PHLOEM-XYLEM WATER-FLOW IN DEVELOPING CLADODES OF OPUNTIA-FICUS-INDICA DURING SINK-TO-SOURCE TRANSITION

Phloem versus xylem water and carbon flow between a developing daughte r cladode (flattened stem segment) and the underlying basal cladode of Opuntia ficus-indica was assessed using net CO2 uptake, transpiration , phloem sap concentration, and water potential of both organs as well as phloem and apoplastic tracers. A 14-d-old daughter cladode was a s ink organ with a negative daily net CO2 uptake; its water potential wa s higher than that of the underlying basal cladode, implicating a non- xylem pathway for the water needed for growth. Indeed, the relatively dilute phloem sap (7.44% dry weight) of a basal cladode can supply all the water (7.1 g d(-1)) along with photosynthate needed for the growt h of a 14-d-old daughter cladode; about 3% of the imported water flowe d back to the basal cladode via the xylem. In contrast, a 28-d-old dau ghter cladode was a source organ whose water potential was lower than that of its basal cladode, so the xylem can supply the water needed (2 5.7 g d(-1))for its growth; about 6% of the imported water flowed back to the basal cladode along with photosynthate via the phloem. The phl oem tracer carboxyfluorescein occurred in the phloem of 14-d-old daugh ter cladodes after its precursor was applied to basal cladodes. When a pplied to basal cladodes, the apoplastic tracers sulphorhodamine G (SR ) and trisodium 8-hydroxy-1,3,6-pyrenetrisulphonate (PTS) failed to mo ve into 14-d-old daughter cladodes within 5 h, but moved into 28-d-old daughter cladodes within 2 h. SR and PTS moved into basal cladodes wi thin 2 h when applied to 14-d-old daughter cladodes. but not within 5- 6 h when applied to 28-d-old daughter cladodes. The tracer experiments therefore confirmed the patterns of water flow determined using water and carbon budgets.