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The following points highlight the four factors controlling translocation and assimilates partitioning in higher plants. The factors are: 1. Competition among Sink Tissues for Available Translocated Assimilates 2. Photosynthesis and Sink Demand 3. Long Distance Signals between Sources & Sinks and 4. Plasmodesmata.
Factor # 1. Competition among Sink Tissues for Available Translocated Assimilates:
Competition among various sink tissues or organs such as young leaves, stems, roots, fruits and seeds for transport sugars is an important factor in determining translocation pattern in whole plant. Experiments have shown that if a sink is removed from a plant, there is increased translocation of assimilates to other competing sinks.
Reproductive tissues such as fruits and seeds for instance, can compete with growing vegetative tissues such as young leaves and roots. On the other hand, sudden and drastic curtailing of sources (such as by shading all the leaves except one) and keeping the sinks intact in sugar-beet and bean plants, resulted in increased supply of assimilates to young leaves than to roots indicating thereby that young leaves are stronger sinks than roots in these plants.
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The sink strength i.e., the ability of the sink to mobilize photosynthetic or assimilates toward it depends on sink size and sink activity:
Sink strength = Sink size × Sink activity
Sink size is the total weight of the sink tissue while sink activity is defined as the rate of uptake of assimilates per unit weight of the sink. Sink activity is in turn governed by various enzymes that are involved in metabolic utilization and storage of assimilates.
Factor # 2. Photosynthesis and Sink Demand:
Rate of photosynthesis (i.e., the net amount of carbon fixed per unit area of leaf per unit time) is strongly influenced by sink demands. A substantial increase in sink/source ratio results in increased rate of photosynthesis in the source leaves. Rate of photosynthesis declines when sink demand decreases or in other words sink/source ratio is decreased.
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Under such condition, rate of photosynthesis is markedly inhibited especially in those plants which usually store starch instead of sucrose during the day. It is believed that under reduced sink demand in plant, assimilates pile up in the leaves (sources) which cause product inhibition of photosynthetic reactions.
Factor # 3. Long Distance Signals between Sources and Sinks:
The signals between sources and sinks may be physical such as turgor pressure or chemical such as phytohormones (plant growth regulators).
(i) Turgor Pressure:
Rapid phloem unloading results in decrease of turgor pressure in sieve elements of phloem in sink tissues which is transmitted to the sources via interconnecting system of sieve elements. Consequently, rapid phloem loading occurs in sieve elements of phloem at the sources which increases their turgor and the translocation of assimilates from sources to sinks is increased.
Rate of translocation of assimilates or organic solutes would be decreased if phloem unloading at the sinks is slow. It is believed that turgor affects transport of assimilates across the plasma membranes by modifying the activities of proton pumping ATPase located in the membranes.
(ii) Phytohormones or Plant Growth Regulators:
Phytohormones such as auxin, gibberellins, cytokinins and ABA are transported throughout the plant in vascular system and evidences are now accumulating that these growth regulators might regulate source-sink relationships at least partially and affect assimilates partitioning by controlling growth of sinks, senescence of leaves and other developmental processes.
The best studied cases involve remobilization of stored reserves in storage tissues such as tap roots or sugarcane stem parenchyma, from where they are directed to new, typically reproductive sinks. Formation of these new reproductive sinks is itself often under the control of growth regulators. These new sink tissues may in turn also synthesize and release growth regulators which act as strong mobilizing agents.
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According to Gifford and Evans (1981), combinations of plant growth regulators may have additive, synergistic or inhibitory effects on assimilates partitioning.
Factor # 4. Plasmodesmata:
Plasmodesmata play very important role in regulating all aspects of phloem translocation including phloem loading and unloading and assimilates partitioning. Large pressure differences between the cells close plasmodesmata, the degree of closure depending upon the pressure difference. Closure of sieve pores (or sealing of plasmodesmata) by deposition of callose which in turn is regulated by cytoplasmic calcium level, markedly inhibits translocation.
