Hydraulics and sugar transport in long pine needles: a challenge for the Münch hypothesis?

The sieve tubes in phloem of the veins of leaves transport sugar from the source in the mesophyll to the rest of the tree. In needles the venation pattern is particularly simple: basically a bundle of parallel semipermeable tubes. A recent paper (Rademaker et al. 2017) pointed out that the simplest models for the sugar transport in a needle, using the Münch mechanism of osmotically driven flow, indicate that long (i.e. more than around 5 cm) pine needles would transport sugar very inefficiently since the sugar near the tip would be stagnant. Most needles (around 75%) are indeed below this limit, but some are significantly longer, e. g. Pinus Palustris (Longleaf pine) with needles up to around 30 cm. In the present work, we include the mesophyll cells producing the sugar and model the coupled water-sugar transport into the phloem tubes. Since the bottleneck for the transport appears to be from mesophyll to phloem, the length of the phloem tubes becomes less important and the results are comparable with observations.