Source-sink relationships in fruit species: A review
Plants with 2 young fruits as compared to a single fruit, and with 4 young fruits as compared to 2 Source-sink relationships in plants have been and still are a. transport of sugars across plants organs is a major factor affecting plant growth. a . Transport in source tissues. Source-sink relationship: During the process follows a shortest path, that is, it flows from nearest source to sink. This movement of. Photoassimilate Distribution Plants and Crops Source-Sink Relationships DownloadPDF MB Read online. Get Citation.
Different environmental factors influence the source-sink relationship.
Leaf drop effects due to stress greatly influence leaf-fruit sink-source relationships and can be caused by environmental air temperature, storm, hail, and chemical or industrial emissions or root temperature, drought, salinity, oxygen deficiency conditions Fischer, Lenz found that in flooded trees, leaves drop less when they were in full fruit development, as compared to those in other physiological stages.
Because carbohydrates are removed with fruits during harvest and the leaves are the organs of high carbon uptake by the plant, after harvest, all practices that favor carbon uptake such as light and health should be optimized Lenz,figure 2.
Source-sink relationships in crop plants 
Fruit removal in apple trees favors more leaf area development compared to those with intact fruits Lenz, and subsequent fruiting in young trees reduces leaf area.
Furthermore, fruitless growing strawberries produced Defoliating trees partially increases the rate of photosynthesis in the remaining leaves because they provide a relatively larger sink Kozlowski and Pallardy, and this depends on the defoliation degree. Leaf area index LAI Minimum quantities of leaf area and shoot structure are required for setting large fruit crops Lakso y Flore, Compared to annual crops e.
The LAI in conjunction with sunlight interception is useful as a basis for analyzing canopy productivity Fischer, Jackson reported that the LAI in the apple lies between 1. The index in the peach is generally higher, between 7 and 10 Faust, The LAI is higher than 1. Moreover, height and type of training define light penetration to the foliage Faust, The leaf area index in the orange can be as high as 9 or 11 Dussi, These authors studied canopy characteristics of 26 Indian mango varieties on year-old trees, planted at 10 x 10 m, measuring an average LAI of 2.
The fraction of the light passing through the canopy DNI, diffuse non-interceptance ranged from 0. The varieties with a low LAI and high DNI 'Fernandin', 'Papatio', 'Malihabad Safeda' and 'Rataul' were better exposed to solar radiation and produced more reproductive stems and good color fruits than varieties with denser foliage Rajan et al. Apart from cultural practices, agro-ecological conditions and age of plants can influence LAI development Fischer, The rapid LAI development at 2, m a.
For a full crop, most fruit species will set more fruit than needed if growing conditions are optimal Westwood, During their development, fruits accumulate carbohydrates, generally as starch, sucrose, or hexose sugars Kozlowski and Pallardy, which are highly dependent on the fruit maturity stage and varies according to cultivar, leaffruit ratio and growing conditions Friedrich and Fischer, The fruit attracts photosynthates and thus increases the photosynthetic production of leaves Kozlowski and Pallardy,while few fruits in the canopy cause accumulation of photosynthates in leaves less photosynthesis activity Hansen, A high fruit load can induce vegetative growth stagnation Kozlowski and Pallardy, While a high fruit load decreases the distribution of assimilates to the roots and other permanent plant organs, the lack of assimilates may also have negative effects on fruit production in the following years Lenz, Poor accumulation of reserves in persimmon Diospyros kaki inhibited flower induction, causing alternate bearing Ojima et al.
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This same phenomenon has also been reported in the literature for species such as citrus Goldschmidt and Golomb, and the apple Lenz, Consequently, vegetative growth must be sufficiently vigorous to enable growth of well-illuminated leaves Gil, Fruits show a strong attraction for photosynthetic products and if the amount of fruit rises, the photosynthate production by leaves is higher Kozlowski and Pallardy, Expanded leaves near the fruit exhibit increased photosynthetic rates Urban et al.
Hansen observed that the distant leaves can serve as an assimilate source and thus, the importance of having more fruits in the thinned branches. Green and immature fruits exhibit substantial surface-to-volume ratios and refix a lot of internally respired carbon but only modest amounts of atmospheric CO2, mainly during early development Blanke and Lenz, Soluble CH in fruit trees are composed of monosaccharides normally glucose and fructoseoligosaccharides mainly sucrosewhereas insoluble carbohydrates consist of starch and hemicelluloses Oliveira and Priestley, Glucose is also a building block for starch, cellulose, and a substrate for synthesis of hemi-celluloses, pectins and gums DeJong and Ryugo, Nonstructural CH in roots and wood fractions are important for tree longevity and quality potential during harvest Zufferey et al.
Starch accumulates whenever a high level of sugar builds up; it is converted to sugars when sugar contents are low Kozlowski y Pallardy, The process whereby the largest fraction of CH is oxidized is respiration, taking place not only in the light but also in the dark DeJong and Ryugo,releasing the energy needed in the synthetic processes associated with growth and plant metabolism Kozlowski and Pallardy, Maintenance respiration occurs continually in living tissues to keep them healthy and functioning, whereas growth respiration occurs to supply energy for the construction of new tissues figure 2 DeJong and Ryugo, The distribution partitioning of CH determines the amounts and patterns of plant growth and yield Lakso and Flore, Translocation is dependent on the developmental state of the plant.
Furthermore, transport direction and volume depend on sink position and relative attraction strength Friedrich y Fischer, Taiz and Zeiger referred to allocation as the differential use of CH in metabolism, transport and storage, in the latter case, starch is synthesized and stored within chloroplasts and is the primary storage form that is mobilized as sucrose for translocation during the night. Carbohydrates and other organic substances are translocated in the sieve tube elements of the phloem following the pressure-flow model as a mass flow of solution driven by an osmotically generated pressure gradient between source and sink organs Taiz and Zeiger, Ninety percent of sap solute molecules are carbohydrates that travel at a speed of about cm h-1 Friedrich and Fischer, The form of CH translocation in fruit plants is primarily sucrose, which is less reactive than reducing sugars such as glucose and fructose Taiz and Zeiger, Important media for translocating CH include sugar-alcohols such as sorbitol in pomaceous fruit species Rosaceae Ryugo, and mannitol in the olive Wolstenholme and Whiley, Other materials translocated in the phloem are amino acids and proteins, hormones, and some inorganic ions Taiz and Zeiger, Assimilates are supplied to the fruit generally by leaf photosynthesis and plant carbohydrate reserves Friedrich and Fischer, In the grapevine, rapid accumulation of total soluble solids TSS in berries at veraison is mainly due the mobilization of non-structural CH previously stored in the permanent organs Zufferey et al.
When a plant develops a heavy fruit load, the fruits seem to have a priority for the photosynthate from most leaves Wardlaw, figure 2both the direction and pathway of assimilate transport change in favor of fruit growth Ho, Generally, root and shoot apices are the principal sinks during vegetative growth, fruits generally become the dominant sinks during the reproductive phase, particularly for adjacent and nearby leaves Taiz and Zeiger, ; Parra, Competition among organs Different tree organs compete for CH, which are mainly produced by leaves.
Fruits have a greater sink strength than other organs table 1 A and D Wardlaw, ; Ho, Sink strength in plants without fruits occur in the stem table 1 B or roots table 1 C which predominantly attract CH. The upper expanded leaves export CH to young leaves even importing and cauline meristem, as the number of leaves increases, the basal leaves send photosynthates predominantly to the roots Taiz and Zeiger, According to Kozlowski and Pallardythe rate of translocation of photosynthates from the sources mainly leaves to the sinks mainly fruits influences photosynthesis.
Carbohydrate partitioning within a tree is not a genetically programmed process, but a result of the unique combination of competing organs and their relative abilities to compete for limited carbohydrates Lakso y Flore, The degree of competition among various sinks depends on the organ activity and distance from the CH source.
Moreover, Parra noted that the four adjacent leaves to the tree tomato fruit were responsible for filling. Fruits demand large quantities of photosynthates and the growth of branches and especially the root system decrease as the fruit load increases Lakso and Flore, Fruit-growing trees build more dry matter per unit leaf area than plants without fruit Lenz, Leaves closest to the fruit have a dominating photosynthetic activity and, moreover, high transpiration rates and stomatal opening Herold, Shoots and roots of young trees receive considerable amounts of CH, the relative amount that roots receive however, tends to decline with tree age and with heavy fruit loads, partitioning of CH to the roots reduces dramatically Lakso and Flore, ; Lenz, On the contrary, in the semi-woody and fruiting cape gooseberry plant, characterized by an indeterminate growth habit, the highest amount of starch reserve is found in the roots and basal stem Fischer et al.
Species and cultivars with an indeterminate growth habit Passifloraceae, Solanaceae and Caricaceaein which the vegetative phase overlaps with the reproductive phase, balance their supply to both sink types vegetative and reproductive Fischer, Fruits closer to the main stem have a tendency to become larger because they better compete for CH, and the farthest from the main stem become smaller because they have less of a chance to compete, as confirmed in the cape gooseberry by Mazorra et al.
The exceptions are the fruit produced in the periphery of the canopy which take advantage of direct sunlight, both on the fruits and the adjacent leaves, as compared to those growing under the canopy Kozlowski and Pallardy, Hansen found higher translocation rates of photosynthates and fruit growth with exposure to full sun conditions in contrast to deficient light conditions within the canopy.
Harvest index The harvest index HI is used in crop physiology as the percentage of total DM partitioned to the harvested portion the fruit Lakso y Flore, The HI increases with the age of the fruit tree and depends on various factors such as variety, root stock, agro-ecological conditions and crop management.
Some external factors Temperature plays an important role in CH partitioning Fischer, Night temperature is of great importance for CH translocation. This is because CH are translocated during night hours and therefore, as in the case of the Rosaceae, it has been reported that growth occurs more during the night than in the day Fischer, Storage of photosynthates in leaves increases as a result of slow growth and low CH demand during low temperatures below the optimum range Fischer, The distribution of assimilates may be affected by a deficiency or imbalance of mineral nutrients and, furthermore, by the initiation and development of sink organs and for source functioning, the plant requires an adequate supply of nutrients Taiz and Zeiger, Potassium is claimed to be essential in the process of loading and unloading the phloem due to high concentrations of K in companion cells of sieve elements Taiz and Zeiger, Potassium deficiency affects vegetative growth because the plant alters the distribution of K to improve the growth of the fruit Ho, In contrast, boron does not facilitate sugar transport via the formation of borate-sugar complexes, because sucrose builds only weak complexes with B and, additionally, B is not involved in sucrose phloem loading Marschner, Optimal leaf-to-fruit ratio varies according to the species and variety, and orchard geographic location Schumacher, Moreover, the capacity of leaf photosynthesis depends on the incidence of light, whereby the shaded parts of the canopy assimilate less and need more leaves than the well illuminated part for optimal fruit development.
Schumacher considered that the leaf-fruit ratio is not totally reliable. Hansen stated that decreasing the leaf-fruit ratio increases the photosynthetic efficiency of the leaves, causing a raised sink-effect. Tree fruits with a high leaf-to-fruit ratio, as in young plants or those with a low fruit load, often form large fruits with a "spongy" tissue which reduces postharvest life and increases susceptibility to diseases Fischer and Friedrich, As fruit density increases, the leaf-tofruit ratio decreases, resulting in a lower supply of photosynthate per fruit; fruit size therefore decreases Dennis, ; figure 4along with insufficient color and flavor Schumacher, Optimal leaf area in several fruit species is cm2 per g of fresh fruit mass for favorable growth and quality Fischer, Furthermore, grapes require twice this value table 2.
The increase in leaf-fruit ratio may facilitate the accumulation of starch reserves, favoring vegetative growth and fruiting in the following season Chacko et al. The leaf-fruit ratio changes with the production area latitude in which the temperature and light have the greatest influence, with lower ratios at sites nearer to the equator Fischer, Sauer and Baumann reported that under the conditions of Franken, for the production of 1 g of berries, 20 cm2 of leaf area are needed for 1 kg2 m2.
Growers can rely on a number of methods which directly or indirectly influence photosynthesis and sink activity fruit growth. Among these, the most important are tree height, distance, fruit thinning, pruning, fertilization, application of growth regulators, irrigation and phytosanitary control Flore and Lakso, ; Fischer, Girdling branch ringingthe removal of a bark ring in the trunk or in the base of lateral growth axes, interrupts photosynthate flow to the roots and thereby increases flower induction and fruit filling, apparently through increased sugar availability in the aerial parts of the tree Iglesias et al.
In deciduous plants renewal of growth and development of new foliage are dependent upon upward transport of food from reserves present in the roots and stems.Source- Sink Movement
Growth of the stem apices, formation of flowers, fruits, etc. It is little except when source and sink lie on the opposite sides. The two types of translocation are believed by many workers to occur in different sieve tubes.
Source and Sink in Phloem Translocation | Plant Physiology
Differences between Diffusion and Translocation Pathway of Translocation: The most common organic nutrient trans-located in plants is sucrose. The channels of transport are sieve tubes in flowering plants and sieve cells in non flowering vascular plants of phloem. It was proved for the first time by Czapek The evidences are as follows: There are only two paths for long distance translocation, tracheary elements and sieve tubes. The former are dead while the latter are living. Translocation of organic solutes seems to be through sieve tubes because it is inhibited by steam girdling which kills living cells.
Source sink relationship during ageing and senescence of solanum tuberosum l
In girdling or ringing experimentsa ring of bark is cut from the stem. It also removes phloem. Nutrients collect above the ring where the bark also swells up and may give rise to adventitious roots Fig. Growth is also vigorous above the ring. The tissues below the ring not only show stoppage of growth but also begin to shrivel Roots can be starved and killed if the ring is not healed after some time.
Killing of roots shall kill the whole plant clearly showing that bark or phloem is involved in the movement of organic solutes which occurs in one direction, i. Girdling experiments are performed in fruit trees to make more food available to fruits.
However, the rings are kept narrow and cambium is not touched so that the incision heals up after some time.
Girdling experiments cannot be carried out in monocots and dicots with bi-collateral bundles because of the absence of a single strip of phloem. Mason and Maskell inserted a wax paper between phloem and xylem.
Parts of the bark were also removed except for a narrow strip. They found evidence that the organic solutes passed through the narrow strip of bark containing the phloem. By means of aphid stylets, Weatherley found that sieve tubes contained a concentrated solution of organic substances under a pressure.
Radio-autographs show that assimilates with incorporated radioactive elements pass out of the leaves and travel towards the sink ends through phloem. Sucrose is most suitable form of carbohydrate translocation as it is non-reducing and chemically stable. It does not react with other substances during translocation. Tonoplast is absent in sieve tube cells so that cytoplasm is in direct contact with vacuolar contents.
Sieve tube cytoplasm can tolerate high concentration of solutes without being plasmolysed. Cytoplasm of one sieve tube cell is continuous with that of the adjacent sieve tube cells through sieve plates so as to form continuous filaments.