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V. Progress and Prospects in the Development of Annual Seed Crops

V. Progress and Prospects in the Development of Annual Seed Crops

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122



C.



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.

B. BARLEY



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



123



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,

1976).



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



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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.

D. MAIZE



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



125



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

seed crop.

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



126



C.



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?

E. SORGHUM



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



127



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).

F. COMMON

OR AMERICAN

BEAN



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

here.

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



128



C. M.



DONALD AND J. HAMBLIN

Table I



Rcspoase of Phase&



v u f g d is to -u



chuacteristic of plant as

_____



Type of selectionb



Wild



DomaSticated



2

2

1



W d Y pmnnial

Short-day plant

Indeterminate

Many nodes

Scrambling habit

Small seeds

Hard-seeded

Physiological dormancy

Testa colors and patterns few

Small leaves

Pods dehiscent

Stems thin



Annual

Day neutral

Determinate

Few nodes

More erect to erect habit

Large seeds

soft-seeded

No dormancy

Testa colors and patterns, many

Large leaves

Pads indehiscent

Stems thick



?



2

1 and 2

2

2

1

2

2

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



129



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

situation.

G. FIELDBEAN

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-



130



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

appreciable.



H. SOYBEAN

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

realized.

I.



PEAS



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



131



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,



132



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



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