Seasonal changes in temperate woody plant phloem anatomy and physiology: implications for long-distance transport

• Published in: AoB plants 2021-08, Vol.13 (4), p.plab028-plab028
• Main Authors: Ray, Dustin M, Savage, Jessica A
• Format: Article
• Language: English
• Subjects: Editor's Choice; Reviews
• ISSN: 2041-2851; 2041-2851
• DOI: 10.1093/aobpla/plab028

Seasonal changes in climate are accompanied by shifts in carbon allocation and phenological changes in woody angiosperms, the timing of which can have broad implications for species distributions, interactions and ecosystem processes. During critical transitions from autumn to winter and winter to spring, physiological and anatomical changes within the phloem could impose a physical limit on the ability of woody angiosperms to transport carbon and signals. There is a paucity of the literature that addresses tree (floral or foliar) phenology, seasonal phloem anatomy and seasonal phloem physiology together, so our knowledge of how carbon transport could fluctuate seasonally, especially in temperate climates is limited. We review phloem phenology focussing on how sieve element anatomy and phloem sap flow could affect carbon availability throughout the year with a focus on winter. To investigate whether flow is possible in the winter, we construct a simple model of phloem sap flow and investigate how changes to the sap concentration, pressure gradient and sieve plate pores could influence flow during the winter. Our model suggests that phloem transport in some species could occur year-round, even in winter, but current methods for measuring all the parameters surrounding phloem sap flow make it difficult to test this hypothesis. We highlight outstanding questions that remain about phloem functionality in the winter and emphasize the need for new methods to address gaps in our knowledge about phloem function. How does sugar movement change seasonally and can sugars be moved during the winter when temperate trees are largely dormant? We review the current knowledge of seasonal changes to the sugar-conducting tissue, the phloem, and use a simple model to facilitate a discussion of whether or not the phloem could be more active than previously assumed in some species.