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Carbon flow and carbohydrate metabolism during sink-to-source transition for developing cladodes of Opuntia ficus-indica

Wang, N., Zhang, H.H., Nobel, P.S.
Journal of experimental botany 1998 v.49 no.328 pp. 1835-1843
source-sink relationships, ribulose-bisphosphate carboxylase, carbohydrate metabolism, sucrose-phosphate synthase, stems, hydrogen ions, Opuntia ficus-indica, phosphoenolpyruvate carboxylase, sucrose, vacuoles, carbon, beta-fructofuranosidase, pH, malic acid, chemical constituents of plants, enzyme activity, chlorophyll, sucrose synthase
Daughter cladodes (flattened stem segments) of Opuntia ficus-indica (L.) Miller at 14-18 d after appearance on the underlying basal cladodes were sinks, requiring carbohydrate import for growth. Import stopped at 25-26 d, and the daughter cladodes became sources at 27-28 d. The activities of Rubisco, PEPCase, and sucrose-Pi synthase as well as the chlorophyll content at 14 d were not less than those at 28 d, suggesting that photosynthetic or sucrose synthesis capacity was not limiting carbon assimilation for sink cladodes. Sucrose synthase (SS) activity was three times higher than that of alkaline invertase, indicating that SS is the major enzyme for cytoplasmic sucrose hydrolysis. The SS activity was correlated with cladode growth, the highest activity coinciding with the highest growth rate. The sink-to-source transition for daughter cladodes was correlated with increases in malate and H+ concentrations in the vacuoles of chlorenchyma cells, with 5-fold higher nocturnal malate production and 10-fold higher H+ concentration in 28- than in 14-d-old daughter cladodes. The vacuolar H+ increase during cladode development would lower cytoplasmic pH, which may trigger metabolic events affecting the sink-to-source transition.