Quantification of symplastic continuity as visualised by plasmodesmograms: diagnostic value for phloem-loading pathways.

The use of plasmodesmatal frequency to correlate cell-cell symplastic transport capacity remains a contentious problem, as variation in cell shape, accurate determination of interface contact area between cell types, distribution (i.e. whether random or aggregated) and shape (i.e. whether single or branched), and state of permeability may confuse the issue. Additionally, variation in the methods used to determine the frequencies compounds the problem further. Data presented in this paper show that plasmodesmograms offer a means to visualise the potential transport pathway from mesophyll cells to sieve tubes. Furthermore, the results allow an instant appreciation of symplastic continuity or discontinuity and, accordingly, the potential symplastic and-or apoplastic stages involved in the overall loading process.