Inter-Plant Competition: Growth Responses to Plant Density and Row Spacing

Abstract

Optimizing cotton yield through manipulation of plant spacing has been the objective of many research efforts. Field research
has shown that increasing plant population density (number of plants per unit ground area) consistently increases leaf area
index (LAI) and light interception but its effects on yield have been inconsistent. Understanding how the arrangement of plants
in the field affects cotton growth requires consideration of many interacting factors, such as genetics, physiology, and canopy
structure.

Full text

I. INTRODUCTION
Optimizing cotton yield through manipulation of plant
spacing has been the objective of many research efforts.
Field research has shown that increasing plant population
density (number of plants per unit ground area) consistently
increases leaf area index (LAI) and light interception but its
effects on yield have been inconsistent. Understanding how
the arrangement of plants in the eld affects cotton growth
requires consideration of many interacting factors, such as
genetics, physiology, and canopy structure.
Plant population density (hereinafter referred to as plant
density) and row spacing are two management decisions
that determine the spatial arrangement of plants within a
eld. Plant density is the number of plants per unit area and
is inversely related to the distance between plants within a
row. Row spacing is the distance between rows. Altering
plant density and row spacing changes the radiative trans-
port within the canopy. Changes in radiative transport al-
ter the interception of photosynthetically active radiation
(PAR), and the distribution and quality of light within the
canopy. Changes in plant spacing have a distinct impact on
physiology, morphology, canopy development, and boll
and ber growth although the specic physiological mecha-
nisms are largely unknown. In order to summarize some of
what is known about cotton and plant spacing, this chapter
will discuss the effects of plant density and row spacing on:
(1) PFD (photon ux density) quality, quantity, and distri-
bution; (2) resource availability; and (3) growth, yield, and
ber quality.
Chapter 17
INTER-PLANTCOMPETITION:GROWTH
RESPONSES TO PLANT DENSITY AND ROW SPACING
J.J. Heitholt1 and G.F. Sassenrath-Cole2
Need afliations for both authors
2. LIGHTQUALITY
2.1 Spectral Changes Due to
NeighboringPlants
As plant density increases, PFD in the middle and lower
sections of the canopy is greatly altered due to penumbral
effects creating regions of shade and deeper shade as light
passes through the canopy (Jones, 1992). Radiation is re-
ected from foliage and soil at lower wavelengths than
incident, and contributes to the diffuse radiation within
the canopy. The changes in incident and reected radia-
tion alter the ratio of red (600 to 700 nm) to far-red (700
to 800 nm) light within the canopy. With increased reec-
tance of far-red light from neighboring plants, an increase
in the number and nearness of neighboring plants can de-
crease the red:far-red ratio of intercepted light (Sanchez et
al., 1993). Changes in the spectral composition of incident
radiation directly impact carbohydrate metabolism (Casal
et al., 1995), but exert a more profound inuence on crop
performance through the photomorphogenetic responses of
plant tissues.
2.2 Photomorphogenetic Responses to
Plant Density
The changes in plant structure and canopy development
with altered plant spacing observed in cotton canopies re-
sult, at least in part, from photomorphogenetic responses to
the altered light quality within the canopy. The photomor-
phogenetic responses are mediated via the phytochrome
system, which provides a mechanism for plants to sense
and respond to the light environment of the canopy (Ballare
J.McD. Stewart et al. (eds.), Physiology of Cotton,
DOI 10.1007/978-90-481-3195-2_17, © Springer Science+Business Media B.V. 2010

180 Heitholt and Sassenrath-Cole
et al., 1992). Documented photomorphogenetic responses
to altered light quality include seed germination and cell
elongation. The exact mechanisms by which plants trans-
late light quality into physiological responses are not com-
pletely understood. Evidence indicates that specic photo-
receptors are capable of translating photoexcitation of the
chromophores into molecular signals triggering cellular re-
sponses (Aphalo and Ballare, 1995 and references therein).
Recently, a novel phytochrome receptor has been identied
(Carabelli et al., 1996) that appears to mediate cell elonga-
tion by induction of a specic gene. The novel phytochrome
is induced under red/far-red light conditions typically found
at dawn/dusk and shade within a canopy.
Research on photomorphogenetic responses in dal-
lisgrass (Paspalum dilatatum Poir.) and annual ryegrass
(Lolium multiorum Lam.) have shown that a lower red/far-
red ratio (as would occur from increasing plant density) re-
duced tillering (Casal et al., 1986). In several other species,
greater plant density increased stem elongation, changed
leaf shape and size, decreased specic leaf weight, and re-
duced chlorophyll per unit leaf area (Aphalo and Ballare,
1995 and references therein). In mustard (Sinapis alba L.)
supplemental far-red light not only increased internode
elongation but also increased internode concentrations of
reducing sugars, starch, hemicellulose, and cellulose (Casal
et al., 1995). There is little data documenting specic
changes in red to far-red light within cotton canopies due to
altered plant spacing. However, decreasing the red:far-red
ratio of incident light through the use of various colored
mulches causes an increase in plant height (Kasperbauer,
1994), especially early in the season. For more details
about spectral composition changes and the phytochrome
response in other species, the reader is referred to other re-
views (Ballare et al., 1992; Sanchez et al., 1993; Kendrick
and Kronenberg, 1993; Aphalo and Ballare, 1995).
3. GROWTH RESPONSES TO
PLANT DENSITY
3.1 LeafAreaandMain-stem
Development Responses to Plant
Density
As in most crop species, cotton compensates for decreas-
es in plant spacing by producing a greater number of leaves
and fruits per plant (Constable, 1986; Morrow and Krieg,
1990) ostensibly through photomorphogenetic processes.
Although previously published reviews have discussed
cotton growth characteristics (Hearn and Constable, 1984;
Mauney, 1986a) and its response to various management
practices (Hearn and Fitt, 1992; Kerby et al., 1996), only
one review (Hearn, 1972) has specically addressed the ef-
fects of plant density. In quantifying some of the effects,
Constable (1986) showed that a high plant density (24 m-2)
had a slower rate of leaf appearance than a low plant density
(2 m-2). The lower plant density also had larger leaves and
more main-stem nodes than the high plant density. The low
plant density had longer sympodial branches (~5.5 fruiting
sites) than the high density (~1.7 fruiting sites). It is thought
that this response is partly due to light quality changes and
partly due to the amount of resources per plant.
Fowler and Ray (1977) showed that plant height at 149
days after planting decreased as plant density increased.
In one year of a three-year study, increasing plant density
(from 5 to 20 m-2) resulted in taller plants early in the season
but shorter plants later (Heitholt, unpublished). In the sec-
ond year, plant height increased as density increased (from
2 to 15 m-2), regardless of the stage of development. In that
same study, the number of main-stem nodes increased as
plant density decreased (Heitholt, 1995). Kerby (1990a)
showed that a full-season Acala cultivar showed a negative
relationship between plant height and density but there was
no relationship for two early maturing “determinate” types.
Because many ndings appear to conict, it appears that
genotypic, environmental, and developmental factors con-
found the effects of plant density on plant height.
3.2 Reproductive Developmental
Responses to Density
Because increasing plant density increases the number
of main-stem nodes per unit area, researchers have proposed
that high plant densities will increase the number of owers
per unit area. Guinn et al. (1981) showed that increasing
plant density of ‘Deltapine 61’ from 5.2 to 9.4 plants m-
2 of increased ower numbers by 5%, but decreased boll
retention by 8% and yield by 6%. Heitholt (1995) showed
that increasing plant density of DES 24-8ne okra and nor-
mal-leaf from 5 to 15 plants m-2 did not appreciably affect
ower numbers, boll retention, or yield. Therefore, increas-
ing plant density may, but does not necessarily, increase
ower numbers.
Boll distribution is also an important consequence of
the effects of plant density on yield. Increasing plant den-
sity from 3 to 15 plants m-2 increased the percentage of
bolls found on rst position fruiting sites from 40 to 80%
(Kerby et al., 1987). In a similar study, increasing plant
density from 5 to 15 plants m-2 increased the percentage of
bolls found on rst-position fruiting sites from 48 to 71%
(Kerby et al., 1990a). The exact physiological causes for
the altered boll distribution are not known. However, al-
tered light environment in the “crowded” conditions expe-
rienced by plants grown at high density probably prevents
sympodial branches from developing many distal fruiting
sites as described earlier (Constable, 1986). The response
is expected considering that a plant grown at high density
generally develops a similar (Heitholt, 1994a) or a slightly
lower (Constable, 1986) plant height and number of main-
stem nodes by the end of the season than plants grown at
lower density. However, plants grown at high densities pro-

Chapter 17. Inter-Plant Competition: Growth Responses to Plant Density and Row Spacing 181
duce fewer bolls per plant than plants grown at low densi-
ties (Guinn et al., 1981).
3.3 Plant Density Effects on Yield
In general, the reported effects of plant density on yield
have been small, inconsistent, or dependent upon environ-
ment (Buxton et al., 1979; Guinn et al., 1981; Kerby et al.,
1990a; Heitholt et al., 1992). In one case where four low
densities were used, four hybrid cottons had a 30% greater
seedcotton yield at the two higher plant densities (2.8 and
5.5 plants m-2) than at the two lower plant densities (1.4
or 1.8 plants m-2) (Jadhao et al., 1993). Using tillage and
seeding rate to alter plant density in the Texas High Plains,
Hicks et al. (1989) showed that yield of Paymaster 404 and
Acala A246 increased as plant density increased from 2 to 8
plants m-2 but decreased above 8 plants m-2. On a very ne
sandy loam in Louisiana with Deltapine 41, Micinski et al.
(1990) found that resulting plant densities explained yield
differences found among planting dates.
In one of the most comprehensive summaries on cot-
ton plant density and yield, Hearn (1972) demonstrated that
optimal plant density increased as yield potential increased.
This is consistent with the idea that with more resources
a canopy can support more plants and is consistent with
the skip-row results (presented later). However, this dogma
does not hold for all situations. In contrast to Hearn (1972),
the optimal plant density in a low yielding environment
(800 kg ha-1) for DES 24-8ne normal-leaf was 15 plants m-2
whereas the optimal density in a high yielding environment
(1100 to 1400 kg ha-1) was 5 plants m-2 (Heitholt, 1994a).
These results can probably be reconciled by looking at LAI
data. In Heitholt (1994a), the 15 plants m-2 was excessive
when the resulting LAI was 5 or greater. The idea that op-
timal density is associated with resulting LAI was also ad-
vanced by Constable (1977). Kerby et al. (1996) also indi-
cated that optimal plant density was greater (i.e., 20 plants
m-2 rather than 10 plants m-2) under severely stressed condi-
tions. Using an alternative approach to determine optimal
plant density of Pima cotton, Kittock et al. (1986) measured
plant height at plant densities ranging from 2 to 20 plants
m-2. The optimal plant density decreased by 1.1 plants m-2
for every 10-cm increase in plant height.
4. PLANT SPACING EFFECTS ON
LIGHTQUANTITY
4.1 Sunlight Interception and Utilization
Differences in plant density and row spacing physically
modify a canopy. Decreasing the distance between nearest
neighbors alters the light environment within the canopy
and the resources available to each plant. An ideal canopy
would capture as much solar radiation as possible and opti-
mize utilization of that radiation for conversion to carbohy-
drate and production of harvestable product. The goal of the
producer/researcher is to manipulate the inherent growth
pattern of plants to optimize growth and development of
harvested components. Directed manipulation of canopy
architecture is a technique used extensively in many crop-
ping systems to optimize radiation use and enhance yield.
Fruit production systems regularly use direct manipulation
of canopy structure (e.g., through pruning), to optimize
yield (Palmer, 1989). In soybean (Glycine max L. Merr.),
Ikeda (1992) attempted to develop an optimal canopy struc-
ture by varying planting pattern so that land area per plant
and plant density were maximized.
Because sunlight is essential to plant production, the
canopy must optimize interception and utilization of radia-
tion to achieve maximum growth. Some areas of the U.S.
Cotton Belt (e.g., Texas and Arizona) can plant earlier than
other regions due to more favorable soil temperatures and
therefore can optimize the interception of solar radiation. By
planting earlier, canopy closure in these regions coincides
with peak insolation periods (near 21 June) whereas in re-
gions with later planting dates, canopy closure occurs later.
By planting at higher plant densities or narrower row spac-
ing, early-season light interception is increased and the crop
can take advantage of the greater insolation levels during
the early growing season. However, increased early-season
light-capture does not necessarily result in increased yield.
Optimizing solar radiation interception has been the
objective of many plant spacing studies. Radiation inter-
ception is dependent on the leaf area and can be dened as
the absorption of solar radiation by the light-harvesting pig-
ment-protein complexes in the leaf chloroplasts. Radiation
that is not absorbed by the topmost layer of leaves passes
through to the lower canopy layers. However, in passing
through the upper canopy layer, the full sunlight intensity
rarely reaches the lower canopy layers due to the penum-
bral effects of light movement through gaps in the canopy.
For a thorough discussion of the movement of light through
plant canopies, the reader is referred to Jones (1992) and
Monteith and Unsworth (1990).
As LAI and light interception increase (with canopy
development or increased plant density), the shape of the
canopy foliage and the arrangement of leaves relative to
one another determine solar radiation interception ef-
ciency. In theory, a continuous layer of leaves at the top
of a canopy, with LAI equal to 1.0, would allow complete
radiation interception. Although this may appear to be an
efcient canopy structure for the interception of radiation,
this strategy does not result in an efcient use of sunlight.
An LAI of 3 to 6, with evenly distributed leaves and more
erectophile leaf angles, greatly increases the efciency of
sunlight utilization. Studies have demonstrated that canopy
photosynthesis is enhanced when more PAR is allowed to
reach the middle portion of the canopy as opposed to nearly
complete radiation interception by the upper layer of leaves
(Wells et al., 1986; Aikman, 1989; Herbert, 1991).

182 Heitholt and Sassenrath-Cole
Assuming 2000 µmol m-2 s-1 PFD is available and all leaf
surfaces intercept equal PFD, theory suggests that optimal
carbon uptake would occur at an LAI of 5 and a PFD of 400
µmol m-2 s-1. This value is slightly greater than found in two
Australian eld studies where maximum crop growth rate
and yield were found to occur at an LAI of 3.5 (Constable,
1977; Constable and Gleeson, 1977). In a Mississippi Delta
eld study, maximum crop growth rate and 95% light in-
terception were reached at an LAI of 4.0 (Heitholt et al.,
1992). Plant densities of 2, 3, and 5 plants m-2 resulted in
an LAI of 4.0 or less and a greater yield than plant densities
of 10 and 15 plants m -2 that had an LAI of 5.0 or greater
(Heitholt, 1994a). Although there is some disagreement on
the optimal LAI, it is clear that plant spacings that allow a
more even distribution of PAR throughout the canopy may
optimize productivity.
Because leaf age affects the photosynthetic light re-
sponse (Sassenrath-Cole et al., 1996), the optimal LAI for
maximal canopy carbon uptake may change during devel-
opment. Peng and Krieg (1991) reported that leaf age, rath-
er than PAR, was primarily responsible for reduced canopy
carbon exchange rate (CER) late in the season. It has been
suggested that extending the optimum CER phase of leaves
by 10 days would lead to a signicant increase in yield
(Landivar et al., 1983). Although a delay in the age-related
decline in CER could indeed increase growth, two research
groups have shown that other factors may prevent it from
being realized. First, Wullschleger and Oosterhuis (1990a)
showed that the CER decline of older leaves found lower in
the canopy was limited to a signicant extent by the inten-
sity of PAR incident to the leaf surface (i.e., CER and PAR
were correlated) and not solely due to leaf aging per se.
Second, we found that the loss of physiological activity due
to leaf age accounted for less than one-half of the reduced
CER that occurred in lower canopy leaves during the late
season (Sassenrath-Cole and Heitholt, 1996). In that study,
low PAR was the primary factor that reduced CER of lower
canopy leaves. In many cotton canopies, the older, lower
canopy leaves receive low PAR and therefore, contribute
little carbon to the plant. Thus, distribution of radiation is
an important consideration in designing an optimal canopy
architecture. For a further discussion of photosynthesis
within cotton canopies, the reader is referred to Chapter 7
(Temporal Dynamics of Leaves and Canopies).
In order to obtain a better PAR distribution, leaf distri-
bution must be optimized. However, some disagreement ex-
ists as to whether leaf area is arranged more efciently (less
mutual shading) in low density or high density. Constable
(1986) showed that canopy extinction coefcients at low
density (2 m-2) were greater than those from high density
(24 m-2). This suggested that leaf arrangement with low
density exhibited less efciency (or in the author’s words
greater “clumpyness”) than at higher plant densities. In ap-
parent contrast, Heitholt (1994a) showed that light intercep-
tion per unit LAI was greater at lower density (5 m-2) than at
20 m-2. In this case, the lower plant density had a better leaf
arrangement than the high density. The differences between
these two studies may reect the growth environment, sug-
gesting that different strategies may be optimal under dif-
ferent growing conditions. For example, the high solar ra-
diation environment found in the southwest U.S. results in
a much denser canopy structure (Alarcon and Sassenrath-
Cole, unpublished). Because improving the efciency of
leaf arrangement is important and there is not a consensus
as to the best arrangement, future research directly testing
the effects of plant density on leaf arrangement is needed.
4.2 LightDistributionDifferences
CausedbyGenotype
Genetic variants with altered leaf and canopy struc-
ture can help reduce the radiation absorption by the up-
per canopy layers and allow greater PAR to reach lower
canopy leaves. These simply-inherited leaf shapes range
from the full-sized normal-leaf types to the narrow-lobed
leaf types called okra-leaf. Leaf morphology variants, such
as okra-leaf (discussed earlier by Stewart, chapter 2 and
Wells, chapter 3) increase light and air movement within
the canopy. There is a signicant difference in PAR prole
within canopies of normal vs. okra-leaf (Wells et al., 1986).
Although the leaves at the very top of the normal-leaf cano-
py receive more PAR, the PAR drops rapidly with decreas-
ing height in the canopy. The decrease in PAR as a function
of canopy depth is less severe in the okra-leaf canopy (i.e.,
light penetrates further into the canopy) than in a normal-
leaf canopy (Wells et al., 1986).
At a given plant density, okra-leaf cottons usually de-
velop a lower LAI than normal-leaf (Kerby et al., 1980;
Heitholt et al., 1992). This raises the possibility that the
optimal plant density for okra-leaf types may be greater
than for normal-leaf types. Meredith (1985) compared the
yield of okra-leaf, normal-leaf, their F1, and their F2 prog-
eny using three seeding rates (9, 19, and 38 seeds m-2) in
four Mississippi Delta environments. Averaged across en-
vironments, yield of okra-leaf cotton types were unaffected
by seeding rate but yield of the normal-leaf, F1, and the
F2 progeny all declined as seeding rate increased above 9
seeds m-2. In a later study, DES 24-8ne okra-leaf exhibit-
ed its greatest yield at 10 plants m-2 whereas yield of the
normal-leaf isoline was greatest at 5 plants m-2 (Heitholt,
1994a). Kerby et al. (1990a,b) also showed that genotypes
differed in optimal plant density. One short “determinate”
type exhibited its greatest yield at 15 plants m-2 (and lowest
yield at 5 plants m-2) whereas yield of the more full-season
type ‘Acala SJC-1’ (with a maximum LAI of 4.4 to 5.1)
was unaffected by plant densities of 5, 10, and 15 plants
m-2. Based on these studies, it appears that increasing plant
density above 10 plants m-2 for low leaf area, early-matur-
ing types can sometimes increase yield whereas increasing
plant density above 10 plants m-2 for normal-leaf, full-sea-
son types usually does not.

183
4.3 Growth Regulators
Directed modication of canopy architecture is pos-
sible through the use of plant growth regulators, such as
mepiquat chloride (1,1-dimethyl-piperidinium chloride), a
gibberellic acid (GA) synthesis inhibitor or conversely by
GA-like substances (Oosterhuis and Zhao, 1995). Mepiquat
chloride can inhibit vegetative growth (Kerby et al.,
1986; Cathey and Meredith, 1988; Fernandez et al. 1992;
McConnell et al. 1992), reducing internode length and LAI.
Gwathmey et al. (1995) showed that mepiquat chloride de-
creased the percentage of light intercepted in upper leaves
and allowed greater light penetration to the middle portion
of the canopy. This alteration in canopy light prole most
probably arises due to a decrease in the penumbral effects
with shortened internodes. Although mepiquat chloride can
lower LAI and light interception (Heitholt et al., 1996), it
has also been found to increase yields in California (Kerby
et al., 1986) and in Mississippi when cotton was planted
late (Cathey and Meredith, 1988; Heitholt et al., 1996). The
LAI in the Heitholt et al. (1996) Mississippi study was rath-
er high (ranged from 4 to 5). Because mepiquat chloride is
more likely to have a positive effect in canopies that would
otherwise develop excess vegetative growth, this raises the
possibility that mepiquat chloride would be more effective
at high plant densities (Atwell, 1996; Atwell et al,. 1996).
However, in four of ve environments, mepiquat chloride
did not increase yield at plant densities ranging from 3.7 to
13.6 plant m-2 (York, 1983). The exception occurred in a test
that used 23 plants m-2 (a plant density often considered ex-
cessive) where mepiquat chloride increased yield by 51%.
4.4 Leaf Orientation
Factors altering leaf orientation may relate to the cot-
ton plant’s response to density. Light penetration within
the canopy depends not only on LAI and leaf distribution
(Constable, 1986), but also on the three-dimensional struc-
ture of the leaves and the heliotropic behavior (Lang, 1973;
Ehleringer and Hammond, 1987; Sassenrath-Cole, 1995).
Studies have demonstrated that changes in leaf angle in the
upper canopy can redistribute PAR to lower canopy leaves
and lead to increased canopy photosynthesis (Wells et al.,
1986; Aikman, 1989; Herbert, 1991; Sassenrath-Cole,
1995). Leaf angle in other species can also be altered by
plant density (Aphalo and Ballare, 1995). Because upper
leaves of a G. hirsutum canopy are diaheliotropic, tracking
the sun particularly in the early morning and late evening,
PAR striking the upper leaves of G. hirsutum during these
times is greater than PAR striking leaves of G. barbadense
that display no heliotropism (Ehleringer and Hammond,
1987). There is a signicant difference in light penetration
within mature canopies of diaheliotropic G. hirsutum from
that in non-heliotropic G. barbadense (Sassenrath-Cole,
1995). Photon ux attenuation in the G. barbadense cano-
py declined gradually with decreasing height in the canopy,
whereas the lower third of the leaves in the G. hirsutum
canopy received low PAR throughout the day (Sassenrath-
Cole, 1995). Although the suntracking of G. hirsutum al-
lowed upper canopy leaves to intercept more total radiation,
the restriction of PAR to leaves lower in the canopy resulted
in less efcient light distribution in canopies of G. hirsutum
compared to that of G. barbadense. In agreement with this
interpretation, leaf heliotropism was suggested to be ben-
ecial to net canopy photosynthesis early in the season, but
detrimental to photosynthesis due to a decreased distribu-
tion of PAR to lower canopy layers as LAI increased (Fukai
and Loomis, 1976).
Although differences in heliotropic response contributed
to differences in light environment within the two canopies,
a more signicant contribution was the degree of curvature
or three-dimensional cupping (i.e., a concave leaf shape)
of the G. barbadense leaves (Sassenrath-Cole, 1995). This
cupping of upper canopy G. barbadense reduced PFD in-
cident to upper leaf surfaces but increased PFD penetra-
tion to lower canopy layers, more closely approximating
an ideal canopy conguration (Kuroiwa, 1970). In addition
to distributing more PAR to lower canopy layers, this strat-
egy may be advantageous by reducing photoinhibition to
upper canopy leaves by reducing the time and intensity of
PAR exposure. Since upper leaves greatly reduce PAR re-
ceived by older leaves lower in the canopy, PAR limitation,
rather than age-induced physiological limitations, might be
a more signicant cause for reduced photosynthetic activ-
ity that occurs with aging (Sassenrath-Cole and Heitholt,
1996). The increase in PAR to lower canopy layers may
signicantly increase the potential carbon uptake of G. bar-
badense. Use of varieties with more erectophile orientation
of the upper canopy foliage, achievable in cotton by greater
three-dimensional “curling” of the leaves, would benet
canopy carbon uptake and potentially be better adapted to
higher densities and narrower row-spacings. These changes
in the vertical orientation of leaves have been successfully
incorporated into corn (Zea mays L.) via traditional breed-
ing methods.
4.5 MicroclimateChangesasAffectedby
Plant Spacing
To understand differences in fruit development with
plant spacing, we must consider the alteration of the mi-
croenvironment at the point of boll development. The infra-
red portion of the electromagnetic spectrum radiates heat
to and from the earth’s surface. This energy of both direct
and diffuse PAR is attenuated (wavelength is increased but
frequency and energy are decreased). This gives rise to
differences in temperature proles within cotton canopies
as a function of canopy structure. Canopy characteristics
that cause changes in radiative transfer, light penetration,
distribution, and quality throughout the canopy will also
change the heat balance within the canopy. Changes in ra-
diative transport have been demonstrated in other cropping
Chapter 17. Inter-Plant Competition: Growth Responses to Plant Density and Row Spacing

184 Heitholt and Sassenrath-Cole
systems as a function of canopy modication (Baldocchi et
al., 1985). Alteration of radiative ux due to modication
of the canopy architecture will drastically affect the mi-
croenvironment (temperature, humidity, and wind speed)
within the crop canopy. These changes in temperature at
the site of fruit development, while often subtle, can have
profound inuence on boll growth and ber development
when taken over the course of boll development (Hessler et
al., 1959; Gipson, 1986; Sassenrath-Cole and Hedin, 1996).
Moreover, additional effects on air movement through the
canopy alter the humidity that will affect the development
of boll rot and other microorganisms. Preliminary evidence
in cotton suggests that daytime mid-canopy temperatures
are cooler in 51-cm rows than in 102-cm rows (Sassenrath-
Cole, unpublished). Although extensive research has been
conducted in other crop canopies examining differences
in radiative transport as a function of canopy architecture
(Baldocchi et al., 1985), cotton has only recently received
attention (Sassenrath-Cole, 1995).
In addition to the plant density and row spacing effects
on mid-canopy boll temperatures, leaf morphology can
also be important. Lower-canopy boll temperatures were
found to be warmer during the day and cooler at night for
okra-leaf plants compared to those measured in normal-leaf
isolines (Sassenrath-Cole, unpublished). This difference in
temperature in okra-leaf canopies would have signicant
impact on boll maturation over the course of the growing
season and may account for the observed increased rate of
boll maturation (i.e., earliness) of okra-leaf types (Heitholt
et al., 1993; Heitholt et al., 1996). Additionally, lint from
okra-leaf plants exhibited lower strength than normal-leaf
cotton (Heitholt et al., 1993), which could result from the
larger diurnal temperature uctuations in the okra-leaf
canopies. More data on the effect of plant density and leaf
morphology on diurnal boll temperatures are needed to help
explain differences in ber quality.
5. RESOURCE AVAILABILITY
When plant density is altered, the amount of resources
(e.g., water, nutrients, soil volume, etc.) available per plant
is changed. At high density, leaf area per plant decreases
and the volume of soil available for each plant decreases.
The negative effects of reduced resources per plant that oc-
cur due to high density are likely to offset any gains realized
from increases in LAI (Sanchez et al., 1993). In this sec-
tion, we cover the effects of plant density on physiological
response to resource availability.
5.1 Soil Resources and Plant Density
Among the resources per plant that are potentially lim-
iting at high density are mineral nutrients. In this discus-
sion, we limit ourselves to how mineral nutrition might
interact with plant density. For a more in-depth discussion
of mineral nutrition, the reader is referred to chapter 14.
Nitrogen fertility effects on cotton growth are well docu-
mented (Wullschleger and Oosterhuis, 1990b; McConnell
et al., 1993). Excess N can lead to unnecessary leaf area,
delayed maturity, and lower ber quality. When high soil N
leads to excess vegetative growth (as described by Boman
and Westerman, 1994), assimilates get distributed toward
unneeded vegetative growing points and away from fruit.
Although potassium deciency can reduce plant biomass
and limit yield (Cassman et al., 1989a, 1990; Pettigrew
et al., 1996; Pettigrew and Meredith, 1997), there are no
reported negative consequences from adding excess K.
Decient N or K results in an insufcient LAI (Wullschleger
and Oosterhuis, 1990b; Bondada et al., 1996; Pettigrew and
Meredith, 1997) and a low number of potential fruiting
sites. Increased density effects on root growth may reduce
cotton’s ability to obtain nutrients from the soil. In some
desert shrubs, increased plant density reduced root prolif-
eration and elongation rate (Mahall and Callaway, 1991,
1992).
Several studies have looked at the possible interaction
between N and plant density but usually little interaction has
been found. However, in one study, Yasseen et al. (1990)
did report N x plant density interaction on plant height. At
low N (60 kg N ha-1), plant height was greater at 34 plants
m-2 than at 17 plants m-2. At 90 kg N ha-1, no differences in
plant height among densities were found. Rao and Weaver
(1976) found no signicant N x plant density interaction
when three N levels (100, 134, and 168 kg N ha-1) and three
plant densities (2, 3, and 7 plant m-2) were compared. Koli
and Morrill (1976) found ber properties were generally
unaffected by N x plant density interaction.
In another study (Sadras, 1996a), plant density effects
on compensatory growth (as a result of oral bud removal
twice weekly until 80 days after sowing) were tested with
‘Siokra S324’ and ‘CS7S’. Under low plant density and high
nitrogen fertility, subsequent fruit growth rate of defruited
plants was only 17% lower than the control. However, un-
der high plant density and low N fertility (a stressed envi-
ronment), the decrease in fruit growth rate was 50%. This
decrease under stressed conditions was associated with an
increased partitioning of dry matter to roots.
In order to better quantify a plant’s effect on other plants,
Sadras (1997b) calculated a factor for an individual plant re-
ferred to as “neighbor interference” which was equal to the
leaf area of the adjacent plant divided by their distance from
each other. A strong inverse linear relationship was found
between “neighbor interference” and the percent of dry mat-
ter partitioned to fruit. This result supports previous obser-
vations by Heitholt (1994a) that showed high plant density
and LAI was associated with reduced fruit numbers.
In two studies, low plant density (about 5 plants m-2)
combined with adequate N supply was compared to a high
plant density (about 15 plants m-2) with minimal N (Sadras
1996b, 1996c). In one study with Siokra V-15 (Sadras,
1996c), vegetative bud removal tended to increase yield of

185
the high density treatment presumably by promoting greater
root growth. The treatments did not affect yield in the low
density treatment. Both low density treatments outyielded
the high density treatments. One explanation for the results
could be improved light quantity and quality striking oral
meristems in the high density treatment. Another explana-
tion could be that larger roots in the high density treatment
allowed those plants to extract more water and nutrients
than the untreated plants.
5.2 WaterAvailabilityandPlantDensity
High plant densities may exhaust the available water
earlier in the season than lower plant densities leaving in-
sufcient water for the boll-lling period. Hearn (1972)
demonstrated that the optimal plant density under irrigation
was greater than under dryland conditions. Likewise, in the
Texas High Plains, Staggenborg and Krieg (1993) found
that the yield decline associated with high plant density was
less severe when more water was available. In a separate
study, Staggenborg and Krieg (1994) varied plant density
and water supply and showed that the optimal density was
one that supplied 30 to 35 kg H2O per plant per season.
The apparent interaction between water supply and plant
density indicates that excessive plant density can cause soil
water depletion that leads to lower yield.
6. PLANT DENSITY AND HARVEST
VARIABLES
6.1 Effects of Plant Density on Harvest
Maturity Date
Data from some studies have suggested that increasing
plant density would increase earliness (Rao and Weaver,
1976) whereas other data have suggested the opposite
(Kerby et al., 1990a). Rao and Weaver (1976) showed
that both the okra-leaf and normal-leaf isolines of ‘Coker
201’ matured earlier when grown at 6.5 plants m-2 than
at 2.2 plants m-2. Using three hand-picking dates, Kerby
et al. (1990a) showed that a tall “indeterminate” cultivar
(‘Acala SJ-2’) grown at 15 plants m-2 matured later (i.e.,
59% picked by rst two harvests) than it did at densities
of 5 and 10 plants m-2 (i.e., 69% picked by rst two har-
vests). Kerby et al. (1990a) also showed that the earliness
of two “determinate” types was unaffected by plant den-
sity. Also using consecutive hand-pickings, in a three-year
study (Heitholt, unpublished), the effects of plant density
on earliness were examined. In 1991 and 1993, no effects
were found. However, in 1992, DES 24-8 ne normal leaf at
2 and 3 plants m-2 reached 65% open bolls ve days earlier
than at 10 and 15 plants m-2. Baker (1976) and Kostopoulos
and Chlichlias (1979) found that plant density did not affect
earliness. Changes in earliness with altered plant density/
row spacing may depend on the differences in heat units
experienced by the developing bolls due to an alteration of
radiative transport within the modied canopy structures.
6.2 EffectsofPlantDensityonFiber
Quality
Developing bolls and subtending leaves on plants
grown under high plant densities are more likely to be
shaded than bolls and leaves on plants grown under lower
densities. The lower leaves that subtend developing bolls
have been suggested to be a primary assimilate source
for those bolls (Constable, 1986). If inadequate radiation
strikes these leaves, boll set or ber quality of these bolls
may be reduced (Pettigrew, 1995). Alternatively, if high
plant density causes abscission of lower bolls, then subse-
quent bolls are more likely to be found in middle or up-
per portions of the canopy. The indeterminate growth habit
of cotton results in bolls developing under very different
environmental conditions throughout the growing season.
Despite numerous studies that have been performed, most
have found that plant density had little direct effect on ber
quality (Gannaway et al., 1995; Minton and Supak, 1980;
Kostopoulos and Chlichlias, 1979; Hawkins and Peacock,
1973). In two cases, increases in plant density were associ-
ated with decreases in micronaire and length (Fowler and
Ray, 1977; Hearn, 1972).
7. ROW SPACING
7.1 RowSpacingandLightDistribution
The effects of row spacing on the ratio of percent light
intercepted to LAI are more consistent than plant density ef-
fects. Row spacing is consistently and negatively related to
light interception per plant (Heitholt et al., 1992; Heitholt,
1994a). Given equal plant densities, narrow rows result
in more equidistant plant spacing. This means more early
season solar radiation is captured (Peng and Krieg, 1991;
Heitholt et al., 1992); leaves on narrow-row plants are less
likely to shade each other (i.e., a plant’s nearest neighbor is
further away and mutual shading is less likely). Although
row spacing is consistently and negatively related to light
interception per plant its effects on growth per plant and
LAI range from no effect (Constable, 1977; Heitholt et al.,
1992) to increases in both (Peng and Krieg, 1991).
7.2 Row Spacing and Yield
The range of cotton row spacings that have been tested
is large. Examples of row spacing comparisons include 18-
cm vs. 102-cm (Constable, 1977), 33-cm vs. 67-cm vs. 102-
Chapter 17. Inter-Plant Competition: Growth Responses to Plant Density and Row Spacing

186 Heitholt and Sassenrath-Cole
cm (Peng and Krieg, 1991), 51-cm vs. 102-cm (Heitholt
et al., 1992), and 76-cm vs. 102-cm (Heitholt et al., 1996;
Williford et al., 1986). Unfortunately, many narrow-row
studies have confounded plant density and row spacing
(Andries et al., 1969; Buxton et al., 1979; Fowler and Ray,
1977; Minton, 1980) by using a greater plant density in the
narrow row spacings. The confounding of plant density and
row spacing in the studies listed above has made it difcult
to separate the effects of the two factors in these studies.
Confounding plant density and row spacing would be justi-
ed if the optimal density for narrow rows was shown to be
greater than that for wide rows. This hypothesis is undoubt-
edly true for some environments. However, using one okra-
leaf and one normal-leaf genotype, Heitholt (1994a) found
that row spacing did not affect optimal plant density in 51-
cm vs. 102-cm spacings or in 76-cm vs. 102-cm spacings.
Although narrow rows generally increase seasonal
light interception by 10% for most crops, this increase does
not necessarily provide production advantages for cotton
(Constable, 1977; Heitholt et al., 1996). The effects of row
spacings, ranging from narrow rows (51-cm to 76-cm) to
skip rows (dened later) on yield, are small in many environ-
ments (Constable, 1977; Heitholt et al., 1992; Kostopoulos
and Chlichlias, 1979; Williford et al., 1986; Williford, 1992).
The effect of row spacing is possibly dependent upon other
management factors. In a three-factor study with mepiquat
chloride (0 to 50 g ha-1), irrigation vs. nonirrigated, and row
spacing (76- vs. 96-cm) in the Missouri Bootheel, Tracy and
Sappeneld (1992) reported that narrow rows outyielded
wide rows if mepiquat chloride and irrigation were applied.
However, row spacing did not affect yield if either mepiquat
chloride or irrigation were withheld.
In addition to the conventional, solid planted row spac-
ings mentioned above, a skip-row planting pattern (e.g., a
row pattern where each pair of 102-cm rows is separated
by a 204-cm spacing), has been a commonly used varia-
tion of row spacing. The skip-row pattern allows a grower
to plant fewer rows and to harvest fewer rows (harvesting
being the most expensive input). Skip rows also allow use
of soil water from the “unplanted” area (Quisenberry and
McMichael, 1996). Like much of the research comparing
solid-planted row spacings, skip-row comparisons to solid
planting are often confounded by plant density (i.e., skip
rows use fewer plants per unit area). Although the yield of
skip-row management per unit area is almost always lower
than solid planted cotton, skip-rows may prove more eco-
nomical than solid planting because the cost of producing a
unit of lint is lower. The yield per plant in skip-row culture
occurs because each row is essentially unbordered on one
side. Therefore, lower leaves receive more light increasing
the chances that yield per plant will increase. This claim
is supported by a two-year study with three cotton culti-
vars that showed aluminum reectors placed in the two row
spaces (of solid planted cotton) bordering the yield row
increased yield by 6% (Pettigrew, 1994). Quisenberry and
McMichael (1996) also showed that the further the distance
between rows up to 204 cm also increased yield per plant.
Yields of skip-row and solid cotton were reported to be
similar when water was applied at a rate of 4 megaliters per
hectare or less (Constable et al., unpublished). However,
at 6 and 8 megaliters of water per hectare, lint yields were
about 500 kg ha-1 greater in the solid cotton. The skip-row
culture appears to be useful when either economic or envi-
ronmental resources are limited.
8. SUMMARY
Plant density, row spacing, and genotype are three factors
that alter the canopy PAR prole, light quality (i.e., spectral
composition), and resources available per plant. Plant den-
sity induced changes in light quality affect canopy devel-
opment and yield. These changes in light quality can also
affect ber quality through photomorphogenetic effects on
cell elongation and differentiation. Development mediated
via the phytochrome system is also altered by plant density.
High plant density can cause excessive PAR interception
by the upper canopy and a reduction in the amount of light
striking mid-canopy leaves. These changes subsequently re-
duce carbohydrate status of subtended bolls.
Commonly tested cotton plant densities range from 5
to 15 plants m-2 (Kerby et al., 1990a,b) although data for
densities on both sides of this range are available (Fowler
and Ray, 1977; Hearn, 1972; Heitholt, 1994a; Hicks et al.,
1989; Sadras, 1996c). Increasing plant density results in
greater LAI but less leaf area per plant and fewer bolls per
plant. Plant densities that result in an LAI between 4 and 5
are usually associated with optimal yield. Increasing plant
density also results in taller plants during early growth but
at maturity the reverse may be true.
The effects of plant density on yield, maturation date,
and ber quality are often small. Optimal plant density
was shown to be altered by soil water, nutrient availability,
growth regulators, and genotype but not by row spacing.
Assuming equal plant densities are used, narrow rows allow
each plant to intercept more light. However, the effects of
row spacing on biomass per plant are small compared to the
effects of plant density.
Although this review indicates that many of cotton’s
agronomic responses to plant density have been thoroughly
characterized, it is clear that the cotton plant’s basic re-
sponses to plant density and row spacing, such as biophysi-
cal, canopy microclimate, and biotic changes remain largely
uncharacterized and need to be researched.
9. ACKNOWLEDGMENTS
The authors thank C.O. Gwathmey, K.E. Lege, W.T.
Pettigrew, V.O. Sadras, and L.J. Zelinski for comments on
early versions of the manuscript.