Phloem loading, plant growth form, and climate

Plasmodesmatal frequencies in the phloem of leaf minor veins vary considerably, suggesting that photoassimilate is loaded into the phloem by different strategies. The ecophysiological basis for multiple loading types is unknown. We updated the analysis of van Bel and Gamalei (Plant Cell Environ 15: 265-270, 1992) with more current phylogenetic data and by treating separately two symplastic loading types, those that load actively by polymer trapping (synthesis of raffinose family oligosaccharides--RFOs), and those that load passively, by diffusion. The results indicate a stronger association between passive, symplastic loading and the tree growth form than previously recognized. Apoplastic loading is highly correlated with the herbaceous habit. There is no correlation between RFO families and growth form. At the family level, there are no correlations between minor vein types and climate that cannot be explained by the dearth of woody plants in the arctic for reasons unassociated with phloem loading. However, at the species level, a floristic analysis uncovered a correlation between the RFO trait and species frequency in tropical and subtropical regions of the world. The correlations between loading types and both growth form and climate are subtle, probably indirect, and poorly understood.