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II. Comparative Responses of Peas to the Crop Environment

II. Comparative Responses of Peas to the Crop Environment

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230



C. L. HEDLEY AND M . J . AMBROSE



A detailed analysis of the data revealed an overriding difference between the

two phenotypes compared with differences between genotypes within a

phenotype. These differences were so marked for biological and economic yield

that it was possible to combine data from genotypes within a phenotype and then

make comparisons between the two phenotypes.

For a given planting density the two phenotypes produced canopies that differed in their ability to intercept light. This was demonstrated by comparing the

two near-isogenic lines for the amount of light intercepted at soil level and at the

level of the first flowering node (Fig. 3a and b). The measurements were made at

a stage when seeds were developing in the early pods. Canopies composed of the

leafed line intercepted almost 90% of the light before soil level (Fig. 3a), even at

the lowest planting density (16 plant/m'). Canopies composed of the leafless

line, however, only approached this level of light interception at the highest

planting density (400 plants/m2), light interception declining progressively as

planting density decreased. Differences were even more marked at the level of

the first flowering node (Fig. 3b), with the leafed genotype intercepting about



100



100



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Square of Distance B e t w e e n Plants (cm'x lo-' )



FIG.3. Comparison of percentage light interception for two near-isogenic pea lines over a range

of planting densities. Conventional leafed (JI 1194, AfAfStSr. 0-0) and leafless (JI 1198,

afafssrsr, 0



4 ) at soil level (a) and at the level of the first flowering node (b). Numbers in

parentheses are the planting densities (plants per square meter).



I



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



23 1



70% of the light at densities down to 25 plants/m2 and more than 50% at 16

plants/m” In contrast, none of the leafless canopies exceeded 30% light interception at the first flowering node level. A similar increase in light penetration has

been shown for the super okra leaf mutant of cotton (Kerby et al., 1980), which

is perhaps the only leaf variant that is comparable to the leafless character.

It is very evident therefore that light will penetrate the leafless canopy and

reach photosynthetic structures below the level of the first flowering node.

Photosynthetic structures produced early in the leafless plant’s development

therefore may remain above the C02 compensation point for longer and compensate to some extent for the poor light interception of the crop above the first

fruiting node. Photosynthetic activity of the lower leaves of soybeans (Johnson et

al., 1969), beans (Phaseolus vulgaris; Crookston ef al., 1975), and alfalfa

(Brown ef al., 1966) have all been shown to be increased as a result of increased

irradiation. Such compensation in the leafless pea canopy, however, may only be

significant at population densities in excess of 44.4 plants/m2, since 50% or more

of the light falling on canopies composed of lower plant populations fails to be

intercepted. The extent to which photoassimilate produced by the photosynthetic

structures below the first flowering node is incorporated into developing fruits is

unknown. It has been shown that the major photosynthetic contribution to the

fruits of spaced plants is derived, in leafless and leafed peas, from the tendrils or

leaflets subtending each developing pod (Harvey, 1974). It has been suggested

that the photosynthetic potential per unit area of the tendrils of the leafless mutant

may be higher than for the leaflets of the leafed phenotype (Harvey and Goodwin, 1978). If this proves correct, then tendrils subtending pods would be more

efficient and compensate to some extent for the poor light interception at the top

of the crop.

One consequence of more light penetrating the leafless canopy to soil level is

an increase in soil temperature compared with corresponding leafed canopies

(Table I). At 100 plantslm’ a soil temperature of 29.8”C in the leafless canopy

was reduced to 24.2”C in a corresponding leafed population. This temperature

difference increased to 8°C when the populations were reduced to 25 plants/m2.

The physiological consequences of this increased soil temperature are not

known. It is likely, however, that there will be a higher rate of evaporation from

the soil surface of the “leafless” crop, although this may be more than compensated for by a decrease in the rate of water utilization by leafless plants (Harvey,

1980). Although soil temperatures differed between the two phenotypes, air

temperature within the canopies were much more similar, and at 100 plants/m2

were not significantly different. Greater air movement in the leafless canopy may

compensate for the increased level of radiation penetrating the crop, although at

present there has not been any research on this aspect of leafless canopy structure.

The differences in light interception between leafed and leafless canopies



C. L. HEDLEY AND M. J . AMBROSE



232



Table I

Comparison of Temperatures (“C) through the Crop Canopies of JI 1194 (Leafed) and

JI 1198 (“Leafless”) at Two Planting Densities

Planting density (plantsh’)

Height

within

canopy (cm)



25



I00



JI 1194



JI 1198



JI 1194



JI 1198



25.6

25.2

25.4



24.9

24.9

26.4



24.6

25.6

26.2



27.2

27.5

28.7



24.2



29.8



27.5



35.5



~



30

20

10

1 cm below



soil level



correlate with the differences between the two phenotypes in total above-ground

biological yield per unit area (Fig. 4a). The biological yield of the leafed

canopies decreased marginally as the planting density was increased. The leafless

phenotype, however, had a biological yield per unit area at the lowest planting

density (16 plants/m2) that was approximately half that attained by the corresponding leafed canopy. At higher planting densities the biological yield per unit

area of the leafless phenotype increased progressively and at densities in excess

of 100 plants/m2 exceeded that attained by the leafed canopies at any of the

population densities.

The responses observed on a unit area basis are obviously a reflection of the

responses of individual plants within the population. As the space available to

individuals is increased (plant density decreased) so plants will take advantage of

increased resources and grow larger. There will therefore be a tendency for low

plant populations to compensate by each plant contributing more biological yield

to the total biological yield per unit area. This explanation, although true, is an

oversimplification of the effect of planting density on the individuals within the

population. The “average” plant response, as determined by dividing the response per unit area by the number of individuals, does not convey the differences between plants within each population, induced by interplant competition. The use of average plant values, while ignoring these complex plant-toplant interactions, does, however, indicate how genotypes (or phenotypes) are,

in general, responding to particular environments.

Over the range of planting densities, the average leafed plant (Fig. 4b) had the

capacity to compensate for a 25-fold increase in the available space [25 cm2/plant

(400 plants/m2)to 625 cm’/plant (16 plantdm’)] by a 30-fold increase in biological yield. The average leafless plant (Fig. 4b), however, could respond to a

similar increase in space by only a 12-fold increase in biological yield. The

biological yield per unit area of the leafless phenotype was therefore greatly



233



DESIGNING “LEAFLESS” PLANTS FOR DWED PEA CROP



reduced at low population densities. An alternative view is that there was a steep

reduction in the biological yield of average leafed plants as the space available

was reduced, whereas the effect on average leafless plants was less severe. The

result of this differential effect on average plants of the two phenotypes was for

the biological yield per unit area of the leafless phenotype to exceed that of the

leafed when the space available was less than 100 cm2/plant (density greater than

100 plants/m2).

Up to a density of approximately 100 plants/m2, the effect of planting density

on the economic yield per unit area (seed dry weight per square meter) of the two

phenotypes (Fig. 5a) was similar to the effect on biological yield (Fig. 4a). Seed

weights per unit area of the leafed phenotype decreased marginally between 16

and 100 plants/m2, whereas those of the leafless phenotype increased progressively up to this density. As with the biological yield, the economic yield per

unit area of the leafless phenotype exceeded that of the leafed at densities of

approximately 100 plants/m2 and greater. The economic yield per unit area of

both phenotypes, however, was significantly reduced between 100 and 400

plants/m2.

7-



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100 400)



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6



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3



2



A



A



A



( 16



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44



1

A



loo4oo)



Square of Distance Between Plants ( c m 2 x



FIG.4.



0

A



Biological yield in grams per square meter (a) and grams per plant (b), of leafed



(0-0) and leafless ( 0 4 )phenotypes, over a range of planting densities. Each phenotype is

the mean response of three genotypes. Numbers in parentheses are the planting densities (plants per

square meter).



234



C. L. HEDLEY AND M. J . AMBROSE



4



3



N

I



b



2



N*



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1



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looroo)



Square of Distance Between Plants ( an2 x



lo-'



)



Economic yield in grams per square meter (a) and grams per plant (b), of leafed

phenotypes, over a range of planting densities. Each phenotype is

the mean response of three genotypes. Numbers in parentheses are the planting densities (plants per

square meter).



FIG.5.



(0-0) and leafless (0-0)



The effect of planting density on the seed yield of average plants (Fig. 5b) was

more extreme than the effect on biological yield (Fig.4b). This was most marked

for average leafed plants that had a 45-fold reduction in seed yield per plant when

space per plant was reduced 25-fold [625 cm2/plant (16 plants/m2) to 25 cm2/

plant (400 plants/m2)] compared with a 30-fold reduction for biological yield.

Average leafless plants had a 16-fold reduction in seed yield per plant compared

with a 12-fold reduction for biological yield.

Although the degree of response of the two phenotypes to planting density was

very different, there was a remarkable similarity between them in the efficiency

with which biological yield was partitioned into economic yield (Fig. 6), often

termed the harvest index (Donald, 1962). Both phenotypes partitioned approximately 50% of their biological yield per unit area into seed at planting densities in

the range 16- 100 plants/m2,this figure declining to nearer 35% at 400 plants/m' .

The reason for the decline in the harvest index at the highest planting density,

however, was different for the two phenotypes. In the leafed phenotype the

decline was due almost solely to a reduction in economic yield per unit area,

whereas in the leafless phenotype it was due partly to a reduction in economic

yield and partly to a continued increase in biological yield per unit area, at this

high planting density.

There was also a close similarity between the two phenotypes in the proportion



A



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



235



of total biological and economic yield produced by basal and axillary branches

(Table 11). Although there was a great deal of variation for this character between

the three genotypes that constituted each phenotype group, the overall trends

were similar in each case. It was apparent that on average, widely spaced plants

of both phenotypes branched profusely and contributed 40-50% of the biological and economic yield per unit area. This proportion was reduced progressively

as the planting density was increased and the average contribution made by

branches at 100 plants/m2 was reduced in both phenotypes to 5-10%. At the very

dense population of 400 plantdm’ all of the measurable biological and economic

yield was derived from the main stem of the plants.

B. CONSEQUENCES

OF GROWING

“LEAFLESS”PEASA T HIGH

PLANTING

DENSITIES



In the previous section it was demonstrated that “leafless” crops have only a

limited ability to produce compensatory increases in biological and economic

yields at planting densities below 100 plantdm’. From the observations on light

60



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50



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A



(16



25



3



2

A

44



1

A



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A



100 4 0 0 )



Square of Distance Between

Plants ( c m 2 x lom2)

FIG. 6 . Percentage harvest index for leafed (0-0) and leafless (0-0) phenotypes over a

range of planting densities. Each phenotype is the mean response of three genotypes. Numbers in

parentheses are the planting densities (plants per square meter).



236



C. L. HEDLEY AND M. J. AMBROSE



Table II

Effect of Planting Density on the Percentage Contribution of Branches to Total Biological

(BY) and Economic (EY) Yield of Leafed and Leafless Peas

Planting

density

(plantslm')

16



25



44.4

100



400



Leafed"



Leaflessb



% BY



% EY



46

23

10.7

5.4

0



BY



% EY



46

20

11.5

7.0



44.1

15

14.3



41.4

31.5

16.8

8.0



0



0



%



5.1



0



aMean of three leafed genotypes.

*Mean of three leafless genotypes.



penetration it is likely that this is due to a low leaf area index, which has been

shown by Watson (1947a,b) to be a major factor governing crop productivity.

As a consequence the biological and economic yield per unit area of leafless

crops will decrease to unacceptable levels at low planting densities, and it is a

necessity therefore to grow the plants in dense populations.

As with other crops, when leafless peas are grown at high planting densities

there will be an increase in the competition between adjacent plants for light,

water, and minerals. As well as affecting the growth and development of individual plants within the crop, interplant competition will also affect the yield of

the crop as a whole per unit area. It has been demonstrated for a number of crops

(Holliday, 1960a-c) that the effect of planting density on biological yield per unit

area is asymptotic (a plateau being formed over a large range of planting densities), while the effect on economic yield is parabolic (yield decreasing at

densities on either side of an optimum). Harper (1961) has suggested that this

differential response occurs because the weight of individual plants can almost

exactly compensate for changes in the number of plants per unit area, but the

allocation of assimilatory products to organs of the plant changes, often to the

detriment of the seed output:

The concept of an asymptotic biological yield curve and a parabolic economic

yield curve in response to increased planting density appears to hold for leafless

peas. Our evidence suggests, however, that unlike other crops, including leafed

peas, the planting density giving maximum biological yield may be higher than

the optimum density for economic yield (Fig. 7). Donald (1963) has suggested

that the maximum biological and economic yields occur at the same density when

the main limiting factor to production is light, and that the relationship may not

hold if water is limiting during seed production. It is possible therefore that

leafless crops at high planting densities may be limited by water before competi-



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



237



tion for light becomes critical and that economic yield, produced late in the

development of the crop, is disproportionately affected.

In addition, our data suggest that the maximum biological yield per unit area of

the leafless phenotype will exceed that of the leafed pea. The basis for this

difference is likely to be a reduced competition between the photosynthetic

surfaces of the leafless canopy for light. Donald (1961) suggests that competition

for light within a crop, unlike competition for water and minerals, does not occur

between plants but between leaves. If a leaf is shaded by another, then the

depression of the photosynthetic rate will be the same whether the superior leaf is

on the same plant or another. Once leaves at the base of a canopy become so

shaded that they are below the compensation point, they will die. When the rate

of death of these lower leaves equals the rate of appearance of new leaves, then

the leaf area index of the crop will become static. The improved light penetration

through the leafless canopy, even at high planting densities, would allow tendrils

at the base of the crop to remain above the compensation point for longer. The

attainment of the static leaf area index would therefore be delayed and a higher

biological yield per unit area would be achieved.

The most obvious effect of planting density on individual “leafless” plants

within a crop is the great reduction in plant size as planting density is increased.

As with other crops, once biological yield per unit area has reached its

76 -



7



0



5 -



c



K



N



4 -



E

3m



\



2 -



7



6

(1:



5



4



3



2



A



A



25



44



1

A



0

A



100 4 0 0 )



Square o f Distance B e t w e e n

Plants (cm2 x

Fic. 7. Effect of a range of planting densities on the biological (A-A),

economic

(0-0). and vegetative (0-0) yield of the leafless phenotype, derived from the mean of three

genotypes. Numbers in parentheses are the planting densities (plants per square meter).



238



C. L. HEDLEY AND M . J . AMBROSE



maximum, the mean weight per plant is inversely proportional to the density.

Kira et al. (1953) have shown that in soybean, once competition between plants

has occurred, the logarithm of the mean individual plant weight plotted against

the logarithm of the reciprocal density is linear and sloping. The slope becomes

steeper with increased plant age until it is 45",and the relationship extends over

the whole range of densities. At that time a constant final yield, irrespective of

plant number, is attained. Similar results were obtained when Kira ef al. (1953)

applied the same transformations to data derived from other sources (e.g.,

Donald, 1951).

The basis of our studies with leafless peas is to identify plant characteristics

that suit the plant to the crop environment. It was mentioned earlier that the use

of mean plant values in studies at high planting densities masks the complex

responses of individual plants. It is essential therefore to understand the relative

responses of individuals within the crop and the effect of specific plant characters

on the relative performance of individuals. There have been very few reports

where the responses of individual plants within a crop have been measured.

Koyama and Kira (1956) have shown for a number of species that a population,

initially of nearly uniform plant weight, will move progressively toward a skew

distribution. As growth continues there will be an increasing proportion of small

plants and a decreasing proportion of large plants within the population. Donald

(1963) suggested that crowding accelerates this process and that it is due to

increased variability of relative growth rate in crowded communities.

An increase in the variation between plants occurs as competition becomes

intense and is reflected in an increase in the coefficients of variation (CVs). For

example, Stem (see Donald, 1963), using subterranean clover, found that the CV

for plant weight was similar at all densities for 90 days and then increased sharply

at the higher densities. Certain plant characters respond more to increased competition than others. It has been shown for Zea mays (Edmeades and Daynard,

1979) that the CV for plant dry weight and for ear components increases with

plant density, whereas the CV for plant height and leaf area per plant were little

affected. Hozumi et al. (1955), in one of the few experiments specifically

designed to study the effect of competition on individual plants, found that

yellow dent corn plants within a row oscillate for weight and shoot length

between negative and positive relative to the overall plant mean. It was apparent

that if a plant grew vigorously, its neighbors were suppressed, and if its growth

were retarded, the neighbors were favored in their growth.

At very high planting densities distributions for plant size become so skewed

that self-thinning occurs. Donald (1 963) demonstrated that self-thinning in

wheat occurs to a density greater than that giving the highest grain yield per unit

area. This suggests that within a dense population survival of individuals has

precedence over total seed production per unit area.

How variation between individual plants in the population relates to the yield



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



239



per unit area of the crop and to stability of crop yield is not clear. It is assumed

that populations where the yields of individuals are distributed normally are to be

preferred to those where the distribution is skewed and the CVs are high. In other

words, a crop where all of the individuals yield something is to be preferred to a

crop where a few individuals yield much while others yield little or nothing.

It is the degree of competition between plants at high planting density that

determines how much individuals differ from each other. The extent to which

individuals interact within a crop is dependent on the competitiveness of the

individuals, the planting density, and the environment. Since leafless plants must

be grown at high planting densities, we can only reduce the interaction between

individuals by selecting genotypes that are less competitive or more tolerant of

high population densities. This is a similar conclusion to that derived for wheat

by Donald (1968), who states that, “The individual plant within the community

will express its potential for yield most fully if it suffers minimum interference

from its neighbours. ” Neighboring plants should therefore be weak competitors,

and the ideotype itself must be of low competitive ability.

It is not, however, sufficient to define a crop plant solely by its tolerance of

other individuals at high planting density. An ideal crop plant should also have a

high efficiency for partitioning its assimilate into economic yield. Donald (1968)

has stated for wheat that, “The successful crop plant should be of low competitive ability relative to its mass and of high efficiency relative to its environmental

resources.” Therefore, in the following sections of this article we discuss those

characteristics of leafless peas that may be incorporated into the ideotype to make

it more tolerant to high planting density and maximize biological yield per unit

area. We then define, to the best of our knowledge and experience, characters

that will maximize the efficiency with which biological yield may be partitioned

into economic yield.



111. AlTAINING MAXIMUM BIOLOGICAL YIELD PER

UNIT AREA

A. IDENTIFYING PLANTSTHAT ARETOLERANT

OF INTERPLANT



COMPETITION



The main conclusion from the previous section was that the most suitable

leafless crop plant will be tolerant of its neighbors at high planting densities.

“Average” plant responses can be used to indicate those characteristics that will

be advantageous to a genotype in a competitive environment. Average plants of

strongly competitive genotypes will greatly increase their biological yield when

grown at low planting density. When such plants are grown at high planting



240



C. L. HEDLEY A N D M. J . AMBROSE



density, however, they will compete so vigorously that the yield per average

plant will be drastically reduced. The density response for an average strongly

competitive plant will therefore be extreme and the slope of this response will be

steep (Fig. 8a). Weak competitive genotypes with the same duration of growth as

strong competitors, will not take full advantage of the resources available at low

planting densities and average plants will therefore have lower biological yields

than strong competitors. When such genotypes are grown as dense populations,

their reduced aggressiveness in competition for resources will reduce the interaction between individuals, and the yield per average plant will be less affected and

may be higher than that of the strong competitor. The density response for an

average weakly competitive plant will therefore be less extreme and the slope of

this response will be shallow (Fig. 8a).

The effect of density on an average individual is determined from measurements made per unit area of the population. The population-density interactions for strong and weak competitors (Fig. Bb), however, will be the inverse of

those for the average individual within a population (Fig. 8a). Yields per unit

area from a monoculture composed of a genotype that is tolerant of high planting

density will therefore show a steep positive response to planting density (Fig.

8b), whereas a population composed of a genotype that is intolerant of competition will show a shallow or even negative response to increased density (Fig. 8b).

Therefore, in theory a genotype's ability to tolerate competition can be determined from a comparison of yields per unit area of populations grown at a low

(noncompetitive environment) and at a high (competitive environment) planting

density. This comparison can be made to test the relative effect of specific

genotype characteristics on tolerance to competition. Comparisons using strongly



a



b

,I



"/



BIl

Unit A r m



Plant



low



High

Plant Density



,

I



low



,,

,,



,,



High



FIG. 8. Biological yield (BY) for weak (WC)and strong (SC) competitors at high and low

planting densities (a) per plant and (b) per unit area.



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II. Comparative Responses of Peas to the Crop Environment

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