Allocation, stress tolerance and carbon transport in plants: how does phloem physiology affect plant ecology?

Despite the crucial role of carbon transport in whole plant physiology and its impact on plant-environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem-phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment. This review highlights the important but understudied role of phloem physiology in mediating how plants interact with their biotic and abiotic environment and shaping larger ecological patterns. We focus on three critical areas of current research: interactions between the xylem and phloem, carbon fluxes both in plants and at the ecosystem scale and interactions between plants and their biotic environment. The goal of this review is to draw attention to the critical role of carbon transport in plant physiological ecology and outline many of the questions that remain to be answered about this critical part of the plant vascular system.