Advances in agronomy volume 44

ADVANCES IN

AGRONOMY
VOLUME 44


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ADVANCES IN

AGRONOMY
Prepared in Cooperation wirh the
AMERICAN
SOCIETYOF AGRONOMY

VOLUME 44
Edited by N. C . BRADY
Science and Technology
Agency for International Development
Department of State
Washington, D . C .

ADVISORY BOARD
N. L. TAYLORH. G . HODGES

E. L. KLEPPERG . L. HORST
R. J . KOHEL R. H. MILLER
G . E. HAM S . MICHELSON
K. H. QUESENBERRY

C. W. STUBER
G . H . HEICHELD. E. KISSEL

ACADEMIC PRESS, INC.
Harcourt Brace Jovanovich, Publishers
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1


CONTENTS
CONTRIBUTORS
..........................................................................

PREPACE ..................................................................................

ix
xi

VARIATION IN TIME OF SEEDLING EMERGENCE WITHIN POPULATIONS:
A FEATURE THAT DETERMINES INDIVIDUAL GROWTH AND DEVELOPMENT

L. R. Benjamin
I.
11.

111.
1V.

..........................................
Introduction ................................
Factors That Influence Time of Seedling Emergence.. .........................
Importance of Variation in Time of Seedling Emergence to Crop
Development ..............................................................................
Conclusions
............................
References .................................................................................

1
2

9
20
21

FORAGE TREE LEGUMES: THEIR MANAGEMENTAND CONTRIBUTIONTO THE
NITROGEN ECONOMY OF WET AND HUMID TROPICAL ENVIRONMENTS

Graeme Blair, David Catchpoole, and Peter Horne
1.
11.

111.
1v.
V.
VI.

VII.
VIII.

Introduction.. ..........................................................
Species of Useful Tree Legumes.
...............................................
Agronomic Performance of Tree Legumes ........................................
Tree Legume Leaf as Animal Feed ..................................................
Management of Tree Legumes........................................................
Nitrogen Yields of Material Harvested from Tree Legumes ..................
Nitrogen Recycling via Leaf and Excreta.. ......................
Conclusion .................................................................................
References ........................................................................

21
28
29
34
36
45
46
49
50

STATISTICAL ANALYSES OF MULTILOCATIONTRIALS

Jose Crossa
I.
11.

Introduction ................................................................................
Conventional Analysis of Variance.. ................................................
V

55

51


vi

CONTENTS

111. Joint Linear Regression.................................................................
IV. Crossover Interactions.. ................................................................
V. Multivariate Analyses of Multilocation Trials ....................................
VI. AMMl Analysis ...........................................................................
VII. Other Methods of Analysis ............................................................
VIII. General Considerations and Conclusions ..........................................
References .................................................................................

61

68
70
76
80
81
82

EVALUATION AND DOCUMENTATION OF GENETIC RESOURCES IN CEREALS

A. B. Damania
I.
11.
111.

IV.
V.
VI.
VII.

Introduction .....................
................................................
Evaluation of Cultivated Wheat ......................................................
Evaluation of Cultivated Barley
.........................................
Genetic Resources from Ethiopia.. ..................................................
Evaluation of Wild and Primitive Forms of Wheat and Barley ...............
Documentation of Genetic Resources ..............................................
Summary and Conclusions.
....................
References .................................................................................

87
90
93
95
96
I02

I05
107

MODELING CROP ROOT GROWTH AND FUNCTION

Betty Klepper and R. W.Rickman
1.
11.

111.
IV.
V.
VI.

Introduction
..............................................
Early Models ..............................................................................
Desirable Model Features..
...................................................
Model Components ......................................................................
Some Existing Root Growth and Function Models .............................
Limitations to Development of Root Growth Models ..........................

...............................

I13
114
115

118

I28
130
131

GENETIC MANIPULATION OF THE COWPEA (Vigna unguiculafa [L.] Walp.) FOR
ENHANCED RESISTANCE TO FUNGAL PATHOGENS AND INSECT PESTS

A. 0. Latunde-Dada
1.
11.
111.

IV.

Introduction ................................................................................
Insect Pests ................................................................................
Fungal Pathogens.. .......................................................................
Tissue Culture Technology ............................................................

133
139
141
142


vii

CONTENTS
V.

Conclusions and Epilogue.. .................................................... ........
References ............................................................ ....................

.

149

I 50

NITROGEN FIXATION BY LEGUMES IN TROPICAL AND
SUBTROPICAL AGRICULTURE

Mark B. Peoples and David F. Herridge

I.
11.

111.
IV.
V.
VI .

.

Introduction.. .................................................................. ...........
Methods of Assessing N2 Fixation
N2 Fixation in Legume Production Systems ......................................
Contribution of Legume N to Plant and Animal Producti
Strategies to Enhance N2 Fixation ...................................................
Concluding Remarks .................................,............

References .............................
..............................................

i56
158
177
190
202
216
216

DISTRIBUTION, COLLECTION, AND EVALUATION OF Gossypium

A. Edward Percival and Russell J. Kohel

.

11.

Introduction.. ........ ........
.......................
Distribution ..........,.....................................................................

Ill.
IV .
V.

Evaluation.. ................................................................................
Concluding Remarks ................................ ..

1.

225
228
235
245
253
253

BREEDING WHEAT FOR RESISTANCE TO Septoria nodorum AND
Septoria tritici

Lloyd R. Nelson and David Marshall

I.
11.
111.
IV.
V.
VI .

Introduction.. ..............................................................................
Identification of Resistance.
.......................
Pathogen Variation.. ........... .........................................................
Genetics of Resistance
.............................................
Sources of Resistance ..... ....,.......................
Discussion and Conclusions ...........................................................

.

INDEX......................................................................................

257
258
268
270
272
272
214

279


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CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.

L. R. BENJAMIN ( l ) , AFRC Institute of Horticultural Research, Wellesbourne,
Wanvick CV35 9EF, England
GRAEME BLAIR (27), Australian Centre for International Agricultural Research, Department of Agronomy and Soil Science, University of New England,
Armidale, New South Wales 2351, Australia
DAVID CATCHPOOLE* (27), Australian Centre for International Agricultural
Research, Department of Agronomy and Soil Science, University of New England, Armidale, New South Wales 2351, Australia
JOSE CROSSA (55), Biometrics and Statistics Unit, International Maize and
Wheat Improvement Center (CIMMYT),06600 Mexico D. F., Mexico
A. B. DAMANIA (87), Genetic Resources Unit, International Centerfor Agricultural Research in the Dry Areas (ICARDA),Aleppo, Syria

DAVID F. HERRIDGE (155), Australian Centre for International Agricultural
Research (Project 8800),New South Wales Agriculture and Fisheries, Tamworth, New South Wales 2340, Australia
PETER HORNE (27), Australian Centre for International Agricultural Research,
Department of Agronomy and Soil Science, university of New England, Armidale, New South Wales 2351, Australia
BETTY KLEPPER ( 1 13), United States Department of Agriculture, Agricultural
Research Service, Columbia Plateau Conservation Research Center, Pendleton, Oregon 97801
RUSSELL J. KOHEL (229, United States Department of Agriculture, Agricultural Research Service, Southern Crops Research Laboratory, College Station,
Texas 77840
A. 0. LATUNDE-DADA (133), Department of Crop Production, College of
Agricultural Sciences, Ogun State university, Ago-Iwoye, Ogun State, Nigeria
DAVID MARSHALL (257), Texas A&M University Research and Extension
Center at Dallas, Dallas, Texas 75252
LLOYD R. NELSON (257), Texas A&M University Agricultural Research and
Extension Center at Overton, Overton, Texas 75684

*Present address: Queensland Department of Primary Industries, Ayr. Queensland 4807, Australia.

ix


X

CONTRIBUTORS

MARK B. PEOPLES ( I S ) , Australian Centre for International Agricultural Research (Project SSOO), CSIRO Division of Plant Industry, Canberra, A . C . T .
2601, Australia
A. EDWARD PERCIVAL (225), United States Department of Agriculture, Agricultural Research Service, Southern Crops Research Laboratory, College
Station, Texas 77840
R. W. RICKMAN ( 1 13), United States Department of Agriculture, Agricultural
Research Service, Columbia Plateau Conservation Research Center, Pendleton, Oregon 97801

PREFACE
In the nearly 25 years I have had the privilege of editing this serial, it has
been an inspiration to witness the desire and willingness of crop and soil
scientists to prepare review articles for their compatriots. Scientists
throughout the world have taken literally thousands of hours of their own
time and energies to prepare articles for Advances in Agronomy gaining
little but the satisfaction that they were doing a favor for their fellow
scientists. They have indeed furthered the cause of science by writing
these reviews.
The authors of the nine articles in this volume have followed the tradition of their predecessors. Located at research institutions in six different
countries, they maintain the international focus of this serial. The subjects
covered vary from genetic resources of cereals and cotton to timing of
seedling emergence and modeling of root growth and function. Two articles focus on tropical crops and agriculture as well as agro-forestry, subjects of keen concern as low-income farmers of the tropics struggle to
increase food production while maintaining the quality of their environment and especially of their soils.
Efforts to increase the host resistance of wheat and cowpeas are covered
in two articles. These are evidence of the increasing focus on alternatives
to the use of chemical pesticides to control plant pests. Host resistance is
one of the characteristics evaluated in multilocational field trials around
the world. The statistical analyses of such trials are the subject of another
article in this volume.
Thanks are due the advisory board of the American Society of Agronomy and the directors-general of three international agricultural research
centers for suggesting topics and authors for this serial. To the 15 authors
of the important reviews contained herein I express my special gratitude.
N. C. BRADY

xi

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ADVANCES IN AGRONOMY, VOL. 44

VARIATION IN TIME OF SEEDLING
EMERGENCE WITHIN
POPULATIONS: A FEATURE THAT
DETERMINES INDIVIDUAL GROWTH
AND DEVELOPMENT
L. R. Benjamin
AFRC Institute of Horticultural Research
Wellesbourne, Warwick CV35 9EF, England

Introduction
Factors That Influence Time of Seedling Emergence
A. Water
B . Temperature
C . Sowing Depth
D . Seed Attributes
E. Conclusions
Ill. Importance of Variation in Time of Seedling Emergence to Crop
Development
A. Total Plant Growth
B. Partitioning between Organs
C . Organ Morphology and Composition
D. Longevity
IV. Conclusions
References
1.

11.

I. INTRODUCTION
The point in time when the growing point of a shoot emerges from the
soil into the aerial environment is one of the most easily observed events in
crop development. The range of percentage and timing of seedling emergence can be large, even in cultivated species, because emergence is the
culmination of a large number of preceding events. For example, FinchSavage (1984) reported in carrots sown at eight different times between 1 1
February and 15 June in the United Kingdom that the number of days
between the 10 and 90 percentile points for emergence ranged from 11 to
24. In noncultivated Umbelliferae. and in some cultivated Umbelliferae
1
Copyright Q 1990 by Academic Press. Inc.
All rights of reproductionin any form reserved.


2

L. R. BENJAMIN

species, there are also dormancy mechanisms that result in periodicity of
emergence times (Roberts, 1979). Much attention has been directed to
unraveling the complex interactions between the agronomic and genetic
factors that influence seedling emergence. The salient features of these
factors and their influence on variation in seedling emergence time will be
reviewed here.
Most plant communities are characterized by intense competition between individuals for growth resources, which leads to the development of
dominance hierarchies (Watkinson, 1985; Benjamin and Hardwick, 1986;
Weiner and Thomas, 1986). These hierarchies are important in crops
because they contribute to variation in weight per plant, which is undesirable for a market that increasingly requires uniformly sized produce for
processing and the fresh market (Anonymous, 1982), and can also contribute to loss of economic yield (Benjamin and Hardwick, 1986). The hierarchies are also important in natural communities because they contribute to

size-dependent fecundity (Pacala and Slander, 1985; Watkinson et al.,
1989) and to size-dependent mortality (Mithen et ul., 1984; Schmitt et ul.,
1987; Thomas and Weiner, 1989).
This review will examine the relevance of time of seedling emergence to
the development of these hierarchies and seek mechanisms to account for
any relationships.

II. FACTORS THAT INFLUENCE TIME
OF SEEDLING EMERGENCE
Crops have been bred and selected for genetic uniformity and elimination of seed dormancy mechanisms. Engineers have developed sophisticated equipment to produce good seedbed tilths and to sow seeds at
uniform depth. So why should there be variation in times of seedling
emergence? About a hundred papers are published annually on germination (Lovato, 1981) and this subject has been reviewed extensively
(Koller, 1972; Heydecker, 1973; Harper, 1977; Heydecker and Coolbear,
1977; Johnston, 1979; Perry, 1982).
The purpose of this review is not to make another exhaustive study but
rather to examine the ideas that are implicit in most of these studies and to
determine how much is already known about the causes of variation in
time of seedling emergence. Although germination is a complex process, it
has only three requirements: water, warmth, and a free exchange of gases.
Seed dormancy is common in most species, but despite some notable
exceptions, it has largely been overcome in commercial crops (Villiers,


SEEDLING EMERGENCE WITHIN POPULATIONS

3

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


4

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

5

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.


6

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

7

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


8

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

9

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


10

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


SEEDLING EMERGENCE WITHIN POPULATIONS

11

whether the plants under consideration are a monoculture or part of a
mixed-species population. Table I presents a summary of these factors and
the percentage weight variation accounted for by spread in time of seedling
emergence in a number of species.
The percentage of weight variation associated with spread in time of
seedling emergence was usually greatest when fewer than 20 days have
elapsed between mean time of seedling emergence and harvest (Table I).
(Gray, 1976; Benjamin, 1987). The paper by Benjamin (1982) (Table I) best

illustrates how the percentage of weight variation associated with spread
in time of seedling emergence increases as spread in time of seedling
emergence increases. This trend is also seen in other work (Table I).
Benjamin (1982) reduced the spread in time of carrot seedling emergence
from 19 to 14 days by using a contact herbicide to kill the first seedlings to
emerge. This treatment had no detectable effect on the frequency distribution of storage roots in different diameter grades, suggesting that relatively
slight changes in spread of seedling emergence time have no detectable
influence on variation in weight. Controlling time of seedling emergence
might allow other sources of weight variability to be expressed more
strongly. However, the residual sum of squares of root weight (after taking
out the effect of emergence time) from plots that had small spreads in
emergence times was no greater than the residual sum of squares from
plots that had large spreads in emergence times (Benjamin, 1982). Therefore, there is no evidence that other sources of variation are able to express
themselves to a greater extent when time of seedling emergence is controlled.
The effect of density on the relationship between spread in time of
seedling emergence and plant weight is difficult to interpret from Table I
because the other factors were varied too. The very high percentage
variation in weight associated with spread in time of seedling emergence
reported by Ross and Harper (1972) is perhaps because they used extremely high densities (Table I). Benjamin (1982) showed that the increase
in coefficient of variation (CV) of carrot shoot and storage root weight at
wider spreads in seedling emergence time was greater at 400 than at 25
plants mP2.
Ross and Harper (1972) noted that the last cocksfoot seedlings to emerge
in a population growing at 30,000 plants m-’ had a weight only a little
greater than that of seeds, even after 35 days of growth. The implication is
that late-emerging seedlings are denied resources for growth by the earlieremerging plants and these competitive interactions are more intense at
high densities.
More direct evidence that relative time of seedling emergence determines the relative amount of growth resources that can be captured by an
individual comes from a second experiment by Ross and Harper (1972) in

Table I
Percentage Shoot Weight Variation Associated with Spread in Time of seedling Emergence

Species
e

N

Dactylis
glomerata
Lactuca sativa

Spread in
time of
seedling
emergence
10

Density
(m-*)

Mean days
from
emergence
to harvest

Accounted
% variation

in weight

Notes

Reference

30,000

40

95

Ross and Harper

13
13
13
13
13
13
13

17
63
69
17

81
63
49

94

Gray (1976)

(1972)
9
9
9
11
11
6
5-10

Lolium
perenne

-a

Impatiens
capensis

23

24.48, or%

seedlings in
34 x 12cm
tray
500- 1000

20

90

22
17

72
80

180

50

ca. 105

<1

Naylor (1980)

Fecundity measured, plants grouped
into six emergence classes

Howell (1981)