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, -amylase; n, -amylase; o, -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.