Abstract
Since Münch in the 1920s proposed that sugar transport in the phloem vascular system is driven by osmotic pressure gradients, his hypothesis has been strongly supported by evidence from herbaceous angiosperms. Experimental constraints made it difficult to test this proposal in large trees, where the distance between source and sink might prove incompatible with the hypothesis. Recently, the theoretical optimization of the Münch mechanism was shown to lead to surprisingly simple predictions for the dimensions of the phloem sieve elements in relation to that of fast growing angiosperms. These results can be obtained in a very transparent way using a simple coupled resistor model. To test the universality of the Münch mechanism, we compiled anatomical data for 32 angiosperm and 38 gymnosperm trees with heights spanning 0.1-50m. The species studied showed a remarkable correlation with the scaling predictions. The compiled data allowed calculating stem sieve element conductivity and predicting phloem sap flow velocity. The central finding of this work is that all vascular plants seem to have evolved efficient osmotic pumping units, despite their huge disparity in size and morphology. This contribution extends the physical understanding of phloem transport, and will facilitate detailed comparison between theory and field experiments. The phloem of gymnosperms differs in some important aspects from the respective sugar-conducting tissue in angiosperms which offers a low-resistance pathway from source to sink, well matching the transport mechanism postulated by Münch 80 years ago. To test universality of the Münch mechanism, we compiled anatomical data for 32 angiosperm and 38 gymnosperm trees with heights spanning less than 1m up to 50m. The species studied showed a remarkable correlation with the scaling predictions found recently, when optimizing the Münch mechanism theoretically using a simple resistor model. The central finding of this work is that all seed plants are geometrically optimized for rapid translocation in the phloem and that the flow conductivity is significantly lower in gymnosperms than in angiosperms of similar height, but compatible with the Münch mechanism.
| Original language | English |
|---|---|
| Journal | Plant, Cell and Environment |
| Volume | 35 |
| Issue number | 6 |
| Pages (from-to) | 1065-1076 |
| Number of pages | 12 |
| ISSN | 0140-7791 |
| DOIs | |
| Publication status | Published - Jun 2012 |