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Complex Liquid Dynamics: Sap Flow and Sugar Transport in Plants

Postdoc-stipendium i Danmark | 14/10/2016

Figure 1: Green plants are chemical machines that harvest solar energy. The sugars produced by photosynthesis in the leaves must travel great distances to reach the root. (Image credit: K. H. Jensen)

Green plants are earth’s primary solar energy collectors. They harvest the energy of the sun by converting light energy into chemical energy, stored in the bonds of sugar molecules. A multitude of carefully orchestrated transport processes are needed to move water and minerals from the soil to sites of photosynthesis, and to distribute energy-rich sugars throughout the plant body to support the metabolism and growth. The long-distance transport takes place in the plants’ vascular system, where water and sugars move along the entire length of the plant. 

However, the physical mechanisms that drive and regulate energy distribution in plants are poorly understood. Thus, critical input is missing for future crop engineering programs needed to address societal, economic, and environmental challenges of the 21st century. In this project, supported by the Carlsberg Foundation Postdoctoral Fellowship program, we shed new light on the dynamic processes that power sugar transport in plants. The project was carried out in collaboration with Professor Michael Knoblauch at Washington State University and Professor N. Michele Holbrook at Harvard University.

Figure 2. Leaf veins contain phloem and xylem tubes. The phloem pipes are responsible for export of sugars, while the xylem conduits supply water taken up by the roots. (Image credit: K. H. Jensen)

Introduction

Plants feed and power the earth, and the vision of enhancing productivity to ensure future population - and food security has stimulated enormous scientific interest. The basic idea is to take advantage of breeding and gene technology to increase yield. The desired outcome is a plant with an enhanced ability to perform photosynthesis and distribute the energy-rich sugars across the plant body. 

However, to succeed in this effort a basic understanding of the biophysical constraints on sugar transport is required. The sensitivity of plant cells has hampered the development of experimental tools to reach this goal. This, combined with the lack of appropriate biomimetic models, has made it exceedingly difficult to shed light on several highly important biological processes. The goal of this project is to develop new methods and theory to resolve this issue. 

Sap Flow in Plants

PHLOEM

The phloem is the plant tissue that carries the products of photosynthesis (sugars) from sites of production (leaves and other green parts of the plant) to sites of consumption. In trees, the phloem is located just below the bark. The phloem comprises cylindrical cells lying end-to-end, forming a fluidic network spanning the entire length of the plant.

Sugar transport in plants occur in the phloem vascular system. The products of photosynthesis are transported from sources (mature leaves) to sinks (e.g. shoots, roots and fruits) in this microfluidic channel network. The sap flowing in the plant veins contains 15% to 25% sugar, mostly sucrose (table sugar), and moves at speeds of approximately one meter per hour. To drive transport, a remarkably large cell pressure is required in the leaf phloem: 10 to 20 atm. This is almost 100 times larger than typical human systolic blood pressure. 

Sugars are believed to play a fascinating dual role in generating this pressure, acting both as an energy carrier and a motile force generator: According to the Münch hypothesis, the osmotic pressure of the phloem sap provides the necessary force to drive a convective flow from sugar sources to sinks. This process was first described by German botanist Ernst Münch in the late 1920s (Stroock et al. 2014). Despite general consensus on the basic principles for this mechanism, however, laboratory techniques to test key aspects of this hypothesis are missing, and the Münch hypothesis has gained wide acceptance based to a large extent on its simplicity and plausibility, rather than on experimental evidence.

Testing the Münich Hypothesis

THE MÜNCH MECHANISM

According to the Münch mechanism, sugar transport in plants is driven by osmotic pressure differences caused by the gradients in sugar concentration between sources and sinks of sugar. 

Four parameters need to be measured simultaneously to test the Münch hypothesis: the rate of sugar transport, the viscosity of the sweet sap, the size and length of the phloem tubes, and the pressure differential between leaf and root. In a long, narrow tube, a relatively large pressure difference is needed to drive flow, making transport in tall trees particularly challenging. 

“Münch’s hypothesis is physically plausible even for large plants measuring tens of metres or more.” Chris Surridge, Chief Editor of Nature Plants, Nature Plants 2 (2016).

Supported by the Carlsberg Foundation, I have collaborated with researchers at Washington State University, and at Harvard University, in developing new techniques to measure phloem sap viscosity (Jensen et al. 2014) and cell pressure (Knoblauch et al. 2014). For instance, we traced the motion of dye molecules to determine liquid viscosity. These new methods – combined with measurements of transport rates and phloem cell geometry – allowed us to test the Münch hypothesis. Our results (Knoblauch et al. 2016) demonstrate the existence of strikingly large pressure differences (17 atm) between shoots and roots, sufficient to drive the observed flow. Moreover, we found significant changes in the phloem tube geometry, which tended to be bigger in larger plants.

“The Münch hypothesis. Accepted? For decades. Experimentally proven? Just now!“ Ulrich Hammes, Regensburg University, twitter 2 June 2016.

Future Research

After almost a century, we finally have experimental evidence that addresses the key challenge to the Münch flow hypothesis. Utilizing this knowledge may become an important tool in future crop engineering programs, and it may lead to completely new directions in research on biologically inspired engineering systems (Jensen et al. 2016). However, two extremely important processes remain to be understood: phloem loading and unloading. In phloem loading, sugars move from photosynthetic cells to the export conduits (the phloem). 

This process must be sufficiently rapid to allow the cells to develop the observed pressures. Most plants use an active mechanism (protein pumps) to deliver sugars to the phloem. However, in most trees, for which transport distances are largest, loading seems to occur via passive diffusion into the phloem; a process which runs much slower than the active pumps. It is currently unknown if the diffusive loading mechanism can account for the extreme pressures needed to drive transport in plants. Similarly, it is unknown if diffusive unloading is sufficient to maintain sugar export at rates necessary to support growth and metabolism.

How the Grant from the Carlsberg Foundation has Affected My Career

With generous support from the Carlsberg Foundation’s Postdoctoral Fellowship, I had the opportunity to join the faculty at the Department of Physics at the Technical University (DTU) following a postdoctoral appointment at Harvard University. Today, I lead the Systems Biophysics group in the section for Fluids and Biophysics at DTU Physics. 

With the Carlsberg Foundation’s support I was able to independently perform tasks that are relevant for pursuing a research career and developing a research group, such as establishing of my own independent research activities, supervising master students, participating in international conferences, and obtaining a degree in university teaching and learning. This project has given me not only the privilege of pursuing basic science, but also the unique opportunity of further developing a broad international scientific network and links to stakeholders in industry. I firmly believe that an interdisciplinary (and intercontinental) approach will be key to addressing the societal, economic, and environmental challenges of the 21st century. For more information on our groups’ research activities, please visit our website:

Systems Biophysics - Jensen Research