Tree Physiology

A tome for the phloem: Tree Physiology’s Special Issue on Phloem

Phloem is a crucial tissue in trees, responsible for transporting carbon and cycling of nutrients throughout the plant. However, there is comparatively little information on phloem physiology compared to other tissues. That is why Tree Physiology has dedicated an entire issue to phloem physiology.

Phloem diagram
Phloem transports nutrients throughout vascular plants. Unlike the xylem, which transports water, phloem tissues are living and require support cells to maintain function. Image: Kelvin Ma / Wikipedia

While the xylem is responsible for transporting water from the roots to leaves of a tree, transport through the phloem is more complex, with carbohydrates moving from ‘sources’ to ‘sinks’. Daniel Epron and colleagues provide an overview of the findings reported in this issue of some of the difficulties in working with phloem. One barrier to phloem research is that when you cut into phloem tissues, special proteins called P-protein rapidly seal the wound and stem the flow of fluids. The use of aphid stylets inhibits the action of P-protein and allows extraction phloem fluids, however only tiny volumes can be obtained, making the technique difficult on a large scale.

Given the importance of phloem in sugar transport, how does drought affect phloem function? To address this question, Yann Salmon and colleagues review what we know about drought impacts on phloem transport. Their key findings are that drought intensity is important to knowing when (or if) phloem transport will respond, that phloem function is critical to understanding carbon flow through trees and ecosystems, and that we urgently need more information on these processes. To address this gap, Benjamin Hesse and colleagues look at the impact of repeated drought on sugar transport through the phloem. Using specially labelled carbon dioxide, their findings suggest that water uptake in the phloem is impaired under drought, reducing the rate of transport of materials through the phloem. Meanwhile, Masako Dannoura and colleagues looked at the impact of drought on the structural development of phloem, finding that the phloem cell size was reduced under drought, directly causing a reduction in the phloem to transport capacity. Lastly, Michiel Hubeau and colleagues present a new method for studying phloem under drought using positron emission tomography (PET, one of the techniques used in medical diagnostics). They were successful in studying phloem function without destructive harvesting, which promises to speed up phloem research.

The special issue isn’t all about drought, and while many questions surrounding phloem remain, the collection of articles makes progress in understanding how phloem responds to changing environmental conditions, and sets a framework for further research