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V. Improving the Efficiency of the Pea Fruit

V. Improving the Efficiency of the Pea Fruit

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266



C. L. HEDLEY AND



M. J . AMBROSE



Unlike other legumes, the illuminated pod of the lupin (Lupinus albus) can

make CO, gains from the atmosphere for all but the last 2 weeks of its life (Pate

et al., 1977). It is also suggested that the pod acts as a temporary reservoir and

agent for remobilization of the respiratory products of the seed.

The pea (Pisurn sarivurn) pod is committed to exporting assimilates, derived

from CO, fixation, to the developing seeds (Lovell and Lovell, 1970). This

carbon is mainly derived from fixation of COBrespired by the seeds into the pod

cavity (Flinn and Pate, 1970). Most of the carbon required for seed development

originates from the subtending leaf and stipule (Flinn and Pate, 1970), the photosynthetic activity of which is modulated by the growth rates of the pod and the

seed (Flinn, 1974). Although the pea pod is capable of a net uptake of CO, from

the atmosphere only during the very early stages of pod development (Harvey et

al., 1976), its role in refixing and recycling carbon to the seed is substantial and

accounts for up to 20% of the fruits assimilate requirement (Flinn et al., 1977).

Both the photosynthetic carbon fixation enzyme, ribulose- 1,5-bisphosphate

(RuBP) carboxylase and phosphoenolpyruvate (PEP) carboxylase, often associated with dark C 0 2 fixation mechanisms, have been demonstrated within the

pod tissue of peas (Hedley er al., 1975). The distribution of these enzymes

within the pod wall emphasizes the refixation role of the pod, the inner epidermis

being rich in both carboxylase enzymes (Atkins el al., 1977). At high light

intensities these enzymes are capable of fixing 60% of the CO, released by the

seeds, although this proportion is reduced at the light levels present in the canopy

(Atkins et al., 1977).

It is likely that the type of pod that will be most efficient in the “leafless”

canopy, with its increased light penetration and improved standing, will be very

different in structure and physiology from that more suited to the leafed pea crop.

A wide range of genetic variation is available for pod type in Pisum sativum.

Large differences exist between genotypes for pod size and pod growth rate

(Hedley and Ambrose, 1980), and also for pod wall thickness (Wellensiek,

1925b) and for the presence or absence of schlerenchyma layers (White, 1917).

Comparisons, using genotypes that differed for wall structure or chlorophyll

content, have been made to assess variation for carboxylase activity (Price and

Hedley, 1980). The activity of both carboxylase enzymes was shown to vary

with pod type and with pod age, the activity of the photosynthetic enzyme (RuBP

carboxylase) correlating with the chlorophyll concentration. Yellow-podded

types had lower activities of this enzyme but also had higher absolute levels of

the dark fixation enzyme (PEP carboxylase). The PEP-carboxylase system was

shown for all pods to comprise a far higher proportion of the total carboxylase

activity than that normally found in leaf tissue exhibiting the C, photosynthetic

system (Price and Hedley, 1980).

As yet there is little information about the significance of different pea pod

types for the seed yield of the plant. Even more important, there is no informa-



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



267



tion about the effect of pod phenotypes when incorporated into “leafless” plants.

There is a suggestion from studies using near-isogenic lines for the uf and st

genes, that the net CO, uptake of the pods from leafless plants is higher than that

for corresponding leafed plants (Harvey, 1978). No clear explanation, however,

has been put forward to explain this observation. There is also little information

for peas on the role of the pod as a temporary storage organ. Although some

studies suggest that pods are committed to the export of assimilate to seeds (Flinn

and Pate, 1970), there is no reason to suppose that variation does not exist for this

characteristic, especially among pods differing for wall thickness.

B . THESEED



Ultimately it is the number and weight of individual seeds that survive through

to maturity that determines the yield of a crop. Although the ability of a genotype

to tolerate interplant competition is important in partitioning assimilate into economic yield, it is of equal importance for the seeds of such a genotype to tolerate

the resulting intraplant competition. As with the competition between plants, if

the competition between developing seeds for diminishing resources within the

plant is high, then the number of seeds that succeed in developing through to

maturity will be low. As discussed in Section IV,B, competition between seeds

developing at different nodes will be reduced if a genotype has a reproductive

indeterminate habit, and competition within reproductive nodes will be reduced

if only a single pod develops at each node. The number and weight of seeds that

develop within each pod, however, will be determined by the tolerance of each

seed for its neighbors. Consequently, if seed variants that are less demanding of

the plant’s resources can be found, then more seeds would be expected to develop successfully.

In general, seeds within a pod develop asynchronously with the largest individuals in the center, tapering to very immature seeds at both ends (Fig. 19). This

asynchronous growth is initiated very early in the development of the fruit. The

cause of the asynchrony is not known, but it does not appear to be due to a lack of

fertilization (Linck, 1961). As development continues there is a tendency for the

seeds at both ends of the pod to abort, and for only the central seeds to continue

development (Linck, 1961). When plants are grown in environments that induce

intensive intraplant competition, further abortion occurs and it is the smallest

seeds that appear to be most susceptible (personal observation). The interactions

between individual seeds within a pod are in many ways similar to those between

individual plants within a population. It is possible therefore to apply to the

behavior of seeds toward their neighbors reasoning that is similar to that applied

to explain the responses of individual plants within the crop (Section 111,C).

It can be assumed that there is a finite rate for the translocation of assimilate



268



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



FIG. 19.



Distribution of seed size within a pea pod



from the plant into the pod. This rate of assimilate input will therefore only

supply a finite sink demand from the seeds, the sink demand being equivalent to

the sum of the growth rates of all the developing seeds within the pod. Initially

the sum of the individual seed growth rates will not exceed the assimilate supply

and all of the seeds will begin to develop. As the seeds within the pod develop,

the sum of the growth rates may eventually exceed the rate of assimilate input

into the pod and competition between seeds will occur. As with plants within a

competitive sward, the seeds with the highest growth rate, situated in the center

of the pod, will continue to grow at the expense of the seeds with lower growth

rates, situated at the two ends of the pod. If this interseed competition occurs at a

critical stage in development, then the seeds with the lowest growth rates will

abort. In large-seeded types abortion occurs even among quite large developing

seeds and accounts for the low numbers of large seeds reaching maturity (personal observations).

It is apparent that the key to improved tolerance between seeds within the pod

is to ensure that the total seed sink demand throughout development does not

exceed the assimilate input into the pod. By definition this will entail maintaining

relatively low seed growth rates. This can be achieved by selecting seed types

that have a decreased relative growth rate (RGR) and that would therefore, for a

given seed size, have an increased duration of growth (Fig. 20a). The overall

effect of a reduced RGR will be to reduce the absolute growth rate and hence the



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



269



sink demand of the seed. Relative growth rate is determined by the interaction of

the rate of cell division and expansion with environment. Seed phenotypes with a

lower RGR would either have decreased rates of cell division and/or cell expansion, or more likely the proportion of seed growth attributed to cell division

would be reduced relative to that attributed to cell expansion. This will lower the

seed RGR because dividing cells have a higher RGR than expanding cells.

Another possibility for improving interseed tolerance, especially later in seed



a



b



Time



I CC

0



m



Q)

Q)



In



Time

FIG. 20. Theoretical variation for seed growth. (a) Seeds of similar size differing for relative

growth rate. (b) Seeds differing for size but with similar relative growth rate. (c) Seeds differing for

size and for relative growth rate. L, large seed; S , small seed.



270



C. L. HEDLEY AND M. J . AMBROSE



development, is to select small seeds, which by definition will mature while their

growth rates are relatively low (Fig. 20b). For a given ovule number per pod the

best seed phenotype will therefore be small-seeded with a low RGR (Fig. 20c).

In preliminary studies using six-leafed genotypes, we have found significant

differences in the RGR of seeds during the early part of development (Hedley and

Ambrose, 1980). We have not, as yet, studied the effect of this variation on

competition between seeds, or the effect of interplant competition on the different seed phenotypes.

Selection for a character such as RGR will be difficult because the seed is not a

genetically homogenous structure. The seed is composed of the embryo, the

testa, which is maternal, and the endosperm, which is triploid and composed of

two maternal and one paternal genome (Cooper, 1938). The seed phenotype is

determined by the development of these tissues and by the developmental interactions between them. The effect of the maternal influence on the development of

the seed can best be observed from the differences between reciprocal crosses of

genotypes that differ for seed size. In such crosses the resulting F, seeds usually

resemble the maternal parent in size (Davies, 1975). Such a maternal control may

act via the testa determining or controlling the transfer of nutrients to the embryo

(Murray, 1979, 1980).

In an attempt to understand the complex development of the seed, we have

studied the development of the component parts, in a range of genotypes (Hedley

and Ambrose, 1980). The physical relationship between the embryo and embryo

sac (the endosperm-filled vacuole formed within the developing testa) is primarily concerned with the rate at which the embryo and embryo sac volumes expand

relative to each other (Fig. 21). The initial increases in volume of the embryo and

embryo sac are exponential and therefore linear on a logarithmic scale. Variation

was found between the genotypes for the slopes of both lines and for the separation in time for the initiation of exponential embryo growth relative to that of the

embryo sac. The slope for increase in embryo volume was always considerably

greater than that for the volume of the embryo sac. The difference in time

between the initiation of the two exponentials, however, determined that initially

the absolute volume increase of the embryo sac was greater than that of the

embryo. The absolute difference between the two volumes therefore increases

initially and liquid endosperm accumulates. The difference in slope between the

embryo and embryo sac, however, determines that a point is eventually reached

where the absolute rates of volume increase are the same. This point corresponds

to the maximum volume of endosperm within the seed. After this point endosperm is absorbed, presumably by the developing embryo.

Variation between genotypes in the growth of the seed and in the final seed

size are determined by differences in the slopes of the exponentials and the

difference in time between their initiation. An understanding of the mechanisms

controlling these three variables will be necessary before seeds with specific



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



27 1



MIN.

END.



Time

FIG. 21. Relationship between the expanding embryo sac (es) and embryo (e), showing point of

maximum (max. end.) and minimum (min. end.) endosperm volume. r is the difference in time

between the onset of embryo sac and embryo exponential growth.



growth characteristics can be selected. This forms the basis of our present research into seed development. As part of these investigations we have studied the

cellular composition of embryos from a range of genotypes. Differences have

been found between genotypes in the number of cells within a given embryo

volume, suggesting differences in the proportion of cells within the embryos that

are expanding. Such variation, as stated earlier, may affect the RGR of the

embryo. It is not known, however, if these observations are due to intrinsic

genetic differences between embryos or if they are the result of a maternal

influence. There is evidence from reciprocal crosses that the cell number of

embryos is greatly modified by the maternal parent (Davies, 1975).

As well as selecting seed phenotypes that are less demanding per unit time of

the assimilate input from the plant, it may also be possible to improve the



272



C . L. HEDLEY AND



M. J . AMBROSE



efficiency with which seeds utilize assimilate. As with the pod wall, both the

testa and embryo have significant levels of PEP-carboxylase activity as well as

much lower levels of RuBP-carboxylase activity. These enzymes may act to

reduce respiratory losses and recycle carbon within the seed (Hedley et ul.,

1975). More conclusive evidence of such a system has been found for the

developing seeds of lupin (Lupinus ulbus; Atkins and Flinn, 1978). The significance of this system for the developing embryo is not known, but it can be

suggested that the provision of C4 acids from such a recycling system will be

important in the synthesis of amino acids at a time when protein synthesis is high.



VI. A PLANT IDEOTYPE FOR IMPROVING YIELDS OF

DRIED PEAS

A. THEPLANT



The preceding sections have described some of the problems associated with

the “leafless” phenotype, the main problem being the necessity to grow leafless

plants at high planting densities in order to attain a sufficiently high biological

yield per unit area. We have also suggested how the efficiency of the plants

within the dried pea crop can be improved. Many of these suggestions will apply

irrespective of whether or not the crop plant has a leafless phenotype.

If we persist with the leafless phenotype (ufafstst), it is evident that the

ideotype must be tolerant of high planting density and therefore by definition

must be a relatively uncompetitive plant. The relationship between a plant’s

competitiveness and its growth rate determines that a plant with a reduced growth

rate is required. Unless variants can be found that grow at a reduced relative

growth rate, which seems unlikely, plant growth rate can only be reduced by

selecting plants that have developed from a small embryonic axis. This will

determine that the ideotype will have small seeds, unless the relationship between the size of the embryonic axis and seed size can be broken.

A modification to the leafless model, and one that we are now considering, is

to incorporate the gene for normal stipule size (St) while maintaining the gene for

converting leaflets into tendrils (uf).There is some evidence that this phenotype

(ufafStSt) has a higher growth rate than leafless plants of comparable seed size

(Snoad, 1981), but it will hopefully maintain some of the improved canopy

characteristics of the leafless phenotype. It is possible that such a modification

will overcome the absolute requirement for high planting densities and many of

the problems, both economic and physiological, that accompany such densities.

Even if such a modified model proves successful, we have no reason to suggest



DESIGNING “LEAFLESS” PLANTS FOR DRIED PEA CROP



213



that other features of our crop ideotype, incorporated to improve plant tolerance

to the crop environment, will change.

Ideally the plant should have a genetically determined nonbranching habit.

Branches do not add significantly to economic yield at high planting densities,

although they do utilize environmental resources and increase biological yield

per unit area. If such a plant type cannot be identified, then plants that branch late

in development should be selected, since such plants will be inhibited physiologically from branching by competition within the sward.

The ideotype will be relatively early-flowering so that partitioning of assimilate into reproductive structures is initiated when competition between plants is

low. Early flowering should be coupled with a reproductive indeterminate habit.

This will allow the ceiling biological yield per unit area to be attained and will

increase the duration of assimilate partitioning into economic yield. An increased

duration of partitioning will reduce the competition between yield components

during development.

Each reproductive node will contain a single pod selected for maximum efficiency of carbon refixation and recycling. The seeds will have a low demand per

unit time of available resources, and therefore will have a reduced relative

growth rate and will mature when small. In addition, seeds that have an improved

efficiency for recycling carbon will be incorporated into the ideotype, if such a

system can be shown to have a beneficial effect on yield.

Any incompatibility between the seed’s dual role as the main reproductive sink

and as an embryo plant must be taken into account, and may result in an appropriate compromise.

B . BREEDING

STRATEGY



The overriding effect of interplant competition on plant characters, such as the

number of reproductive nodes and overall plant size, determines that the assessment of an individual’s suitability as a crop plant must be made in an environment akin to that encountered by the plant within the crop. If, however, a

segregating population of individuals that differ for competitiveness is grown as a

microplot at commercial planting densities, then strongly competitive individuals

will thrive relative to weak competitors. Therefore in such an environment the

weak competitors would perform much worse and the strong competitors much

better than if each were grown in a crop of genetically similar individuals. This

presents the breeder with a problem, since for reasons discussed earlier, those

individuals within a segregating population that are most likely to make successful crop plants will be weak competitors. In order that such plants not be discarded early in the breeding program, it is essential that selection be delayed until



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