Transition
The second phase in seed germination
See also: Crusts: Structural; Cultivation and Tillage; Nutrient Availability; Plant–Water Relations; Rhizosphere; Root Architecture and Growth; Structure
Further Reading Arndt W (1965) The nature of the mechanical impedance to seedlings by soil surface seals. Australian Journal of Soil Research 3: 45–54. Benesch-Arnold RL and Sanchez RA (1995) Modeling weed seed germination. In: Kigel J and Galili G (eds) Seed Germination and Development, pp. 545–566. New York: Marcel Dekker. Bewley JD and Black M (1982) Physiology and Biochemistry of Seeds. Berlin, Germany: Springer-Verlag. Bradford KL (1995) Water relations in seed germination. In: Kigel J and Galili J (eds) Seed Development and Germination, pp. 351–396. New York: Marcel Dekker. Corbineau F and Come D (1995) Control of seed germination by dormancy and by the gaseous environment. In: Kigel J and Galili J (eds) Seed Development and Germination, pp. 397–424. New York: Marcel Dekker. Hadas A (1982) Seed soil contact and germination. In: Khan AA (ed.) The Physiology and Biochemistry of Seed Development, Dormancy and Germination, pp. 507–527. Amsterdam, the Netherlands: Elsevier. Hadas A (1997) Soil tilth – the desired soil structural state obtained through proper soil fragmentation and reorientation processes. Soil Tillage Research 43: 7–40. Hadas A and Stibbe E (1977) Soil crusting and emergence of wheat seedlings. Agronomy Journal 69: 547–550. Koller D and Hadas A (1982) Water relations in the germination of seeds. In: Lange OL, Nobel PS, Osmond CB, and Zigler H (eds) Encyclopedia of Plant Physiology, vol. 12B, pp. 410–431. Berlin, Germany: Springer– Verlag. Kutilek M and Nielsen DR (1994) Soil Hydrology. Cremlingen-Destedt, Germany: Catena-Verlag. Marshal TJ, Holmes JW, and Rose CW (1996) Soil Physics. Cambridge, UK: Cambridge University Press. Mayer AM and Poljakoff-Mayber A (1989) The Germination of Seeds, 4th edn. London, UK: Pergamon Press. Van Duin RHA (1956) On the Influence of Heat, Diffusion of Air and Infiltration of Water in the Soil, Versl. Landb. K. Onderz. 62, pp. 1–82. Pudoc: Wageningen, the Netherlands.