Scaling of phloem structure and optimality of photoassimilate transport in conifer needles

TL;DR

This study investigates the structure of phloem in conifer needles, revealing common traits and a power law distribution of conductive elements, and proposes a minimal model based on energy minimization for transport efficiency.

Contribution

The paper introduces a minimal model explaining phloem structure in conifer needles, emphasizing energy efficiency over speed, supported by empirical measurements across species.

Findings

Phloem size follows a power law distribution.

Common structural traits are observed across diverse conifer species.

Transport energetics dominate over translocation speed in structural optimization.

Abstract

The phloem vascular system facilitates transport of energy-rich sugar and signaling molecules in plants, thus permitting long range communication within the organism and growth of non-photosynthesizing organs such as roots and fruits. The flow is driven by osmotic pressure, generated by differences in sugar concentration between distal parts of the plant. The phloem is an intricate distribution system, and many questions about its regulation and structural diversity remain unanswered. Here, we investigate the phloem structure in the simplest possible geometry: a linear leaf, found, for example, in the needles of conifer trees. We measure the phloem structure in four tree species representing a diverse set of habitats and needle sizes, from 1 cm (\textit{Picea omorika}) to 35 cm (\textit{Pinus palustris}). We show that the phloem shares common traits across these four species and find that the size of its conductive elements obeys a power law. We present a minimal model that accounts for these common traits and takes into account the transport strategy and natural constraints. This minimal model predicts a power law phloem distribution consistent with transport energy minimization, suggesting that energetics are more important than translocation speed at the leaf level.