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V. Progress and Prospects in the Development of Annual Seed Crops
M. DONALD AND J. HAMBLIN
contributor to increased yields (Athwal, 1971). Yield potential also increased
(Austin et al., 1980; L. T. Evans, 1980; Perry and Reeves, 1980; Kulshrestha
and Jain, 1982). However, this yield potential increased markedly with the
incorporation of the Norin 10 gene in wheat. These short, fertilizer-responsive
wheats had been grown in Japan long before scientific plant breeding and were
successfully used in Italy 60 years ago (Athwal, 1971). However, it was only
when Vogel at Pullman and then Borlang at CIMMYT used this material that the
so-called semidwarf wheats made such an impact on world wheat yields in the
1960s and 1970s. These wheats are resistant to lodging and have many tillers and
grains per spikelet.
Reduced stature and resistance to lodging are characteristics sought in the
“all-crops ideotype;” increased number of grains per spikelet in the semidwarf
wheats defines the expression of increased yield. On the other hand, the freetillering and relatively broad, lax leaves of these highly successful semidwarf
varieties are a challenge to the all-crops ideotype.
Many workers believe that only a small number of tillers is needed to give both
maximum yields and sufficient plasticity to permit adaptation to the environment
(MacKey, 1966; Hurd, 1969; Bingham, 1972; Jones and Kirby, 1977). This
view was extended by Donald (1968a,b), who described a wheat ideotype for
high grain yields with a short, strong stem, few small, erect leaves, a large ear in
relation to the total dry matter (i.e., a high harvest index), an erect ear, awns, and
a single culm. (In view of the authorship of that article, this wheat ideotype
conforms to the common ideotype for all annual seed crops described in this
article). Atsmon and Jacobs (1977) have produced uniculm wheat lines of medium height, high harvest index, and resistance to lodging; they appreciably outyielded the standard cultivar of the region. Further evidence for the potential of
controlled tillering was presented by Islam and Sedgley (1981), who examined
the effects of manually detillering wheat plants in the field to give biculms. The
performance of these was compared with normal-tillered control plots of the
same variety. The detillered plants outyielded the controls by 14 and 22%,
respectively, in 1978 and 1979.
Barley (Hordeurn distichurn) is a cereal with a growth form and physiology
similar to wheat, so that most considerations of canopy structure, tillering, and
leafiness are applicable to both species. Cultivated barleys vary in height from
brachytic forms (40 cm), widely grown in the Middle East, to tall forms (1.5 m)
(Reid and Weibe, 1968). They tiller freely at low density, especially the tworowed types, but there is a mutant form with a single stem per plant (uniculm).
There are considerable differences in leaf size, with extremes of leaf shortness in
CONVERGENT EVOLUTION OF ANNUAL SEED CROPS
brachytic kinds and of leaf narrowness in the mutant form governed by a single
gene. Two-rowed barleys generally have narrower leaves than six-rowed forms.
During this century there have been two principal trends relating to plant form
and productivity, as illustrated among varieties released, in the United Kingdom.
There has been a progressive reduction in height from about 1 m (cv. Spratt,
pre-1900) to semidwarfs of about 70 cm. This has been accompanied by an
increase in harvest index from about 0.4 to 0.5 (Cannell, 1968; Hayes, 1970;
Donald and Hamblin, 1976). This increase has not been consciously sought but
is the result of a substantially constant biological yield concomitant with advances achieved by selection for grain yield, early flowering, and reduced plant
height (Hamblin and Rosielle, 1983).
Interest in plant form as a contributory feature to future yield relates to height
(further reduction seems probable), reduced leafiness, and less tillering. Jones
and Kirby (1977) believe that although tillering is invaluable for adaptation to the
environment, it can serve this role adequately with only a small number of tillers.
Donald (1977), using his wheat ideotype as his model, has produced semidwarf,
uniculm barleys which, when sown at about double the standard seed rate,
outyield the leading local cultivars by 15-20%. However, these lines were not
evaluated for grain quality.
The initial attempts to produce radically new cereal plants (Atsmon and Jacobs, 1977; Donald, 1979) are sufficiently promising to warrant further effort. As
well as the increases in yield that are evidently attainable through dwarfing and
the elimination of tillering, further substantial increases may be possible through
the development of nonleafy lines with short, narrow, erect leaves (Hamblin and
Donald, 1974). The retention of awns seems desirable (Frey, 1971). The use of
biological yield and harvest index as a means of interpreting behavior during
breeding programs for yield has been strongly advocated (Donald and Hamblin,
Until 1900, the typical cultivated rice was a tall, strongly competitive plant
which had emerged by natural selection within man’s crops because of its capacity to suppress weeds and more dwarf kinds of rice (Jennings, 1964; Athwal,
1971). It had long, broad, drooping leaves and thick culms, was strongly photoperiodic, and was subject to serious lodging, particularly if fertilizer was
applied. The first development of a more productive rice of communal habit was
in Japan early this century, when cultivars of Oryzu juponicu were bred with
erect habit, reduced height, short, stiff straw, and fewer tillers. These varieties
did not lodge with heavy applications of nitrogen. This was followed by 0.
indicu varieties of similar noncompetitive habit, first in Taiwan with the release
C. M.DONALD AND J. HAMBLIN
of the cultivar Takhung Native 1 (TN1) in 1956, and then in 1966 at the
International Rice Research Institute with the release of IR8, a variety that
transformed rice yields over a great part of Southeast Asia. These two 0.indica
cultivars, TNl and IR8, each derived their semidwarf habit and erect leaf growth
from the variety Dee-geo-woo-gen, a mutant from an old Chinese variety, Woogen (Athwal, 1971).
Additional features of IR8 in relation to the common seed crop ideotype were
its nonphotoperiodicity,permitting use over a much greater geographic area, and
an increased harvest index. Six older, tall, competitive varieties had a mean
harvest index of 0.36, whereas the dwarf, erect, short-leaved varieties had an
index of 0.53 (Chandler, 1969). Poor tiller survival because of intense mutual
shading and the cessation of growth after flowering were believed to contribute
to the low harvest index of the tall, leafy varieties.
The other features of the common ideotype and its culture that may offer
opportunity in rice production are nontillering (Japanese cultivars already show
duction in tiller number) add the use of high plant densities. These features are
of course Wed. As long as most of the world’s rice is transplanted by hand at
enormous human effort from seed bed to paddy field at low plant densities (about
20 plants/m2), heavy tillering is essential. But in situations where rice is broadcast or aerially sown there may be potential gains in yield from less freely t i l l e d
or even uniculm rices of higher harvest index sown at heavier seeding rates.
The wild progenitor of maize was probably relatively dwarf, with several
tillers each having a terminal inflorescence carrying both male and female
flowers and several small ears in leaf a i l s (Mangelsdorf, 1965). The terminal
inflorescence broke easily, assisting seed dispersal. With the exception of the
United States corn belt dent types, the principal Commercial types of maize were
fully developed by the American Indians; little genetic advance was made until
the development of commercial hybrid corn in the 1930s (Mangelsdorf, 1965;
Galiiat, 1965). During domestication strong artificial selection by man for large
ears occurred, but natural selection of fecund plants probably occurred in fertile,
man-made environments (Wilkes, 1977). The trend to a single stem and large ear
was a consequenceof man’s preference for large, easily hand-harvested cobs and
easy cultivation between rows and of natural selection for tall competitiveplants
with many offspring.
Tall competitive plants were regarded favorably,’ but a direct consequence
was low optimal plant stands [e.g., 26,0001ha was considered a high density in
‘An Iowan would boast, “I’m from Iowa where the tall corn grows!”
CONVERGENT EVOLUTION OF ANNUAL SEED CROPS
Iowa in 1924 (Stringfield, 1964)l. However, during the 1950s a growing appreciation of the interaction between genotype, density, and fertility occurred
(Stringfield, 1964), particularly when it was found that dwarf plants suffered
much less sterility (5%) than normal plants (62%) at high densities [105,000
plantslha (Sowell, 1960)]. The importance of leaf distribution was also realized.
With leaves more vertically disposed above the cob at high densities, light
penetrates deeper into the canopy and yields are higher (Pendleton et al., 1968;
Winter and Ohlrogge, 1973; Vidovic, 1974; Pepper et ul., 1977). Horizontal
leaves were better at low densities, whereas at intermediate densities or in widely
spaced rows leaf angle was not important. If no response to leaf angle is found,
this probably results from sampling too narrow a density range, from leaves that
are not stiff enough along their entire length to maintain a constant leaf angle or
from the range of leaf angles that are too small to allow differentiation (Mock and
Pearce, 1975). Nonetheless, responses to high leaf angles occur only at leaf area
indices rarely obtained in commercial crops.
Mock and Pearce (1975) presented features for a maize ideotype that included
(1) stiff and vertical leaves above the ear and horizontal leaves below; (2)
maximum photosynthetic efficiency; (3) efficient conversion of photosynthates
to grain; (4) short interval between pollen shed and silk emergence; (5) ear shoot
prolificity; (6) small tassel size; (7) photoperiod insensitivity; (8) cold tolerance
for germinating seeds and seedlings (in areas where soils are cold and wet at
planting); (9) a grain-filling period as long as is practical; and (10) slow leaf
senescence. Features (4) and (6) relate specifically to maize and (8) relates to
summer crops. All other features (assuming that ear shoot prolificity and small
tassels are components improving harvest index) are common to every annual
Temperature maize production now uses many of these ideas, and similar
objectives are currently being applied to tropical maize. Tropical cultivars are
often tall (up to 3.5 m) and leafy, an competitive evolutionary response. They
lodge easily and have low harvest indices (less than 0.35). Selection at CIMMYT
for reduced height and leafhess within the cultivar Tuxpeno (CIMMYT, 1979)
has reduced height by 8 cm/cycle so that plants now are only 60% of their
original height; at the same time yield increased 190 kg/ha or 3% per cycle (cf.
comments on Gardner’s mass selection program, Section IV,A,5). This increase
is associated with reduced lodging, higher harvest index [0.35-0.48 during 15
cycles; cf. comments of Hamblin and Rosielle (1983) on the height-harvest
index relationship], and increased crowding tolerance (optimum planting density
of 45,000 plantslha at cycle 12 and of 60,000at cycle 15). Flowering was 13
days earlier and there were three leaves less below the ear. Thus similar plant
type-density relationships occur in both temperate and tropical regions.
Questions for future investigation are, What will be the equilibrium situation
between these features and yield in maize? Should we examine the potential of
M. DONALD AND J. HAMBLlN
maize crops sown at 160,OOO plants/ha (25-cm square planted) producing leaf
area indices similar to other cereal crops but with improved canopy-light relationships, a low incidence of barrenness, and a high harvest index?
Only circumstantial evidence is available regarding the early history of
sorghum (Sorghum bicolor). Doggett (1965) proposed that domestication first
occurred in the Abyssinia-Sudan region about 5000 years ago. However, Harlan
(1971) considers that sorghum had a more diffuse sub-Saharan origin. From
there it spread to other parts of Africa, to India before lo00 B.c., and to China
about A.D. 1300. The wild relatives of sorghum, widely distributed over the
African continent, characteristically have large, pyramidal, loose inflorescences
with spreading branches. Although mainly annual, some are perennial with short
rhizomes; the racemes articulate at maturity, assisting natural spread; and they
have small grains (de Wet and Huckabay, 1968; de Wet and Schechter, 1977).
Cultivated grain sorghums have heads of varying degrees of compactness,
from loose to extremely dense with tough rachises and persistent spikelets,
features ascribable to selection by man and to natural selection, respectively.
Competition for light within sown crops gave tall types a powerful advantage, so
that village crops in Africa may be as tall as 3.5 m (Goldsworthy, 1970). Grain
sorghum in the United States prior to 1928 was commonly 140-180 cm tall
(Quinby and Martin, 1954). These cultivars were annual or weakly perennial,
although without rhizomes. They were capable of regrowth from the crown to
produce a second crop, permitting ratooning. Because of the wide geographic
distribution of cultivated sorghums and the free hybridization between genotypes, many distinctive races can now be found (de Wet and Huckabay, 1968;
de Wet and Harlan, 1971).
Breeding programs with sorghum have had several clear-cut objectives.
Through the use of dwarfing genes, striking reductions in height have been
achieved with associated increases in grain yields. By 1953, 98% of the grain
sorghum cultivars in the United States were of dwarf stature and harvested by
combine (Quinby and Martin, 1954). Reductions from 1.5 to 1.2 m in singledwarf material and to 0.75 m in double-dwarf lines were typical of the reductions
in height (Queensland Department of Agriculture, 1970). In Africa the use of
dwarfing genes occurred later, partly because of the value of the tall stems for
building and fodder in village life. Reductions in height are, however, now
occurring in that continent (Goldsworthy, 1970).
Another objective has been the incorporation of one or more genes for insensitivity to photoperiod, giving earlier flowering and permitting the progressive
extension of sorghum to cooler areas of shorter season. At least three additive
genes are involved (Quinby and Karper, 1945; Quinby and Martin, 1954). Since
CONVERGENT EVOLUTION OF ANNUAL SEED CROPS
1954, the great advance has been the discovery of cytoplasmic sterility, permitting commercial use of hybrid sorghums with yields about one-third greater than
those of pure lines. The heterotic manifestations are higher metabolic efficiency,
increased height, earlier flowering and longer grain-filling period, greater vegetative yield, and increased grain size and yield (Quinby, 1963).
With reduced height, increased vigor, and higher soil fertility, there has been a
growing need to manage the crop so as to regulate panicle number per square
meter, taking account of the seeding rate, estimated seedling establishment, and
the probable tillering behavior (Ross and Eastin, 1972). The row spacing adopted
is commonly as close as will permit cultivation (75 cm, or double rows 30 cm
apart, at 100 cm); dry-land populations of 50,000-80,000 and of 250-300,OOO
plants/ha under imgation have been adopted in the United States.
The natural evolution, under cultivation, to very tall competitive plants has
thus been followed by a controlled move toward communal plants, that is,
toward dwarf stature and much-reduced tillering. Some sorghum cultivars are
described as “single-stemmed,” although they tiller at low densities. The opportunities for further progress toward communal plants and higher grain yields
seem to lie in further increases in plant density; the development of lines of
strictly uniculm habit, shorter, narrower leaves, more erect leaf disposition, and
markedly narrower rows without interrow cultivation (Clegg, 1972).
The earliest known cultivation of the common bean (Phaseolus vulgaris) was
at least 7000 years ago (Kaplan and McNeish, 1960; Kaplan el a l . , 1973). Beans
probably evolved over a wide area (Harlan, 1971; A. M. Evans, 1980); they
were a valuable component of the American Indian diet, and their use extended
over much of central and north America to about 42”N and over western South
America. P . vulgaris has been used both for green beans and as dry beans; it is
with the latter use, as a seed-bearing, annual field crop, that we are concerned
The wild progenitor is P . vulgaris f. aborigineus, the climbing thicket bean, a
perennial form with strong branching and a tuberous root. The cultivated species
is very variable in growth habit, ranging from indeterminate climbing types to
determinate bush types with 3-6 nodes on the primary stem (A. M. Evans, 1980).
The climber, grown on a trellis or in association with maize, was the earlier
cultivated form, with a branching, indetenninate habit of growth. The dwarf or
bush type, used for seed production in presentday mechanized agriculture, is
known (from vegetative remains) to have been grown as a crop in indigenous
Mexican agriculture at least 800 years ago. It is of determinate growth habit, the
main stem and each branch having a terminal flower after 3-6 nodes.
The responses to domestication in American Phaseolus beans are summarized
DONALD AND J. HAMBLIN
Rcspoase of Phase&
v u f g d is to -u
chuacteristic of plant as
Type of selectionb
W d Y pmnnial
Testa colors and patterns few
More erect to erect habit
Testa colors and patterns, many
1 and 2
“From A. M. Evpos (1980),Smartt (1969).aad Purseglove (1968).
’1, pmbably conscious selection by man; 2, natural selection in agricultural environment.
and classified in Table I. The mechanisms of some of the changes may be
debated, and some changes may have multiple causes, but the grouping of most
of them is self-evident. Only man himself could have selected the dwarf determinate form; as Smartt (1969) remarks, “In nature or mixed cultivation the
dwarf determinate mutant would have been effectively lethal . . . man has preserved and propagated a key mutant.” We thus see that the American Indians
deliberately selected and developed for cropping a shorter, less competitive
plant; a selection of the unfit. This step has been repeated in wheat and rice by
modem workers loo0 years later. The dramatic increase in seed size, although
undoubtedly leading to a reduced number of propagules, must have also been
attained through deliberate selection. Some seed colors may have had natural
selective advantage (fungistatic properties of pigments, less predation by birds),
but the choice of particular colors by man has been the all-powerful factor in the
local evolution of color patterns.
Various consequences result from man’s conscious selection, particularly effects on growth form related to selection for reduced height. However, many
important characteristics of modem field beans result from natural selection
within the climatic or cultural environment of man’s crops. The most notable of
these is the annual habit, an ability to complete the life cycle before killing frosts
prevent the production of viable seeds. The perennial habit suffices in subtropical
areas, but did not permit survival as the cultivation of the bean extended northward.The responses in time of maturity, photoperiodism, and ready germination
CONVERGENT EVOLUTION OF ANNUAL SEED CROPS
were all responses to the climatic or man-made environment, and large leaves
gave competitive advantage over neighbors for light. The loss of pod dehiscence
enabled survival of the harvested seed to be sown the following year (A. M.
Evans, 1980). There is no doubt that the evolution of the bean under domestication in the Americas was in many ways more advanced, by several centuries,
than the evolution of rice as a crop in Asia or of wheat as a crop in Europe.
What developments offer further increases in seed yields in the common bean?
It was suggested by Adams (1973) that major reduction in branching is desirable,
so that each plant has a main stem and a few short lateral branches with many
pods at each nude. Smaller leaves are also indicated as a means of securing
deeper light penetration into the canopy. To be effective, these changes must be
accompanied by increased density of the stand and strong pursuit of improved
harvest index of communal plants growing in a strongly competitive crop
Viciufubu includes the field bean and the broad bean; the former, Viciufubu
var. minor, is here considered. It is an erect annual with a main stem and,
depending on plant density, one to several lateral stems. Each stem is indeterminate in growth, with 5-10 basal vegetative nodes and about 10 nodes with
axillary inflorescences followed by a continuing production of vegetative nodes
(Poulsen, 1977; Chapman and Peat, 1978). Most of the seed is produced by the
main stem, with one or two pods per inflorescence and four to six seeds per pod.
There is competition both among the developing pods and between the pods and
the further vegetative growth (Chapman and Peat, 1978), a situation closely
comparable to the tall sorghum genotypes discussed earlier.
The weaknesses in this plant structure are evident, namely, unnecessary height
and vegetative growth associated with the indeterminate production of sterile
nodes above the pods. Various useful genes, principally simple recessives, are
available, including those for dwarf stature and for a terminal inflorescence.
Crossing has shown (Chapman and Peat, 1978) that there are excellent prospects
for developing field beans of reduced, erect stature with upright pods borne
terminally and leaf sizes reduced to no more than two leaflets; the problem of
massive amounts of vegetative tissue passing through the harvesting machinery
would thus be alleviated.
The yields of these semidwarf determinate types so far are less than those of
current tall cultivars because of the inadequate yield of seed per node; more seeds
per pod are sought. Two points may be made: first, the reported tendency of
determinate forms to produce more branches (Chapman and Peat, 1978), may
cancel the gains achieved through reduction of the vegetative tissues of the main
stem. Nonbranching determinate forms seem highly desirable. Second, any test-
C. M.DONALD AND J. HAMBLIN
ing of such types should be undertaken within high density communities with
continued emphasis on harvest index as a guide to efficiency within the biomass
of the crop. The efficiency or yield of isolated plants is irrelevant. A disadvantage of reduced branching and increased density may be seed requirements, as
the large seed weight of this species will mean that seed costs will be
The wild ancestor of the cultivated soybean (Glycine man) is believed to be
Glycine soja, indigenous to China, the Soviet Union, Korea, Japan, and Taiwan.
G . l l u u ~and G . soju have few barriers to hybridization and on this and other
grounds are regarded as conspecific (Hymowitz and Newell, 1980). Both species
are annuals, but although the wild G . soju is a slender twiner, characteristic of
hedges and roadsides, the cultivated soybean is a bushy shrub. The domesticated
plant differs also in having reduced dehiscence of the pods and larger seeds of
higher oil content. The soybean was probably domesticated in the eastern half of
north China in the eleventh century B.c., spreading to Southeast Asia in the early
centuries A.D. It was not known to European agriculture until the early eighth
century nor to North American agriculture until the 1850s (Hymovitz and Newell, 1977, 1980).
. Three growth habits are present in soya: determinate, semideterminate, and
indeterminate; genetic control is by two genes. There are marked differences in
the source-sink relationships between these types (Shibles, 1980). Narrow rows
and higher plant populations often produce yield increases (Costa et ul., 1980).
This may result from improved light relationships (Shaw and Weber, 1967) or
improved water-use efficiency (Peters and Johnson, 1960; Timmons et al.,
1967). Narrow leaf types give better light penetration, but this was not associated
with increased yields (Hicks et al., 1969), although they had high water-use
efficiencies (Hiebsch et ul., 1976).
The potential for manipulating the soybean plant to develop communal plants
appears excellent. However, as they will be poor competitors in mixtures, and as
competitive ability is related to branching, height, and late maturity (Mumaw
and Weber, 1957; Hinson and Hanson, 1962; Schutz and Brim, 1967), care must
be taken to ensure their retention in segregating populations. Also, they must be
yield tested in pure culture at high densities if their full yield potential is to be
Davies (1977a,b) has reviewed the dramatic developments in the pea (Pisum
surivum) crop. There has been a marked reduction in stature from a height of 1-2
CONVERGENT EVOLUTION OF ANNUAL SEED CROPS
m for garden peas to 0.3-0.6 m for field peas. Yet even with this considerable
dwarfing, two major physical problems remain: the great bulk of vegetative
material to be handled during harvest and the frequency of severe lodging
(amounting almost to certainty) with loss of the canopy structure and further
deterioration of the light profile. These dwarf pea crops, despite their reduced
height, have much in common with the old, tall rice varieties (large, horizontally
disposed leaves and poor physical stability). Two mutant genes now offer the
prospect of improved canopy structure. The first reduces the leaflets to tendrils
and the second reduces the leafy stipules to small bracts. With only the f m t of
these genes the plant is known as “semi-leafless”; with both, it is “leafless.”
Here then is a dramatic reduction in leafhess. Leafless, and particularly semileafless, crops promise to outyield standard varieties (Davies, 1977a,b; Hedley
and Ambrose, 1981). The advantages for seed crops may be several. First,
leaflessness permits a much deeper penetration of light into the crop and thereby
a more effective mean illumination of the photosynthetic surfaces; second, the
interlocking tendrils give such effective mutual support that lodging cannot occur; third, the reduction of vegetative parts contributes to a higher harvest index;
and forth, leafless peas may use water more efficiently than leafy types. Perhaps
the radical structure of the canopy of these peas may offer, for the f m t time,
prospects of yields from legumes more closely comparable to those of cereals.
A feature of the pea crop that warrants fuller examination is the extent of
vegetative branching, which has already been reduced in some dwarf genotypes.
Increased sowing rates of leafless, nonbranching plants probably would improve
the crop productivity by these most unusual plants. It is of interest at this point to
note the features that existing leafless pea plants have in common with the
semidwarf rice varieties: reduced stature, reduced leafhess, better light profile,
improved physical stability, improved synchrony of flowering, and, almost certainly, better harvest indices.
In pre-Columbian times, all the cultivated cottons of Central and South America were perennial shrubs confined to tropical regions. These perennial American
cottons founded the crops of southern Europe, Africa, and India, but since the
mid-eighteenth century, three annual forms have evolved within these crops;
upland cotton (Gossypium hirsutum), and sea island and Egyptian cottons (barbadense) (Phillips, 1976). There was a progressive change under cultivation
from xeric, wild species to cultigens adapted to more fertile soils and more
abundant water (Stebbins, 1974). A major selective force was the extension of
cropping into temperate regions, where the frost-free season was progressively
shorter. Differential seed production, in favor of plants adapted to the climatic,
C. M. DONALD AND J. HAMBLIN
soil fertility, and water regimes of new environments, ensured the natural selection of mesic, early-flowering annuals.
Cotton culture, as exemplified by that in the United States, has been of
branched, annual shrubs, typically in rows about 1 m apart with about 5-15 cm
between plants. Where irrigation is practiced, these rows run centrally along flattopped “hills,” between irrigation furrows spaced at 1 m. In the mid-l960s,
however, a major change in cotton culture was foreshadowed through the study
of “narrow-row cotton.”
In the f m t paper on narrow-row cotton, Ray and Hudspeth (1966) stated that
the primary objective was to determine whether yields could be increased substantially through high popylations with large amounts of fertilizer and irrigation
water. They found (Brashears et al., 1%8)that population increase was effective
in raising yield only if it was achieved by the closer spacing of the rows rather
than by increasing plant number within the row. When the population was
increased to about 250,000/ha and the mean row width decreased to 50 cm (i.e.,
with 2 rows 40 cm apart on each 1-m hill), the following changes ensued:
reduced plant stature, nonbranching, frost avoidance through earlier maturity
(8-10 days), more simultaneous ripening of the bolls (more uniform cotton
quality), and higher yield. Similar results have been reported elsewhere (for a
review see Low and McMahon, 1973).
The study of narrow-row cotton culture led cotton workers to ask themselves
two questions: Can a more suitable genotype be developed for use in narrow
rows at high population density? and, Can machinery be developed to harvest
narrow rows, pferably in a “once-over” operation? The outcome has been a
trend toward dwarf, determinate cotton varieties, with bolls borne on short
fruiting stalks so that they lie close to the stalk. These varieties are also described
as “stom proof.” They can, as was hoped, be harvested at a single stroke.
These cotton cultivars and the system under which they are grown are parallel
to the common ideotype to be discussed in the next section in many aspects.
Bhardwaj et af. (1971), working in India, reported a negative correlation between yield of seed cotton and both plant height and leaf area. They emphasized
the need for more dwarf cultivars with fewer branches and less leaf area. Constable (1977) states that in Australia there is also a need for varieties bred specifically for narrow-row culture with reduced leafhess.
There are clear opportunities to improve the light profde of the crop through
the use of Okra types of cotton, which have deeply cleft leaves of much reduced
area, but the field evidence in favor of such foliage is as yet inconclusive
(Andries et al.. 1969, 1970; Constable, 1977; Pegelow et al., 1977). Certainly,
Okra leaf is unlikely to offer advantages in 1-m rows, but it may well prove of
significant value at high density in narrower rows. There has been no clear
statement regarding the influence of narrow-row culture on the ratio of lint or