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II. Factors That Influence Time of Seedling Emergence

II. Factors That Influence Time of Seedling Emergence

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SEEDLING EMERGENCE WITHIN POPULATIONS



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1972; Maguire, 1983). Seeds are usually sown directly into soil, often at

varying depths, and the subsequent germination is in conditions of fluctuating temperature and water supply. The effects of these factors on time of

seedling emergence will be considered in the next sections.

A. WATER

Water influences the spread in time of seedling emergence in a number of

ways. First of all, water is essential for germination, so any restriction on

its supply reduces the rate and final percentage of seed germination. The

supply of water to the seed is governed by the conductivity of the soil

water (Collis-George and Sands, 1959; Williams and Shaykewich, 1971;

Hadas and Russo, 1974a), the degree of seed-soil water contact (Sedgley,

1963; Manohar and Heydecker, 1964; Collis-George and Hector, 1966;

Harper and Benton, 1966; Hadas and Russo, 1974a), and the osmotic and

matric potentials (Collis-George and Sands, 1959, 1962; Manohar and

Heydecker, 1964; Williams and Shaykewich, 1971; Dasberg and Mendel,

1971; El-Sharkawi and Springuel, 1979; Ross and Hegarty, 1979; Willat

and Struss, 1979; Tipton, 1988).

Increasing the supply of water can restrict the supply of oxygen that is

necessary for germination. Oxygen is sparingly soluble and its solubility

decreases with increasing temperature, whereas the metabolic demand

for this gas increases with temperature. In addition, any complementary buildup of carbon dioxide has a poisoning effect on germination

(Heydecker, 1958). Consequently, even slight excesses in the amount of

moisture can have large inhibitory effects on germination (Heydecker et

al., 1969; Dasberg and Mendel, 1971). Hanks and Thorp (1956) reported

that emergence of wheat was restricted when the oxygen diffusion rate

g cm-2 min-I. This corresponded to an air

(ODR) fell below 75-100 x

pore space of 16% in a silt clay and 25% in a fine-sandy loam. Subsequent

work shows that the rate of germination is restricted by an ODR as little as

20 x lo-' g cmP2min-l (Dasberg and Mendel, 1971). Insufficient oxygen

supply has been alleviated by coating seeds with calcium or zinc peroxides, or by incorporating calcium peroxide in the growing medium, but the

beneficial effects varied greatly with species and occurred only when the

moisture content of the growing medium was very high. Furthermore, the

addition of peroxides in drier media often had a detrimental effect on

germination and emergence (Brocklehurst and Dearman, 1983; Langan et

af., 1986).

Even if germination proceeds rapidly and uniformly, there can be a wide

spread in time of seedling emergence due to a restriction of seedling growth



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L. R. BENJAMIN



by the strength of the soil, which is largely governed by its water content

(Arndt, 1965; Collis-George and Williams, 1968; Royle and Hegarty, 1977;

Hegarty and Royle, 1978). In addition, slow drying gives closer packing of

soil particles, resulting in high soil strengths (Gerard, 1965). However, the

more important restriction to seedling emergence is the creation of crusts

by surface drying (Hanks and Thorp, 1956,1957; Royle and Hegarty, 1977;

Nuttall, 1982).

Spread in time of emergence is determined by soil strength because the

emergence force that seedlings exert develops dynamically and is a linear

function of volumetric soil-water content and the cross-sectional area of

the seedlings (Gerard, 1980). Taylor and Broeck (1988) measured the

emergence force exerted by nine vegetable species at 25°C in sand at 15%

moisture content and showed that the time taken to exert maximum force

ranged from 4 hr in red beet to 21 hr in snap bean.

Increasing the salinity of soil water caused a reduction in the size of the

emergence force exerted by seedlings and increased the time required to

exert the maximum force (Sexton and Gerard, 1982). Adding nitrogen

fertilizers to soils has reduced percentage emergence, presumably because

of osmotic effects (Hegarty, 1976a; Page and Cleaver, 1983). Nearly all

studies of the effects of fertilizers on seedling growth have examined only

final percentage emergence. However, Henriksen (1978)showed that addition of 75 or 150 kg N ha-' prior to sowing onions increased the standard

deviation of emergence times by about half a day as well as reduced the

percentage emergence compared with addition of the nitrogen after emergence. The salts that increase the salinity of the soil water can have the

opposite effect of stimulating seedling growth by supplying essential mineral nutrients, such as phosphorus (Costigan, 1984).

Most studies of seedling emergence have imposed constant soil moisture

conditions. In nature, however, seeds are exposed to a fluctuating supply

of water. This fluctuation affects variation in mean time of seedling emergence between populations in both weed (Roberts, 1984) and cultivated

species (Hegarty, 1976b; Finch-Savage, 1986), but is also liable to be a

major determinant of the individual-to-individual variation in time of seedling emergence within a population.

B. TEMPERATURE

In some species, seeds require exposure to low temperatures to break

dormancy (see Roberts, 1972 for a review). All species show a qualitative

relationship between germination parameters and temperature. The usual



SEEDLING EMERGENCE WITHIN POPULATIONS



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responses are an approximately linear increase in rate (reciprocal in time

taken to start of or some percentage of germination) with increasing temperature from a threshold to a maximum, with or without a plateau,

followed by a linear decline at superoptimal temperatures. As a consequence of this linearity, it is convenient to describe the effect of temperature on the mean time of seedling emergence in terms of temperature sums

(often erroneously called heat sums) (Hegarty, 1973; Bierhuizen and

Wagenvoort, 1974; Garcia-Huidobro et al., 1982a).

Only a few studies have examined the effect of temperature on the

variability in the time of germination, but there is evidence in carrots that

the spread in time of germination decreases with increase in temperature

over the range 5°C to 25°C (Gray, 1979).

In nature, seeds are exposed to fluctuating temperatures and, for

noncultivated species, this might be a requirement for germination

(Thompson, 1974). However, for most cultivated species, fluctuations in

temperature have negligible practical effects on time of germination

(Wagenvoort and Bierhuizen, 1977; Garcia-Huidobro et al., 1982b).

In moist seedbeds, the rate of seedling emergence has a relationship with

temperature similar to that described for germination (Muendel, 1986;

Finch-Savage, 1986), and temperature sums have been used to describe

the effects of temperature on the timing of emergence (Khah et al., 1986;

Tenhovuori, 1986).

However, the timing of seedling emergence is not governed entirely by

the relationship between germination and temperature, because low temperatures also increase the time taken by seedlings to exert maximum

force (Gerard, 1980). An example of this interaction between temperature

and soil compaction was found in calabrese by Hegarty and Royle (1978).

They showed that as temperature decreased from 20°C to 6"C, percentage

emergence decreased from 93% to 78% when 0.6 N cm-* pressure had

been applied, but the percentage emergence decreased from 90% to 33%

when 4.8 N cm-* had been applied. The interaction between temperature

and soil-water matric potential was quantified by Tenhovuori (1986), who

showed that the temperature sum required for 50% emergence increased

linearly above a threshold value as the soil-water matric potential increased.

Gummerson ( 1989) examined the influence of seedbed preparation practices on the influence of moisture content, impedance, aeration, and temperature on the emergence of sugar beet. Of all four factors, he reported

that temperature was the one that consistently limited rate of seedling

emergence. There appear to be no studies that have quantified the relative

importance of temperature for the spread in time of seedling emergence.



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L. R. BENJAMIN



C. SOWING

DEPTH

The amplitude of diurnal variation in temperature lessens and the time of

maximum and minimum daily temperature shifts with increasing depth

(Orchard and Wurr, 1977). Hence, deep-sown seeds experience a more

uniform temperature than shallow-sown seeds. Similar considerations

would apply to moisture content of the soil. However, deep-sown seeds

would be expected to take longer to emerge and small seeds often do not

have the ability to penetrate through a deep layer of soil (Moore, 1943;

Black, 1956; Stickler and Wassom, 1963; Arnott, 1969; Snyder and Filban,

1970; Bedford and MacKay, 1973; Wagenvoort and Bierhuizen, 1977;

Abul-Fatih and Bazzaz, 1979; Buckley, 1982;Nuttall, 1982). This inability

could be due to insufficient stored materials to generate the osmotic gradient necessary to overcome the pressure exerted by the soil (Black, 1956)or

the seedling might be too weak to withstand the forces necessary to

overcome the resistance of the soil, with the hypocotyl breaking as it drags

the cotyledon through the soil (Rathore et al., 1981). Nuttall (1982) attributed better emergence of rape from small seeds to the requirement for less

energy to push small cotyledons through the soil crusts. However, the

differences in seed sizes were confounded with differences in cultivar in

his experiments and the optimum sowing depths for cabbage, lettuce,

carrot, and onion were 1.5-2.5 cm, despite differences between these

species in seed size, presence of endosperm, and being mono- or dicotyledons (Heydecker, 1956).

D. SEEDATTRIBUTES

The previous sections have concentrated on the effects of the external

environment on variation in time of seedling germination and emergence,

but attributes of the seed also influence the rate of germination and emergence.

Even in a favorable, uniform environment, seeds do not germinate

synchronously but display a probability of germinating in a unit length of

time (Thornley, 1977; Bould and Abrol, 1981).The effect of environmental

factors such as temperature and water supply is to influence this probability of seed germination (Harper, 1977; Bould and Abrol, 1981). This stochastic nature might be an inevitable consequence of germination being a

chain of many physical, biochemical, and physiological events (Thornley,

1977; Tipton 1984). For example, in carrots, germination was faster in

seeds containing large embryos (from primary umbels) than in those containing small embryos (from secondary umbels) (Gray, 1979). Also the



SEEDLING EMERGENCE WITHIN POPULATIONS



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standard deviation of germination time was less in seed lots with a low

seed-to-seed variation in embryo length (mature seed lots) than in those

containing variable embryo lengths (immature seed lots).

Although dormancy is not considered to be a problem for germination in

most cultivated species, it is well known in many natural species. Furthermore, there is a well-known inhibition of germination in some cultivated

species by specific environmental stimuli. For example, light can inhibit

germination of some cultivars of lettuce, tobacco, and tomato (Pollock,

1972). The corky capsules that surround beet seeds contain a watersoluble inhibitor to germination (but see Morris et al., 1984 for an opposing

view). Carrot seeds were considered not to contain such inhibitors, but

recent work on seed priming has revealed their presence (Pill and FinchSavage, 1988). Thus, these inhibitors of germination may be more ubiquitous in cultivated crops than previously suspected.

Attributes of the seed also interact with environmental factors to determine the rate of germination. For example, Harper and Benton (1966)

showed that the germination of all types of seeds was restricted by low

matric potential when placed on sintered glass disks, but mucilaginous

seeds were least sensitive, spiny reticulate seeds were the most sensitive,

and smooth seeds showed a graded response to water tension. Small seeds

were less sensitive to water tension than large seeds.

Time of seedling emergence is controlled by genetic constitution (Eagles, 1988; Lafond and Baker (1986) and seed size (Lafond and Baker,

1986). However, some studies showed no effect of seed size on time of

seedling emergence (Naylor, 1980; Stanton, 1984). These inconsistent

results might be attributed to the use of different growing media in the

different studies. The optimum seed-soil water contact for germination is

achieved when the mean aggregate size of soil particles is one-fifth to

one-tenth of the seed’s diameter (Hadas and Russo, 1974b).

Adverse soil conditions might be partially overcome by using seeds that

are “robust.” Some seed lots have persistently high field emergence over a

wide range of soil conditions (Hegarty, 1974). Osmotic priming of seeds

often improves seedling establishment, presumably by bringing all seeds to

a uniformly mature state (see Bradford, 1986 for a review of this technique). It might be possible to breed for specific seed properties that favor

germination, for example, small seeds and cracked testas (Whittington,

1978). However, these factors that favor germination might be detrimental

to emergence of seedlings in field conditions.

It is interesting to consider whether seed attributes or environmental

factors dominate time of seedling emergence. Only a few studies have

addressed this question directly, but indirect studies indicate an overriding

importance of the environment. For example, varying soil texture has



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L. R. BENJAMIN



large effects on seedling emergence (Hammerton, 1961 ;Wurr e f al., 1982).

There are even large interactions between the method of watering (by

capillary action or by surface watering) and soil moisture content on the

percentage of seedling emergence (Heydecker, 1961). However, attributes

of the seed can influence emergence in unexpected ways. For example, oil

seed rape seedlings adapt to high soil impedance by decreasing the time

taken to develop their full emergence force. This response to soil impedance was enhanced or inhibited by substances that affected ethylene production or action (Clarke and Moore, 1986). When such subtle interactions occur between seed and environment, it is perhaps naive to

determine their relative importance. However, the relative importance of

various attributes of seeds for variation in time of seedling emergence has

been estimated (Waller, 1985). Waller collected seeds from cleistogamous

(self-pollinating) and chasmogamous (cross-pollinating)flowers of jewelweed (Impatiens capensis) and found that between a third and a quarter of

the variation in time of seedling emergence was associated with seed

weight, seed type, maternal parent, and their interaction.

E. CONCLUSIONS



Variation in time of seedling emergence arises because it is the culmination of a large number of preceding processes whose rates can differ

between individuals. Differences between seeds in their genetic constitution, development on the mother plant, and exposure to extraneous

factors, such as fungal attack, produce variation in time of germination in

uniform environments. In nature, an additional source of variation in time

of germination occurs from heterogeneity of the soil. Harper et al. (1965)

suggested that germination of broadcast seeds depends on available sites

of warmth and moisture. This idea is a useful concept also for buried seeds.

Hegarty and Royle (1978) noted that there was greater seedling emergence

in a dry soil that had been compacted than in a similar soil that had not been

compacted. They speculated that compaction had improved the water

supply to the seeds, presumably by increased seed-water contact, which

effectively increased the number of sites for germination. Dasberg and

Mendel (1971) claimed that “the rate of seed-water uptake governs germination. This rate is determined in general by the energy status of the water

in the germination medium, by its conductivity, and by the area of contact

between seed and medium, which is a function of pore geometry and

surface tension.” Seed death is another important aspect of the effects of

soil conditions on seedling emergence (Harper, 1955; Hegarty, 1978).

In a system as multifaceted as the seed-soil complex, it is inevitable that



SEEDLING EMERGENCE WITHIN POPULATIONS



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there is a wide spread in seedling emergence times, even in a crop sown

synchronously (Hegarty, 1976b). The foregoing studies indicate that manipulation of any one of a number of processes would reduce the spread in

time of seedling emergence, but no one treatment would produce

synchrony of emergence. Spread in time of seedling emergence can be

reduced by improved seed production techniques, by laboratory techniques to bring all seeds to maturity (priming), and by improved engineering to give uniform depth and optimum seed-soil contact in the seedbed.

However, the most pragmatic way of reducing the spread in time of

seedling emergence is to ensure a continued supply of soil moisture at a

level that is optimum for germination during the period of imbibition,

radicle emergence, and eruption of the shoots through the soil surface.

The purpose of the rest of this review is to examine the importance of

this spread in time of seedling emergence to the subsequent development

of the plant community.



111. IMPORTANCE OF VARIATION IN TIME OF SEEDLING

EMERGENCE TO CROP DEVELOPMENT

In the remaining part of this review, I shall examine the importance of

seedling-to-seedling variation in time of emergence within populations on

the subsequent development of each plant. Although much work has been

done to compare the effects of treatments that influence mean time of

seedling emergence in separate populations (Hegarty, 1976b; Symonides,

1978; Gummerson, 1989), this is of little value for determining the importance of time of seedling emergence on the interactions between individuals within a population. Therefore, the remainder of the review will be

confined to those few studies that examined the development of individual

plants.

Is the time of seedling emergence truly a boundary in the course of plant

development, or is it just an event of no consequence to the plant but easily

observed by humans? Certainly there is a switch from carbon for growth

provided by the mobilization of seed reserves to carbon provided by

photosynthesis. However, presumably some mineral nutrient and water

absorption occurs through the roots of a seedling whose shoots have not

yet broken the soil surface.

Black and Wilkinson (1963) appear to be the only workers to have

experimentally distinguished between time of seedling emergence and

preemergence relative growth rate. Pregerminated subterranean clover

seeds were sown either synchronously or in plots containing a mixture of



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L. R . BENJAMIN



two sowing dates. In mixed sowing date plots, the plants were sown on a

square grid with the early and late sowings alternating in a checkerboard

design. The difference in sowing time was either 2, 4, or 8 days and the

sowing positions were 1.5 cm apart. Despite sowing seeds carefully at

uniform depth in boxes of compost, there was a spread in emergence time

of 8 to 14 days for the early-sown plants and 5 to 10 days for the late-sown

plants. Thus, there was a wide range of preemergence growth rates, which

resulted in a wide overlap in the seedling emergence times from the different sowings. When the plants had a foliage height of 30 cm, the logarithm of

dry weight was negatively correlated with time of seedling emergence. The

novel point about this work was that the effect of time of sowing on the

regression of weight per plant at harvest on seedling emergence time was

examined and was found to have only a slight effect. Thus, time of seedling

emergence, and not any correlation with preemergence growth rate, was

responsible for the subsequent effects on plant weight.

In the following sections the effect of time of seedling emergence on the

growth, form, and composition of individuals in populations is examined.



A. TOTALPLANT

GROWTH



At the point of seedlingemergence, plant weight is still minute compared

with the potential and probable weight that the plant will attain. Plant

growth at this time is nearly always exponential because there is virtually

no self-shading or competition for growth resources from neighbors. Consequently, a difference of a few days in seedling emergence time can result

in a manyfold difference in weight between plants. For example, Black and

Wilkinson (1963)found that a delay in emergence of 5 days brought about a

reduction in subterranean clover weight of about 50%, and a delay of 8 or 9

days produced a reduction in weight of at least 75%. Gray (1976) showed

that, in lettuce at 26 days after sowing, seedlings that had emerged at 7

days were five to six times larger than those that had emerged at 15 days.

When considering the importance of relative times of seedling emergence for the subsequent dry matter increment of individuals within a

population, a number of factors have to be taken into consideration. First,

the spread in time of seedling emergence in a population is important, but

most studies relied on the natural spread in time of seedling emergence

rather than attempting to modify it artificially. The time from seedling

emergence to harvest is also relevant. As this time interval increases, there

is a greater probability that individual plant growth will be influenced by

some extraneous factor, for example, herbivory. Finally, the “state” of

the population must be considered, such as the density of plants, or



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