Figure 9.
Hypothetical models for the impact of the difference in PMF between SE/CCs and PPCs in transport phloem (ΔC = ΔΨm SE/CC − ΔΨm PPC) on photoassimilate partitioning in symplasmically (A, B) and apoplasmically phloem-loading (C) species. The size of the horizontal arrows quantifies release from the SE/CCs and retrieval by SE/CCs and uptake by PPCs. In symplasmically phloem-loading species, more photoassimilates are accumulated by the PPCs and adjacent axial sinks, which are invested (A) or temporarily stored (B) in axial sinks. As a consequence, photoassimilates are invested to a lower degree (A) or invested with a delay (dashed line, B) into terminal sinks. The high retrieval by SE/CCs in apoplasmically loading species enables more direct photoassimilate investment into terminal sinks. The vertical arrows quantify the extent of investment in terminal sinks (A–C).