Source–Sink Relationships


The term source–sink relationship refers to the integration of suger and amino acid production in photosynthesis with sugar and amino acid utlisation in growth, storage, maintenance and production.

Keywords: source; sink; carbon–nitrogen interactions; carbohydrate; amino acid; metabolism; climate change

Figure 1.

Sources and sinks: (a) wheat grains; (b) sunflower seeds; (c) sugar beet plant with storage root; (d) potato plant with tubers.

Figure 2.

Carbohydrate metabolism in sink cells. Enzymes: a, alkaline invertase; b, sucrose synthase; c, acid invertase; d, hexokinase (includes glucokinase and fructokinase); d, fructokinase; e, phosphoglucomutase; f, phosphoglucose isomerase; g, sucrose‐phosphate synthase; h, sucrose‐phosphate phosphatase; i, UDP‐glucose pyrophosphorylase; j, ADP‐glucose pyrophosphorylase; k, starch synthase, l, starch‐branching enzyme; m, α‐amylase; n, β‐amylase; o, α‐glucosidase; p, starch phosphorylase.

Figure 3.

Phloem loading and unloading. (a) Pathways of phloem loading. (b) Pathways of phloem unloading.

Figure 4.

Photosynthesis. (a) Photosynthetic electron transport. (b) Photosynthetic carbon assimilation pathway (the Calvin cycle). Enzymes: a, ribulose‐1,5‐bisphosphate carboxylase/oxygenase; b, fructose‐1,6‐bisphosphatase. E4P, erythrose 4‐phosphate; Fd, ferredoxin; FNR, ferredoxin‐NADP reductase; F6P, fructose 6‐phosphate; PQ, plastoquinone; R5P, ribose 5‐phosphate; S7P, sedohetulose 7‐phosphate; SBP, sucrose bisphosphate; TP, triose phosphate; Xu5P, xylulose 5‐phosphate.

Figure 5.

Pathways to sucrose and starch synthesis in source leaves. Enzymes: a1, phosphoglycerate kinase; a2, NAD(P)‐glyceraldehyde 3‐phosphate dehydrogenase; b, aldolase; c, fructose‐1,6‐bisphosphatase; d, hexose phosphate isomerase; e, phosphoglucomutase; f, UDP‐glucose pyrophosphorylase; g, sucrose phosphate synthase; h, sucrose phosphatase; i, ADP‐glucose pyrophosphorylase; j, starch synthase; k, starch‐branching enzyme; DPGA, 1,3‐bisphosphoglycerate; F6P, fructose 6‐phosphate; G1P, glucose 1‐phosphate; G6P, glucose 6‐phosphate; PGA, 3‐phosphoglycerate; UDPG, UDP‐glucose.

Figure 6.

Reductive assimilation of nitrate in photosynthetic cells. Enzymes: a, nitrate reductase (NR); b, nitrite reductase; GS‐GOGAT, glutamine synthetase‐glutamate synthetase.

Figure 7.

Integration of photosynthetic carbon and nitrogen assimilation in photosynthetic cells. The pathway of nitrogen assimilation is marked by red arrows, carbon respiratory flow by black arrows, carbon assimilation and carbohydrate production by green arrows, and the anapleurotic pathway by blue arrows. Enzyme a, phosphoenolpyruvate carboxylase; OAA, oxaloacetate; 2OG, 2‐oxoglutarate.

Figure 8.

Sugar sensing leading to the expression of sucrose synthase. The predominant sugar transported is sucrose. Sucrose or hexoses (produced by sucrose hydrolysis) may be sensed. A sucrose nonfermenting‐one‐related protein kinase (SNF1) is required for sugar‐induced expression of the sucrose synthase gene.


Further Reading

Farrar JF (1999) Acquisition, partitioning and loss of carbon. In: Press MC, Scholes JD and Barker MG (eds) Physiological Plant Ecology. Oxford: Blackwells.

Foyer CH and Galtier N (1996) Source–sink interaction and communication in leaves. In: Zamski E and Schaffer AA (eds) Photoassimilate Distribution in Plants and Crops: Source–Sink Relationships, pp. 311–340. New York: Marcel Dekker.

Foyer CH and Quick WP (1997) A Molecular Approach to Primary Metabolism in Higher Plants. London: Taylor & Francis.

Halford NG (1999) Metabolic signalling and the partitioning of resources in plant storage organs. Journal of Agricultural Science 133: 243–249.

Koch KE (1996) Carbohydrate‐modulated gene expression in plants. Annual Review of Plant Physiology and Plant Molecular Biology 47: 509–540.

Smeekens S and Rook F (1997) Sugar sensing and sugar‐mediated signal transduction in plants. Plant Physiology 115: 7–13.

Stitt M (1996) Metabolic regulation of photosynthesis. In: Baker NR (ed.) Photosynthesis and the Environment. Dordrecht: Kluwer Academic.

Stitt M, Krapp A, Klein D, Roper‐Schwarz and Paul M (1995) Do carbohydrates regulate photosynthesis and allocation by altering gene expression? In: Madore MA and Lucas WJ (eds) Carbon Partitioning and Source–Sink Interactions in Plants. Rockville, USA: American Society of Plant Physiologists.

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How to Cite close
Foyer, Christine H, and Paul, Matthew J(Apr 2001) Source–Sink Relationships. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001304]