CARBON FLOW AND CARBOHYDRATE-METABOLISM DURING SINK-TO-SOURCE TRANSITION FOR DEVELOPING CLADODES OF OPUNTIA-FICUS-INDICA

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 a ctivities of Rubisco, PEPCase, and sucrose-Pi synthase as well as the chlorophyll content at 14 d were not less than those at 28 d, suggesti ng 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 tha t SS is the major enzyme for cytoplasmic sucrose hydrolysis. The SS ac tivity was correlated with cladode growth, the highest activity coinci ding with the highest growth rate. The sink-to-source transition for d aughter cladodes was correlated with increases in malate and H+ concen trations in the vacuoles of chlorenchyma cells, with 5-fold higher noc turnal malate production and 10-fold higher H+ concentration in 28- th an in 14-d-old daughter cladodes. The vacuolar H+ increase during clad ode development would lower cytoplasmic pH, which may trigger metaboli c events affecting the sink-to-source transition.