Journal Article

Maintenance of carbohydrate transport in tall trees

Beecher, Sierra D., Holbrook, N. Michele, Clerx, Laura, Knoblauch, Michael, Savage, Jessica A., Knoblauch, Jan, Losada, Juan M., Jensen, Kaare H., Gersony, Jessica T.

2017

Overview

Trees present a critical challenge to long-distance transport because as a tree grows in height and the transport pathway increases in length, the hydraulic resistance of the vascular tissue should increase. This has led many to question whether trees can rely on a passive transport mechanism to move carbohydrates from their leaves to their roots. Although species that actively load sugars into their phloem, such as vines and herbs, can increase the driving force for transport as they elongate, it is possible that many trees cannot generate high turgor pressures because they do not use transporters to load sugar into the phloem. Here, we examine how trees can maintain efficient carbohydrate transport as they grow taller by analysing sieve tube anatomy, including sieve plate geometry, using recently developed preparation and imaging techniques, and by measuring the turgor pressures in the leaves of a tall tree in situ. Across nine deciduous species, we find that hydraulic resistance in the phloem scales inversely with plant height because of a shift in sieve element structure along the length of individual trees. This scaling relationship seems robust across multiple species despite large differences in plate anatomy. The importance of this scaling becomes clear when phloem transport is modelled using turgor pressures measured in the leaves of a mature red oak tree. These pressures are of sufficient magnitude to drive phloem transport only in concert with structural changes in the phloem that reduce transport resistance. As a result, the key to the long-standing mystery of how trees maintain phloem transport as they increase in size lies in the structure of the phloem and its ability to change hydraulic properties with plant height. The phloem is the system of ‘blood vessels’ that translocates carbohydrates from the leaves to different plant organs. Here, using new structural imaging and pressure measuring tools, the researchers show interesting phloem structural changes that ensure a passive transport mechanism in tall trees.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/449/1736 / 631/449/1870 / Anatomy / Biological Transport / Biomedical and Life Sciences / Blood vessels / Carbohydrate Metabolism / Carbohydrates / Deciduous trees / Height / Hydraulic properties / Hydraulics / Imaging techniques / Leaves / Life Sciences / Organs / Phloem / Phloem - anatomy & histology / Phloem - metabolism / Plant Leaves - metabolism / Plant Roots - metabolism / Plant Sciences / Plant tissues / Quercus - metabolism / Sieve plates / Species / Sugar / Trees / Trees - metabolism / Turgor / Vascular tissue / Vines / Xylem - anatomy & histology / Xylem - metabolism