Testing the Münch hypothesis of long distance phloem transport in plants.

Michael Knoblauch,Adam C Dodgen,Daniel L Mullendore,Jessica A Savage,N Michele Holbrook,Kaare H Jensen,Sierra D Beecher,Benjamin A Babst,Jan Knoblauch

doi:10.7554/elife.15341

Abstract

Long distance transport in plants occurs in sieve tubes of the phloem. The pressure flow hypothesis introduced by Ernst Münch in 1930 describes a mechanism of osmotically generated pressure differentials that are supposed to drive the movement of sugars and other solutes in the phloem, but this hypothesis has long faced major challenges. The key issue is whether the conductance of sieve tubes, including sieve plate pores, is sufficient to allow pressure flow. We show that with increasing distance between source and sink, sieve tube conductivity and turgor increases dramatically in Ipomoea nil. Our results provide strong support for the Münch hypothesis, while providing new tools for the investigation of one of the least understood plant tissues.

Highlights

Vascular systems allow organisms to distribute resources internally by bulk flow and to overcome size limitations set by diffusion
The pressure flow hypothesis introduced by Ernst Munch in 1930 describes a mechanism of osmotically generated pressure differentials that are supposed to drive the movement of sugars and other solutes in the phloem, but this hypothesis has long faced major challenges
To test whether phloem parameters scale in accord with the Munch hypothesis, we chose to study morning glory (Ipomoea nil) vines because this species exhibits indeterminate growth, allowing us to test the predictions of the Munch model for a wide range of tube lengths (L)

Summary

Introduction

Vascular systems allow organisms to distribute resources internally by bulk flow and to overcome size limitations set by diffusion. The evolution of vascular tissues enabled the development of trees and forests and was accompanied by a major increase in the productivity of terrestrial ecosystems. Plant vascular tissues are of two types: the xylem allows water to be pulled from the soil to maintain the hydration of leaves surrounded by air, while the phloem distributes the products of photosynthesis throughout the plant, allowing non-photosynthetic structures, such as roots, to be formed. A fundamental issue bedeviling the pressure flow hypothesis is the long standing question of whether sieve tubes have sufficient hydraulic conductivity to allow sugars to be transported from leaves to roots in the largest and longest of plants. Increasing distances between sources and sinks would appear to require greater pressure to drive flow, but measurements suggest lower source turgor in trees compared to smaller, herbaceous plants (e.g., crops or weeds; Turgeon, 2010). It is important to note that such challenges derive in large part from mathematical models of phloem

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