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 daughter 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 sink organ with a negative daily net CO2 uptake; its water potential was 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 gd−1) along with photosynthate needed for the growth of a 14-d-old daughter cladode; about 3% of the imported water flowed back to the basal cladode via the xylem. In contrast, a 28-d-old daughter cladode was a source organ whose water potential was lower than that of its basal cladode, so the xylem can supply the water needed (25.7 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 phloem tracer carboxyfluorescein occurred in the phloem of 14-d-old daughter cladodes after its precursor was applied to basal cladodes. When applied to basal cladodes, the apoplastic tracers sulphorhodamine G (SR) and trisodium 8-hydroxy-1,3,6-pyrenetrisulphonate (PTS) failed to move 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 within 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.