Partitioning of Assimilate in Plants | Crop Plants | Botany

In this article we will discuss how assimilate moves in plants and how it is partitioned among different organs of the plant.

Partitioning of assimilate is generally to the sinks closest to the source. For example, upper leaves export principally to the shoot apex, lower leaves to roots, and middle leaves to both. Since phloem sieve connec­tions are on one side of the stem, the leaves on one side may be more efficient at exporting assimilate to sinks on the same side.

This has been shown for many crops; for example, the upper, expanding leaves of soybean will import more assimilate from the second leaf below them, which is on the same side of the stem, than from the closest leaf, which is on the other side of the stem. Cross-linking of sieve tube elements occurs in most species, but some seem more efficient than others. Grasses have extensive cross-linking at nodes, which essentially eliminates a preferred route by assimi­late from any leaf to any particular sink.

Sink-Source Relationships and Partitioning:

The movement of assimilate from source to sink is currently believed to occur something like the following. The photosynthetic source cell produces the sugars, which can move symplastically to the sieve tubes. Phloem loading increases the sugar concentration of sieve tubes above that of the apoplast.

At the sink, carbohydrates are being absorbed and either actively parti­tioned into cell constituents (e.g., starches) or changed to other carbohydrates that have little effect on hydrostatic pressure of the phloem. Phloem unloading lowers the concentration of sugars in sieve tubes. The buildup of sugars at the source and the removal of sugars at the sink establish a hydrostatic pressure gradient, which moves water and sugars from sources to sinks.

Where the limitations for movement are assimilate from sources to sinks? According to the mass flow hypothesis, anything increasing photosyn­thesis increases hydrostatic pressure and translocation rate. However, this is true only if sinks have the ability to utilize more assimilate. If they are unable to utilize the increased production there would be a steady buildup of sugars in the system, causing a feedback inhibition resulting in reduced photosynthesis.

Presumably, photosynthetic rate would be reduced to the rate at which sinks could accept assimilate. For leaf photosynthesis to be at maximum potential rates, sinks must be able to utilize all the assimilation pro­duced. Under these conditions partitioning would be controlled by sink strength, that is, sink availability and the rates at which available sinks can utilize assimilate.

Factors that control sink strength also control the partitioning in crop plants. The effect of hormones on enzymatic activity and the elasticity of sink cells can have a dramatic effect on partitioning. Indoleacetic acid (IAA), cyto­kines, ethylene, and gibberellic acid, when applied to cut stem surfaces, causes assimilate to accumulate in the region of application.

In bean seedlings, the main control over the distribution of sucrose between root and shoot sinks can be attributed to auxin and cytokinin concentrations in various sinks. Hormonal influences on initiation, development, and abortion of flowers and seeds have a significant effect on the source-sink relationships in crops.

Although there is some evidence that hormones may have a direct effect on translocation rates, most results show an indirect influence through affect­ing sink demand.

Assimilate Partitioning during the Vegetative Phase:

Leaves and other green tissues are the original sources of assimilate. Some remains in the green tissue for cell maintenance and, if translocation is slow, can be converted to starch or some other form of storage. The rest is exported (translocated) to vegetative sinks, which are composed of growth, main­tenance and storage functions.

During vegetative growth, roots, stems, and leaves are competitive sinks for assimilate. The proportions of assimilate partitioned to these three organs can influence plant growth and productivity.

The investment of assimilate into greater leaf area development results in greater light interception. However, the leaves also require water and nutrients, so investment in root growth is necessary. Some crop plants, such as most grasses, have essentially no stem growth during vegetative development and favor partitioning to leaves and roots.

Some meristems are in more favorable positions to intercept assimilate. For example, the intercalary meristems of leaves are in a better position to intercept translocated assimilate than are the peripheral root and shoot meris­tems.

Young developing leaves need imported assimilate to provide the energy and carbon skeletons for growth and development until they produce enough assimilate to handle their own requirements.

Thrower (1962) and Webb and Gorham (1964) have shown that the leaves of soybean and squash are largely self-sufficient when 50% of their final area is developed. After full expansion and under good environmental conditions for photosynthesis, leaves may export 60 to 80% of their assimilate to other areas of the plant.

As the leaf gets older and begins senescence, it may fail to support its own energy requirements because of age or shading or both. Under these conditions the leaf does not export or import assimilates. Instead, cell maintenance requirements (respiration) are often greatly reduced, which allows the leaf merely to survive. Before death, many of the inorganic and organic compounds in the leaf are remobilized and translocated to other parts of the plant.

The early growth of branches and tillers requires importing assimilate from the main stem or other branches until they become autotrophic. In oats, this usually occurs between the two- and four-leaf stages. Whether a branch or tiller becomes completely independent of the rest of the plant is variable among species.

In timothy, the tillers behave as separate units once they become autotrophic. Little interaction between timothy tillers occurs even under stress conditions, and roots are supplied only by the tillers to which they are attached.

When under stress the autotrophic tillers of some species, such as ryegrass and oats, will again start transporting assimilate from the main culm. How partitioning of assimilate among tillers affects total yield is influenced by how much the additional leaf area of the tiller contributes to the total dry weight of the plant and how much the tiller contributes to harvestable yield; for example, tillers of maize do not usually produce grain.

Assimilate Partitioning during the Reproductive Phase:

Reproductive growth is often the primary part of the plant harvested for yield. Crops, whose flowers, fruits, and seeds (and their products) are the economic yield, have been selected over time to partition large amounts of their total dry matter into reproductive parts. In such plants a large photosynthetic surface and supporting structure are required prior to fruiting.

After flowering the reproductive sink becomes extremely strong, which limits the assimilate partitioned for additional leaf, stem, and root growth. In determi­nate species, leaf and stem growth cease at flowering, while indeter­minate species may have vegetative and reproductive growth occurring si­multaneously. Thus indeterminate species are variable in the relative strength of their vegetative and reproductive sinks. If there is much vegetative growth during reproductive development, reproductive yield may be reduced.

In determinate grain crops early growth is vegetative, allowing the plant to intercept more light energy for photosynthesis as it increases in size and allowing for adequate water and nutrient absorption to support leaf growth. The number of leaves is established at the initiation of inflorescence and is affected by temperature and photoperiod. Shortly after seed initiation, seeds become the dominant sink of annual plants. Therefore, during seed filling, the major part of assimilate, both current and stored, is used for increasing seed weight.

Harvest Index:

Two useful terms used to describe partitioning of dry matter by the plant are biological yield and economic yield. The term biological yield was pro­posed by Nichiporovich (1960) to represent the total dry matter accumulation of a plant’s system. Economic yield and agricultural yield have been used to refer to the volume or weight of those plant organs that constitute the product of economic or agricultural value.

The proportion of biological yield repre­sented by economic yield has been called the harvest index, the coefficient of effectiveness, or the migration-coefficient. All these terms characterize the movement of dry matter to the harvested part of the plant.

The harvest index, the most widely used term, is defined as –

Harvest index = economic yield/biological yield × 100 biological yield.

(It must be remembered that the biological yield total often does not include root weights because of the difficulty of obtaining those values.)

Crop yield can be increased either by increasing the total dry matter produced in the field or by increasing the proportion of economic yield (the harvest index) or both. There is potential for increasing yields by both methods. In oats a large genetic population showed variability in both biological yield and harvest index.

Oat lines with high biological yield and a harvest index of 40 to 50% showed the highest grain yield. Crosbie and Mock (1981) should that increases in both biological yield and harvest index were responsible for increased grain yield of three maize populations.

In some grain crops, the increase in seed yield has been primarily due to increases in harvest index. In other words, the plants are not producing any more total dry matter but are rather partitioning more of their dry matter into seed yield. Donald and Hamblin (1976) reported that increased grain yields in small grains were primarily due to increases in the harvest index. Research on peanuts showed the same phenomenon.

‘Dixie Runner,’ developed in 1943, had a harvest index of 23 and a biological yield of 10.8 Mg (metric ton)/ha. In 1952, ‘Early Runner’ showed a 50% increase in seed yield over ‘Dixie Runner,’ primarily due to an increased harvest index of 36. In 1969, ‘Florunner’ showed a 20% increase in seed yield over ‘Early Runner,’ due to a harvest index of 41; and in 1977 ‘Early Bunch’ was intro­duced with a 10% increase in seed yield over ‘Florunner,’ due to a harvest index of about 51.

Yet the total dry matter yields of all these varieties are essentially the same. The question now for peanut breeders is whether they should select for plants with a harvest index even higher than 51, if possible, or instead breed for plants that will yield more dry matter and still have a harvest index of 51. Obviously, there must be a limit to the amount of total dry matter that can efficiently be converted to seeds.

Yield Components:

Grain yield is a product of a number of sub-fractions called yield compo­nents and can be expressed as:

Y = N_rN_gW_g

Where Y = grain yield, N_r = the number of reproductive units (e.g., heads, ears, panicles) per unit of ground area, N_g = the number of grains per repro­ductive unit, and W_g = the average weight per grain.

Yield components are affected by management, genotype, and environ­ment, which often help explain why a reduction in yield occurred.

The genotype can influence emergence capabilities and sets the potential for tillering, flower number, number of flowers that develop into grain, amount of assimilate produced, and assimilate partitioning.

The environment affects the ability of the plant to express its genetic potential. Managerial factors include the number of seeds planted and the ability of the crop man­ager to produce a growth environment favoring maximum yield. Water, nu­trients, temperature, light, and other environmental factors at levels other than optimum can reduce one or more yield components.

Remobilization:

Once produced, assimilate is transported to many areas in the plant. It can be transformed into many compounds, some of the structural com­pounds, such as cellulose and hemicellulose that provide for the physical structure of the plant and usually remain where they are synthesized. Plant cells do not have enzyme systems to degrade structural compounds, but many storage compounds that can be changed back into forms that can be translo­cated to other parts of the plant are also produced.

These storage compounds are significant in maintaining growth and development constancy despite photosynthetic fluctuations. Storage compounds are mostly composed of carbo­hydrates but often include significant amounts of lipids and proteins.

The movement of compounds from an area where they were once deposited to an area where they can be reutilized is referred to as remobilization. During certain phases of development more assimilate is being produced than is used in growth and development, and this excess can be directed to storage com­pounds.

At a later phase, for example, fruiting, when photosynthesis is not able to furnish the assimilate requirements of plant sinks, storage compounds can be remobilized and moved to active sites, such as seed development. Remobilization occurs with both organic and inorganic compounds. During leaf senescence carbohydrates, nitrogenous compounds, phosphorus, sulfur, and other mobile elements are remobilized and translocated to current plant sinks.

During the heading and flowering stage of the plant, the assimilate produced by photosynthesis is more than is required by these processes. The extra assimilate is moved to the stem and stored primarily as starch. However, as the plant goes into grain fill, starch is converted to sugars and translocated to the filling grain.

Assimilate Partitioning during Grain Fill:

Photosynthate deposited in grain can come from three major sources- current leaf photosynthesis, current photosynthesis from non-leaf parts, and remobilization of assimilate deposited in other plant organs. How much each of these factors contributes to final grain yield is affected by species and environment.

Partitioning has been extensively studied in small grain crops. Work in wheat and barley has shown that photosynthesis of the flag leaf, stem, and head, which are the closest sources to the grain, is the primary contributor to the grain.

Lower leaves supply the needs of lower stem and roots. The strength of the grain as a sink and the relative availability and strength of sources affect the assimilate partitioning. If the top leaves are removed, the lower leaves will supply assimilate to the grain; if the lower leaves are removed, the flag leaf will transport assimilate to roots.

It would be helpful to know just how much each source contributes to grain yield and the variability involved. Early work on shading the head of wheat or barley showed a 20% to 30% reduction in grain weight.

Using shading and measuring photosynthesis, these investigators calculated the contribution of different photosynthetic sources to final grain yield. They estimated that the contribution of parenthesis photosynthesis (remobilized assimilate) was 25%, current leaf and stem photosynthesis was 45%, and head photosynthesis was 30%.

These percentages were confirmed by recent studies using more sophisticated procedures. Drought stress during grain filling reduces grain yield through reduced photosynthesis. Thus the sink demand for grain filling uses more remobilized stored assimilate, which results in a much higher proportional contribution by remobilization.

Although remobilization is an important component of grain yield, photosyn­thesis during the grain filling period is normally the most important source of weight for grain yield. This is because most assimilate is used for vegetative or flower production before grain filling, whereas during grain filling most as­similate is used for that process.

Since the heads of small grains are located at the top of the canopy in the best light conditions for photosynthesis, and since the assimilate produced is next to the grain, head photosynthesis would be expected to contribute heavily to grain yield. Primitive wheat types have lower yields, and thus lower sink demand, than modern wheat. They rely primarily on head photosynthesis for grain yield, partitioning very little from leaves.

Wheat developed with greater yield has an increased partitioning of photosynthate from upper leaves. Increasing photosynthesis of small grain heads could also increase yields; one way is to add awns (thin extensions of the glume or lemma), which have been shown to double the photosynthesis rate of heads. Several elaborate studies have shown that in semiarid and arid environments isolines of wheat with awns will out-yield isolines without awns by as much as 12%.

Illustrating the same effect, awn removal can decrease yield as much as 21%. The primary effect of awns on yield components is to increase kernel weight. Lupton (1966) showed that the amount of assimilate from glumes and flag leaves partitioned to the seeds increased as grain filling progressed and that glumes, which are closest to the seeds, partitioned a larger percentage of assimilate to seeds than did flag leaves. Awns have shown no yield advantage in humid climates, possibly because of an increased susceptibility to disease or lodging.

In maize, in which the ear is located in the middle of the stem, almost all the assimilate produced is from leaves or sheaths. During grain filling, the upper leaves distribute about 85% of their assimilate to the ear. Lower leaves contribute to root growth and stem and leaf maintenance as well as to ear weight. In contrast to small grains, all leaves of maize may contribute some assimilate to grain yield.

Remobilization of stalk reserves in modern maize is not much different from that in primitive maize. Stalk reserves are positively corre­lated with stalk strength; lower reserves mean weaker stems, favoring stalk rot disease organisms. Apparently, modern maize cultivars have not been selected for strong remobilization, so the plants maintain resistance to lodging.

In soybean, in which almost every node provides seed growth and devel­opment, the pattern of translocation from each leaf is similar. The greatest amount of assimilate remains with the pods at the node of the applied leaf, with the rest transported to upper and lower nodes. Lower leaves retain more assimilate than upper nodes, which may be due to the lower light levels reducing the amount of assimilate produced.

Conclusion:

Partitioning of assimilate in the plant takes place in the phloem where assimilate translocates from photosynthetic sources to sinks. Source cells syn­thesize sugars, which move to the apoplast around the phloem.

These sugars are actively taken up by the phloem (phloem loading), which increases the hydrostatic pressure in that area of phloem by increasing water uptake. As­similate moves to sinks, where the removal of sugars in the apoplast around the phloem causes sugars and water to move out of the phloem (phloem unloading), keeping a low hydrostatic pressure in that area of the phloem.

In terms of yield, assimilate partitioning is important in both the vegeta­tive and reproductive growth phases. Partitioning during the vegetative phase will determine the final leaf area, root development, and branching. Invest­ment of assimilate into plant growth during the vegetative period determines the productivity at later stages of development, including seed number just prior to anthesis.

Partitioning during reproductive development is important for flower, fruit, and seed crops. Assimilate can be distributed from current leaf photosynthesis, non-laminar photosynthesis, and remobilization of stored assimilate. The proportion of assimilate coming from each source depends on genotype and environment.

For high yield, the crop should quickly produce enough leaf area indexes to intercept most of the light for maximum dry matter production, after which it should maintain high light interception and should partition assimilate in the largest quantities possible to the organs of economic value without affecting quality or harvestability.